11 Apr
2010
11 Apr
'10
11:25 a.m.
Has anyone on the list seen or work with one of these? I received a
Morrow M-20 8-inch hard disk along with my N* Horizon and found the
Memorex 102 disk mechanism inside (what a beast!).
At initial power-up, a 10-ohm 5W resistor near the power connector let out
a puff of smoke (the drive was in the process of spinning up). I killed
power and started checking components carefully. There were no ominous
readings across the 5V logic power rails and I traced things out enough to
determine that this resistor was between the +24V power input and whatever
lies downstream (probably motor servo).
The platters move freely, so the motor certainly wasn't bound up. When I
checked power supply voltages (it's an analog supply), I discovered that
the +24V rail was delivering +44V (!) and the -12V rail was -15V. The 5V
rail was right on spec.
For a quick reality check, I plugged the supply into my bench variac and
adjusted input voltage to yield +24V at the motor supply. Both other
rails were within spec at this point.
When I tried again to spin the drive up at the correct voltage, the series
resistor became extremely hot although, again, the drive was on its way to
full rotational speed at the time. I measured 20V across the 10-ohm
resistor, which calculates to 2A and a dissipation of 10W. The latter
number explains the heat. Not wanting to tempt fate, I shut things down
quickly.
So, opinions and suggestions please? A two-amp draw does not seem
excessive to me, although clearly it's outside the design parameters given
the rating of the dropping resistor. The power supply is certainly
reparable, and before anyone asks it is delivering clean DC - ripple is on
the order of 10mv. Why it's doubling its output is yet to be determined,
but I did find a diagram at bitsavers.
One thought that occured to me is that the +24V rail is probably powering
the voice-coil positioner in addition to the motor. I felt around
extensively, and none of the power devices involved with head motion or
servo is excessively warm. None of them appear shorted or open.
Not sure where to go from here.
Steve
--
11 Apr
11 Apr
5:51 p.m.
On Sun, 11 Apr 2010, Steven Hirsch wrote:
Why it's doubling its output is yet to be
determined, but I did find a
diagram at bitsavers.
One mystery solved: 2N6576 power Darlington with Emitter-Collector short.
Still need to figure out why the drive is pulling so much current...
--
12 Apr
12 Apr
1:08 p.m.
Has anyone on the list seen or work with one of these? I received a
Morrow M-20 8-inch hard disk along with my N* Horizon and found the
Memorex 102 disk mechanism inside (what a beast!).
At initial power-up, a 10-ohm 5W resistor near the power connector let out
a puff of smoke (the drive was in the process of spinning up). I killed
power and started checking components carefully. There were no ominous
readings across the 5V logic power rails and I traced things out enough to
determine that this resistor was between the +24V power input and whatever
lies downstream (probably motor servo).
It would be worth knowing what that resistor feeds. The obvious things to
need the 24V rail are the spindle motor and positioner (which could be a
stepper nmotor in an old/small drive like this, or it might be a voice
coil). I would also suspect that the 24V line is regualted down to 12V
for some of the analogue circuitry.
The platters move freely, so the motor certainly wasn't bound up. When I
checked power supply voltages (it's an analog supply), I discovered that
the +24V rail was delivering +44V (!) and the -12V rail was -15V. The 5V
rail was right on spec.
Ouch. It's possible there's some kind of overvoltage protection
downstream of that resistor (like a fat zneer diode) which has shorted.
Or maybe somthign took a dislike to the high votlage.
Or perhaps there is a real fault. Since it spins up, I guess the spindle
motor is OK. Can you diconnect the positioner actuator and see if the
current drops then?
Also, find some of the analouge chips in the read amplifier circuit and
measure their supply voltages. They should almost certainly be
symmetrical about ground. If the +ve voltage is low or missing, trace it
back to where it comes from (maybe a 3 terminal regulator like a 7812)
For a quick reality check, I plugged the supply into my bench variac and
adjusted input voltage to yield +24V at the motor supply. Both other
rails were within spec at this point.
When I tried again to spin the drive up at the correct voltage, the series
resistor became extremely hot although, again, the drive was on its way to
full rotational speed at the time. I measured 20V across the 10-ohm
resistor, which calculates to 2A and a dissipation of 10W. The latter
Am I missing something? 2A and 20V is a power dissipation of 40W.
number explains the heat. Not wanting to tempt fate,
I shut things down
quickly.
So, opinions and suggestions please? A two-amp draw does not seem
Firstly, fix the PSU. I would be much happier knowing that that was
nothign to do with the problem.
Measure the current drawn from the 24V line (I susepct it's rather more
than the 2A measured through that resistor, and that some parts fo the
drive are not suplied via it)
Remove the reissotr and power up. Does the spindle run? Can you find
which parts of the drive are now lacking a +ve supply line?
excessive to me, although clearly it's outside the
design parameters given
the rating of the dropping resistor. The power supply is certainly
reparable, and before anyone asks it is delivering clean DC - ripple is on
the order of 10mv. Why it's doubling its output is yet to be determined,
but I did find a diagram at bitsavers.
One thought that occured to me is that the +24V rail is probably powering
the voice-coil positioner in addition to the motor. I felt around
extensively, and none of the power devices involved with head motion or
servo is excessively warm. None of them appear shorted or open.
If you know it's a voice coil, try measuring the voltage across the voice
coil. If ti's being driven hard, it will draw a high current from the
supply (after all the coil has a pretty low DC resistance). Maybe the
heads have stuck so the thing is not managing to move the heads to find
the servo data (but it's still trying). Maybe there's something wrong
with the driver circuitry, but not the output stage.
-tony
2:03 p.m.
On Mon, 12 Apr 2010, Tony Duell wrote:
I traced things a bit further. The 24V line from the power connector runs
directly to the NO contact of a relay. The leg that runs through the
scorched resistor ends up at a TO220 transister. I'll try to get more
detail on its downstream side.
I'm wondering if the resistor might be there to limit inrush current?
Maybe the relay closes at servo lock and feeds power directly to the
driver transistors at that point?
I haven't been able to find a low-resistance path - it's one of the first
things I checked for. Also, nothing else on the board is getting even
slightly warm. That's more fuel for this being a startup mode?
Slip of the fingers: It's 10V, not 20. At 40W, that poor resistor would
have gone up in a ball of fire :-)
The heads are not stuck. If I tip the drive, I can watch them slide
across the platter from their own weight. Should have mentioned before
that this thing has a translucent plastic top :-).
Thanks very much for your suggestions, Tony.
Steve
--
At initial
power-up, a 10-ohm 5W resistor near the power connector let out
a puff of smoke (the drive was in the process of spinning up). I killed
power and started checking components carefully. There were no ominous
readings across the 5V logic power rails and I traced things out enough to
determine that this resistor was between the +24V power input and whatever
lies downstream (probably motor servo).
It would be worth knowing what that resistor feeds. The obvious things to
need the 24V rail are the spindle motor and positioner (which could be a
stepper nmotor in an old/small drive like this, or it might be a voice
coil). I would also suspect that the 24V line is regualted down to 12V
for some of the analogue circuitry. The platters
move freely, so the motor certainly wasn't bound up. When I
checked power supply voltages (it's an analog supply), I discovered that
the +24V rail was delivering +44V (!) and the -12V rail was -15V. The 5V
rail was right on spec.
Ouch. It's possible there's some kind of overvoltage protection
downstream of that resistor (like a fat zneer diode) which has shorted.
Or maybe somthign took a dislike to the high votlage. Or perhaps there is a real fault. Since it spins up, I
guess the spindle
motor is OK. Can you diconnect the positioner actuator and see if the
current drops then?
If I can figure out which pins are which. The motor and positioner leads
are in the same harness with a common multipin connector.
Also, find some of the analouge chips in the read
amplifier circuit and
measure their supply voltages. They should almost certainly be
symmetrical about ground. If the +ve voltage is low or missing, trace it
back to where it comes from (maybe a 3 terminal regulator like a 7812)
Will try this.
For a quick
reality check, I plugged the supply into my bench variac and
adjusted input voltage to yield +24V at the motor supply. Both other
rails were within spec at this point.
When I tried again to spin the drive up at the correct voltage, the series
resistor became extremely hot although, again, the drive was on its way to
full rotational speed at the time. I measured 20V across the 10-ohm
resistor, which calculates to 2A and a dissipation of 10W. The latter
Am I missing something? 2A and 20V is a power dissipation of 40W. Firstly, fix the PSU. I would be much happier knowing
that that was
nothign to do with the problem.
I have the power Darlington on order (shorted).
Measure the current drawn from the 24V line (I susepct
it's rather more
than the 2A measured through that resistor, and that some parts fo the
drive are not suplied via it)
Remove the reissotr and power up. Does the spindle run? Can you find
which parts of the drive are now lacking a +ve supply line?
That will be my next move - I'm betting it won't spin up.
One thought
that occured to me is that the +24V rail is probably powering
the voice-coil positioner in addition to the motor. I felt around
extensively, and none of the power devices involved with head motion or
servo is excessively warm. None of them appear shorted or open.
If you know it's a voice coil, try measuring the voltage across the voice
coil. If ti's being driven hard, it will draw a high current from the
supply (after all the coil has a pretty low DC resistance). Maybe the
heads have stuck so the thing is not managing to move the heads to find
the servo data (but it's still trying). Maybe there's something wrong
with the driver circuitry, but not the output stage.
13 Apr
13 Apr
12:11 p.m.
On Mon, 12 Apr 2010, Tony Duell wrote:
I traced things a bit further. The 24V line from the power connector runs
directly to the NO contact of a relay. The leg that runs through the
scorched resistor ends up at a TO220 transister. I'll try to get more
detail on its downstream side.
I'm wondering if the resistor might be there to limit inrush current?
Maybe the relay closes at servo lock and feeds power directly to the
driver transistors at that point?
I've had a look at the Memorex 112 manual on Bitsavers. If your drive is
similar, you might bave big problems.
The relay contact is part of a 4-pole relay used to turn on the spindle
motor (the other 3 poles connect braking resistors ot the windings) The
10 Ohm (5W) resistor is the fred to the collector of a power transistor,
whcih is the pass transistor for a simple 12V regulator. The emitter of
that transistor is the output of the 12V regualtor and feeds a lot of
analuge circuitry.
My guess is that the transsitor is OK (but it would be worth checking) and
that you have something downstream of it that is drawing exceessive
current. It might be a shorted decoupling capacitor, it might be an IC.
And to make things worse there seem to be some custom analogue ICs in
this drive.
First test that I would do is to measure the voltages (wrt ground) on the
3 leads of that transsitor.
At
initial power-up, a 10-ohm 5W resistor near the power connector let out
a puff of smoke (the drive was in the process of spinning up). I killed
power and started checking components carefully. There were no ominous
readings across the 5V logic power rails and I traced things out enough to
determine that this resistor was between the +24V power input and whatever
lies downstream (probably motor servo).
It would be worth knowing what that resistor feeds. The obvious things to
need the 24V rail are the spindle motor and positioner (which could be a
stepper nmotor in an old/small drive like this, or it might be a voice
coil). I would also suspect that the 24V line is regualted down to 12V
for some of the analogue circuitry. The heads are not stuck. If I tip the drive, I can
watch them slide
across the platter from their own weight. Should have mentioned before
that this thing has a translucent plastic top :-).
RIght..The Model 112 is a stapepr motor positioned, BTW. That is what I'd
expect in a low capacity, but large platter drive like this. The trackls
would be suufficiently wide that a stepper motor would be fine. You need
a voice coil positioner taking a servo signal from the disk if you have a
high tranck density, of course.
-tony
2:47 p.m.
On Tue, 13 Apr 2010, Tony Duell wrote:
I've had a look at the Memorex 112 manual on
Bitsavers. If your drive is
similar, you might bave big problems.
I believe the drives are very similar, but haven't had much time to peruse
the schematics yet.
The relay contact is part of a 4-pole relay used to
turn on the spindle
motor (the other 3 poles connect braking resistors ot the windings) The
10 Ohm (5W) resistor is the fred to the collector of a power transistor,
whcih is the pass transistor for a simple 12V regulator. The emitter of
that transistor is the output of the 12V regualtor and feeds a lot of
analuge circuitry.
My guess is that the transsitor is OK (but it would be worth checking) and
that you have something downstream of it that is drawing exceessive
current. It might be a shorted decoupling capacitor, it might be an IC.
And to make things worse there seem to be some custom analogue ICs in
this drive.
I'll keep my fingers crossed on that.
> The heads are not stuck. If I tip the drive, I
can watch them slide
> across the platter from their own weight. Should have mentioned before
> that this thing has a translucent plastic top :-).
>
RIght..The Model 112 is a stapepr motor positioned,
BTW. That is what I'd
expect in a low capacity, but large platter drive like this. The trackls
would be suufficiently wide that a stepper motor would be fine. You need
a voice coil positioner taking a servo signal from the disk if you have a
high tranck density, of course.
Ugh. I'm not sure what I was smoking when I claimed it was a voice-coil
positioner... It is indeed a stepper motor. Sorry about that!
Steve
--
12 Apr
12 Apr
7:01 p.m.
On Mon, 12 Apr 2010, Tony Duell wrote:
The drive is not particularly small, BTW. This is a beast of an 8" drive
mechanism with five platters and a voice-coil positioner.
I removed the resistor and it does not try to spin. Traced things out as
best I could (it's a three-layer board with limited visibility to the
interior plane). The +24V input runs directly and almost solely to the
armature of one SPDT relay contact set. The NO position goes directly to
the motor / armature connector and the NC position goes nowhere. This
gives some support to my theory that the resistor and associated circuitry
is there to limit inrush current. I think this relay is supposed to pull
in rather quickly and feed power directly to the motor.
The downstream leg of the 10-ohm power resistor goes to a 2SD768 power
device and I'm having a lot of trouble following it from there. I think
your guess about a regulator for the 12V circuitry is probably correct.
Steve
--
Has anyone on the list seen or work with one of these? I received a
Morrow M-20 8-inch hard disk along with my N* Horizon and found the
Memorex 102 disk mechanism inside (what a beast!).
At initial power-up, a 10-ohm 5W resistor near the power connector let out
a puff of smoke (the drive was in the process of spinning up). I killed
power and started checking components carefully. There were no ominous
readings across the 5V logic power rails and I traced things out enough to
determine that this resistor was between the +24V power input and whatever
lies downstream (probably motor servo).
It would be worth knowing what that resistor feeds. The obvious things to
need the 24V rail are the spindle motor and positioner (which could be a
stepper nmotor in an old/small drive like this, or it might be a voice
coil). I would also suspect that the 24V line is regualted down to 12V
for some of the analogue circuitry.
13 Apr
13 Apr
12:19 p.m.
I removed the resistor and it does not try to spin.
Traced things out as
best I could (it's a three-layer board with limited visibility to the
interior plane). The +24V input runs directly and almost solely to the
armature of one SPDT relay contact set. The NO position goes directly to
the motor / armature connector and the NC position goes nowhere. This
gives some support to my theory that the resistor and associated circuitry
is there to limit inrush current. I think this relay is supposed to pull
in rather quickly and feed power directly to the motor.
The downstream leg of the 10-ohm power resistor goes to a 2SD768 power
device and I'm having a lot of trouble following it from there. I think
your guess about a regulator for the 12V circuitry is probably correct.
THis does sound very like the schematic that I was looking at in the 112
manual. The relay there eneables the spindle motor (and maybe the
positioner drivers too), the resistor feeds a power transistor that's the
pass tranasitor for the 12V regualtor.
The fact that the drive fails totally if the resistor is removed would
seem to indicate that the otuput of that 12V regulator is somewhat
present.
I have suggested in my least message that iy would eb worth measuring the
votlages on the leads of that transistor.
What if the transistor was shorted? The regualtor would then become a
simple resistor + zner circuit. The reissotr would ahve to drop 10 or
12V, and woul;d get hot and bothered!. The zener diode wouldn't like it
much either (are you sure _nothing_ else is getting hot?). Anyway, it's
worth checking this.
-tony
7:05 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
On Tue, 13 Apr 2010, Tony Duell wrote:
As you surmised, the problem was on the 12V rail, which measured 0.5 ohms
to ground. It runs to quite a number of chips, but was surprising easy to
follow around and through the various via holes. I dragged out the dremel
tool and did a binary search by slicing traces. Took the full logN tries,
but on the last cut I nailed a 4.7ufd / 16V tantalum cap with a dead short
:-).
I could _really_ learn to start hating the blasted things. It seems like
almost every failure of this type has a tantalum cap behind it - on both
audio and computer gear. Just finished resurrecting a KLH/Burwen phono
noise suppressor in which EVERY blasted one of the things was shorted.
My eyes are crossed from staring through the board at a bright light...
Will patch traces and reassemble tomorrow. I'll let you know if things
cooperate.
Thanks again for getting me headed in the right direction!
Steve
--
The downstream
leg of the 10-ohm power resistor goes to a 2SD768 power
device and I'm having a lot of trouble following it from there. I think
your guess about a regulator for the 12V circuitry is probably correct.
THis does sound very like the schematic that I was looking at in the 112
manual. The relay there eneables the spindle motor (and maybe the
positioner drivers too), the resistor feeds a power transistor that's the
pass tranasitor for the 12V regualtor.
The fact that the drive fails totally if the resistor is removed would
seem to indicate that the otuput of that 12V regulator is somewhat
present.
I have suggested in my least message that iy would eb worth measuring the
votlages on the leads of that transistor.
What if the transistor was shorted? The regualtor would then become a
simple resistor + zner circuit. The reissotr would ahve to drop 10 or
12V, and woul;d get hot and bothered!. The zener diode wouldn't like it
much either (are you sure _nothing_ else is getting hot?). Anyway, it's
worth checking this. 
14 Apr
14 Apr
7:45 a.m.
New subject: Memorex 102 20MB Hard disk - Found It!
From: snhirsch at gmail.com
---snip---
As you surmised, the problem was on the 12V rail,
which measured 0.5 ohms
to ground. It runs to quite a number of chips, but was surprising easy to
follow around and through the various via holes. I dragged out the dremel
tool and did a binary search by slicing traces. Took the full logN tries,
but on the last cut I nailed a 4.7ufd / 16V tantalum cap with a dead short
:-).
Hi
Ahhhh!
I have a none destructive method of locating such shorts. It requires
a bench power supply and a good DVM.
You set the supply on a low voltage and current limit at about 2 amps.
Place the outputs of the supply from end to far end of the 12V rail
( not through the short ).
A small voltage will drop across the trace from end to end. With
a DVM on a 200 mv scale, one should be able to see this drop.
Place one meter lead on the ground. With the other lead, probe
along the 12V rail.
When the voltage on the DVM goes to 0, you've reached the location
of the short.
To develope enough voltage across the length of the trace, you
may need to increase the current. Check the trace after each
increase to make sure it is not gettin to hot. Power rails can usually
take quite a bit.
If the rail has a number of fingers, first connect the supply along the
main rail and locate the shorted finger. Next move one of the supply
leads to the end of that finger and repeat the probing.
This method can be extended to power planes ( I've done this )
but it is a little hard to describe in text and easier to show.
I have used this metode many times and never failed to locate
the short. It works better with a 5 digit voltmeter but I've used
a 4 digit in a pinch.
Dwight
_________________________________________________________________
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7:51 a.m.
New subject: Make magazine interviews Forest Mims about Ed Roberts
11:06 a.m.
New subject: Make magazine interviews Forest Mims about Ed Roberts
I read the 1975 Popular Electronics but I never owned an Altair
computer. I did build an IMSAI 8080 for someone else and used a
Processor Technology SOL; my system of choice was the Southwest
Technical Products 6800. I few years ago I started improving the Altair
8800 article on Wikipedia and ended up doing a lot of detailed research
on Micro Instrumentation and Telemetry Systems. Bruce Damer of DigiBarn
gave Forrest Mims my name and I ended up writing several more Wikipedia
articles on MITS and the Altair. I would get Forrest to review my work
and he also provided details and photographs. In November of last year,
Forrest told Bruce and me about Ed Roberts's illness. I started a new
biography on Ed Roberts that I finished in March and placed it on
Wikipedia.
The only MITS product I ever owned was a copy of Altair 680 BASIC that I
bought to use on my SWTPC6800 computer. I ended up being a historian on
Altair and MITS.
Here are some of my Wikipedia articles:
http://en.wikipedia.org/wiki/Micro_Instrumentation_and_Telemetry_Systems
http://en.wikipedia.org/wiki/Ed_Roberts_(computers)
http://en.wikipedia.org/wiki/Model_Rocketry_(magazine)
http://en.wikipedia.org/wiki/Forrest_Mims
http://en.wikipedia.org/wiki/Open_Letter_to_Hobbyists
http://en.wikipedia.org/wiki/Altair_8800
I could use some pictures of Altair boards, especially a 4K Dynamic RAM
board.
Michael Holley
swtpc at comcast.net
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org] On Behalf Of Christian Liendo
Sent: Wednesday, April 14, 2010 7:52 AM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Make magazine interviews Forest Mims about Ed Roberts
http://blog.makezine.com/archive/2010/04/remembering_ed_roberts_the_fath
er_o.html
12:22 p.m.
New subject: Make magazine interviews Forest Mims about Ed Roberts
Michael,
The only MITS product I ever owned was a copy of
Altair 680 BASIC that I
bought to use on my SWTPC6800 computer. I ended up being a historian on
Altair and MITS.
Really glad to see that someone is doing this.
I could use some pictures of Altair boards, especially
a 4K Dynamic RAM
board.
I've got many pictures of my Altairs up on my site, both closed and opened
up. I've also got photos of a number of MITS cards, including the 4K DRAM
board - click on the "S100" header to go to the S100-cards library, and from
there you will find photos and manuals for many different S100 cards. Please
feel free to use any photos you wish. If you need larger/higher-res photos
of anything, contact me and I'll see what I can do.
Dave
--
dave09 (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Classic Computers: http://www.classiccmp.org/dunfield/
8:10 a.m.
New subject: Locating short circuits (was Memorex 102 20MB Hard disk)
On Wed, 14 Apr 2010, dwight elvey wrote:
I have a none destructive method of locating such
shorts. It requires
a bench power supply and a good DVM.
Has anyone here used an ESR meter to locate a short across the power
rails? If so, what's the technique?
Mike Loewen mloewen at cpumagic.scol.pa.us
Old Technology http://sturgeon.css.psu.edu/~mloewen/Oldtech/
9:25 a.m.
New subject: Locating short circuits (was Memorex 102 20MB Hard disk)
Mike Loewen wrote:
On Wed, 14 Apr 2010, dwight elvey wrote:
I guess you mean this
http://www.electronicrepairguide.com/esr-meter.html
But you really want a Milli-Ohm Meter to find shorts with more precision
when it comes to connections with several components on there, about 0.1
milli-ohm resolution. That lets you pinpoint the short directly to the
sport on the pcb, even between the next pin on a chip. I just found a
place online the other week that has one for $150 from Ruby Electronics,
other costs 3 times as much, but I still have to wait to get one
myself--other bills still have priority :)
http://tinyurl.com/y4d7op5
=Dan
I have a none destructive method of locating such
shorts. It requires
a bench power supply and a good DVM.
Has anyone here used an ESR meter to locate a short across the
power rails? If so, what's the technique?
10:52 a.m.
New subject: Locating short circuits (was Memorex 102 20MB Hard disk)
From: ragooman at comcast.net
Mike Loewen wrote:
Hi
It is obvious that you have never tried my method if you truly
believe that a Milli-Ohm meter is easier to find shorts than the
method I describe. I've spent many years using various techniques
to find shorts on all kinds of boards and systems.
I've use TDRs, Milli-Ohm meters, thermal paper and even expensive
HP current probes.
Of these, the method I describes was the simplest and cheapest method.
It requires no special equipment and is far superior to a milli-ohm meter.
The other methods I'd mentioned are superior to the milli-ohm meter
method.
Dwight
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On Wed, 14 Apr 2010, dwight elvey wrote:
I guess you mean this
http://www.electronicrepairguide.com/esr-meter.html
But you really want a Milli-Ohm Meter to find shorts with more precision
when it comes to connections with several components on there, about 0.1
milli-ohm resolution. That lets you pinpoint the short directly to the
sport on the pcb, even between the next pin on a chip. I just found a
place online the other week that has one for $150 from Ruby Electronics,
other costs 3 times as much, but I still have to wait to get one
myself--other bills still have priority :)
http://tinyurl.com/y4d7op5
I have a none destructive method of locating such
shorts. It requires
a bench power supply and a good DVM.
Has anyone here used an ESR meter to locate a short across the
power rails? If so, what's the technique?
9:27 a.m.
New subject: Locating short circuits (was Memorex 102 20MB Hard disk)
Mike Loewen wrote:
On Wed, 14 Apr 2010, dwight elvey wrote:
I guess you mean this
http://www.electronicrepairguide.com/esr-meter.html
But you really want a Milli-Ohm Meter to find shorts with more precision
when it comes to connections with several components on there, about 0.1
milli-ohm resolution. That lets you pinpoint the short directly to the
sport on the pcb, even between the next pin on a chip. I just found a
place online the other week that has one for $150 from Ruby Electronics,
other costs 3 times as much, but I still have to wait to get one
myself--other bills still have priority :)
http://tinyurl.com/y4d7op5
=Dan
I have a none destructive method of locating such
shorts. It requires
a bench power supply and a good DVM.
Has anyone here used an ESR meter to locate a short across the
power rails? If so, what's the technique? 
9:48 a.m.
New subject: Locating short circuits (was Memorex 102 20MB Hard disk)
On 14 Apr 2010 at 12:27, Dan Roganti wrote:
But you really want a Milli-Ohm Meter to find shorts
with more
precision when it comes to connections with several components on
there, about 0.1 milli-ohm resolution. That lets you pinpoint the
short directly to the sport on the pcb, even between the next pin on a
chip. I just found a place online the other week that has one for $150
from Ruby Electronics, other costs 3 times as much, but I still have
to wait to get one myself--other bills still have priority :)
http://tinyurl.com/y4d7op5
As long as you don't require calibration, isn't a milliohmmeter for
this application pretty much a matter of wiring up an inexpensive op-
amp with a small current source?
http://baec.tripod.com/DEC90/ohmmeter.htm
Here's a neat little circuit that uses an LM317 regulator. You could
build it out of components in your hellbox in a matter of minutes:
http://www.edn.com/article/CA408390.html
The basic idea is that you don't actually need an accurate
quantitative measurement, but rather a relative one as you probe down
the PCB.
Alternatively, one could use a wheatstone or kelvin bridge with a
microammeter.
While a lab milliohmmeter would be a cool thing to have, it's not
necessary.
Cheers,
Chuck
10:16 a.m.
New subject: Locating short circuits (was Memorex 102 20MB Hard disk)
----- Chuck Guzis wrote:
As long as you don't require calibration, isn't a milliohmmeter for
this application pretty much a matter of wiring up an inexpensive op-
amp with a small current source?
http://baec.tripod.com/DEC90/ohmmeter.htm
Here's a neat little circuit that uses an LM317 regulator. You could
build it out of components in your hellbox in a matter of minutes:
http://www.edn.com/article/CA408390.html
Hey, I like that, especially when it saves me money !!
I didn't have much luck googling that before when I needed one.
Thanks again for the link.
You must be the Grandmaster Google Search Meister
=Dan
--
http://www.vintagecomputer.net/ragooman/
6:49 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
On Wed, 14 Apr 2010, dwight elvey wrote:
Thanks for the tip, Dwight! I've actually mused about doing something
like that, but never quite had the nerve. Will keep it in mind for the
future.
Steve
--
From: snhirsch at gmail.com
---snip---
As you surmised, the problem was on the 12V rail,
which measured 0.5 ohms
to ground. It runs to quite a number of chips, but was surprising easy to
follow around and through the various via holes. I dragged out the dremel
tool and did a binary search by slicing traces. Took the full logN tries,
but on the last cut I nailed a 4.7ufd / 16V tantalum cap with a dead short
:-).
Hi
Ahhhh!
I have a none destructive method of locating such shorts. It requires
a bench power supply and a good DVM.
You set the supply on a low voltage and current limit at about 2 amps.
Place the outputs of the supply from end to far end of the 12V rail
( not through the short ).
A small voltage will drop across the trace from end to end. With
a DVM on a 200 mv scale, one should be able to see this drop.
Place one meter lead on the ground. With the other lead, probe
along the 12V rail.
When the voltage on the DVM goes to 0, you've reached the location
of the short.
To develope enough voltage across the length of the trace, you
may need to increase the current. Check the trace after each
increase to make sure it is not gettin to hot. Power rails can usually
take quite a bit.
If the rail has a number of fingers, first connect the supply along the
main rail and locate the shorted finger. Next move one of the supply
leads to the end of that finger and repeat the probing.
This method can be extended to power planes ( I've done this )
but it is a little hard to describe in text and easier to show.
I have used this metode many times and never failed to locate
the short. It works better with a 5 digit voltmeter but I've used
a 4 digit in a pinch. 
15 Apr
15 Apr
2:25 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
dwight elvey wrote:
I have a none destructive method of locating such shorts. It requires
a bench power supply and a good DVM.
You set the supply on a low voltage and current limit at about 2 amps.
Place the outputs of the supply from end to far end of the 12V rail
( not through the short ).
This is a great system Dwight. The only problem I have with it is the
requirement for a five digit DVM, and the fact that after you find the
short, you need to fix it. This can be time-consuming.
My solution uses much the same procedure, except that I use an ARC
welder and connect that up. With all of that current available, I don't
need to diddle around looking for the short -- the short gets vaporized.
So not only will I find the short (by the flash) but the repair is done
at the same time. This procedure never fails and can be a real time-saver.
Barry
p.s. Please note that I'm kidding! I don't post very often but I
really do enjoy reading what you're all working on. I love the old kit
(particularly DEC stuff) and there's always lots to read about here.
2:40 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
From: blkline at attglobal.net
dwight elvey wrote:
LOL!
The beauty of my method is that it doesn't depend on the resistance of
the shorting element. Often times the short will change every time
you move the lead of an ohm meter. Other times, the short may be
on the order of an ohm or more. My method still works because
it doesn't depend on current through the short. To use an Ohm meter
effectively requires a 4 point system. Most ESR meters that I've seen
are not 4 point.
If you don't have a 5 digit, you can crank the current up more.
Most traces can handle 5+ amps or so.
You do need 5 digits to find shorts on ground planes with my method.
It just takes a lot of current to get a good voltage drop across the board.
I do know several people that believe in using the ARC welder method.
Dwight
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I have a none destructive method of locating such shorts. It requires
a bench power supply and a good DVM.
You set the supply on a low voltage and current limit at about 2 amps.
Place the outputs of the supply from end to far end of the 12V rail
( not through the short ).
This is a great system Dwight. The only problem I have with it is the
requirement for a five digit DVM, and the fact that after you find the
short, you need to fix it. This can be time-consuming.
My solution uses much the same procedure, except that I use an ARC
welder and connect that up. With all of that current available, I don't
need to diddle around looking for the short -- the short gets vaporized.
So not only will I find the short (by the flash) but the repair is done
at the same time. This procedure never fails and can be a real time-saver.
Barry
p.s. Please note that I'm kidding! I don't post very often but I
really do enjoy reading what you're all working on. I love the old kit
(particularly DEC stuff) and there's always lots to read about here.
16 Apr
16 Apr
11:10 a.m.
New subject: Memorex 102 20MB Hard disk - Found It!
My solution uses much the same procedure, except that
I use an ARC
welder and connect that up. With all of that current available, I don't
need to diddle around looking for the short -- the short gets vaporized.
So not only will I find the short (by the flash) but the repair is done
at the same time. This procedure never fails and can be a real time-saver.
Barry
p.s. Please note that I'm kidding! I don't post very often but I
Just in acase somebody takes this seriously, I should point out that the
big problem is that ther connector that vaporuises is the wwakest part of
the ciercuit which is not necessarily the short you want to remove.
And of course when the connection does open you get thefull voltage of
the arc welder across it. This is high enough to do a lot of damage...
-tony
12:03 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
Tony Duell wrote:
Just in acase somebody takes this seriously, I should
point out that the
big problem is that ther connector that vaporuises is the wwakest part of
the ciercuit which is not necessarily the short you want to remove.
And of course when the connection does open you get thefull voltage of
the arc welder across it. This is high enough to do a lot of damage...
That is why you use Condensor Discharge. :)
-tony
14 Apr
14 Apr
12:07 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
As you surmised, the problem was on the 12V rail,
which measured 0.5 ohms
That is 'guessed' :-). Please remember I've never seen this type of
drive, I've certainly not seen the faulty one, and I was working from a
possibly incorrect scheamtic. No, even 'guessed' is too strong a word..
to ground. It runs to quite a number of chips, but
was surprising easy to
follow around and through the various via holes. I dragged out the dremel
tool and did a binary search by slicing traces. Took the full logN tries,
but on the last cut I nailed a 4.7ufd / 16V tantalum cap with a dead short
:-).
I see. I hope that's the only fault. A capaacitor is easy to replace.
I could _really_ learn to start hating the blasted things. It seems like
almost every failure of this type has a tantalum cap behind it - on both
I know the feeling. Often power supply decouplers at least annouce their
failure with smoke signals. But yours didn't even do that.
audio and computer gear. Just finished resurrecting a
KLH/Burwen phono
noise suppressor in which EVERY blasted one of the things was shorted.
My eyes are crossed from staring through the board at a bright light...
Will patch traces and reassemble tomorrow. I'll let you know if things
cooperate.
I hope it works....
-tony
21 Apr
21 Apr
2:58 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
On Wed, 14 Apr 2010, Tony Duell wrote:
A quick update: The drive spins up, comes ready and is passing the seek
test in Morrow's FORMATHD.COM utility. The header read fails on all
sectors and all cylinders, so something is still amiss. I tried
re-writing the sector headers (if I interpret correctly, it is possible to
simply rewrite the headers without disturbing data in the sector proper -
a new one on me!) but that didn't help. No error on write, but they
cannot be read back.
I'll start poking around in the read logic next. Getting closer, anyway.
Steve
--
My eyes are
crossed from staring through the board at a bright light...
Will patch traces and reassemble tomorrow. I'll let you know if things
cooperate.
I hope it works....
22 Apr
22 Apr
1:44 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
A quick update: The drive spins up, comes ready and
is passing the seek
test in Morrow's FORMATHD.COM utility. The header read fails on all
Sinc this drive has a transparent cover, I assume you can see the heads
moving during the seek trst.
sectors and all cylinders, so something is still
amiss. I tried
re-writing the sector headers (if I interpret correctly, it is possible to
simply rewrite the headers without disturbing data in the sector proper -
That is not unheard-of. Howeever, I would be a little wary of writing to
a drive that didn;t read correctly. If something is faulty, it's quite
possible it's going to write somewhere it shouldn't. And since the drive
fails on every sector, I would think the fualt is unlikely to be simply
bad formatting data on the disk.
a new one on me!) but that didn't help. No error
on write, but they
cannot be read back.
What error do you get if you try to read them? Does it manage to find
anything?
I'll start poking around in the read logic next. Getting closer, anyway.
I've not looked at the schematics, but I guess I'd start by seeing if (a)
it was reading anything (do you get any sensible-looking pulses at the
read data output -- and rmemeber that in 99% of drives the pusle width is
irrelevant, it's set by a monostable on the logic board, it's the pulse
spacing that defines the data), and (b) is it selecting one (and only
one) head
-tony
23 Apr
23 Apr
2:23 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
On Thu, 22 Apr 2010, Tony Duell wrote:
Yes, and it tries to walk off the table :-). Serious amount of moving
mass involved!
Ah, well. I doubt there was anything on there of value to me.
No, it fails 64 times to read every sector on every track.
Yes, I'm planning to poke through it with a scope tomorrow.
Steve
--
A quick
update: The drive spins up, comes ready and is passing the seek
test in Morrow's FORMATHD.COM utility. The header read fails on all
Sinc this drive has a transparent cover, I assume you can see the heads
moving during the seek trst. sectors and
all cylinders, so something is still amiss. I tried
re-writing the sector headers (if I interpret correctly, it is possible to
simply rewrite the headers without disturbing data in the sector proper -
That is not unheard-of. Howeever, I would be a little wary of writing to
a drive that didn;t read correctly. If something is faulty, it's quite
possible it's going to write somewhere it shouldn't. And since the drive
fails on every sector, I would think the fualt is unlikely to be simply
bad formatting data on the disk. a new one on
me!) but that didn't help. No error on write, but they
cannot be read back.
What error do you get if you try to read them? Does it manage to find
anything? I'll start
poking around in the read logic next. Getting closer, anyway.
I've not looked at the schematics, but I guess I'd start by seeing if (a)
it was reading anything (do you get any sensible-looking pulses at the
read data output -- and rmemeber that in 99% of drives the pusle width is
irrelevant, it's set by a monostable on the logic board, it's the pulse
spacing that defines the data), and (b) is it selecting one (and only
one) head
24 Apr
24 Apr
11:28 a.m.
New subject: Memorex 102 20MB Hard disk - Found It!
On Thu, 22 Apr 2010, Tony Duell wrote:
Yes, and it tries to walk off the table :-). Serious amount of moving
Right This reminds me of the time a friend got an old (even then)
high-speed idot matrix impact printer/ After every carrige return it
moved a little way across his workbench. And thus halfway through
printing a large file if fell off the bench (and carried on printing.).
Anyway, I think we can assume for the mom,ent that the head possitioner
in ytour drive is working peoperly.
A quick
update: The drive spins up, comes ready and is passing the seek
test in Morrow's FORMATHD.COM utility. The header read fails on all
Sinc this drive has a transparent cover, I assume you can see the heads
moving during the seek trst. mass involved!
What error do
you get if you try to read them? Does it manage to find
anything?
No, it fails 64 times to read every sector on every track. Yes, I'm planning to poke through it with a scope
tomorrow.
Let us know how you get on :-)
-tony
12:42 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
On Sat, 24 Apr 2010, Tony Duell wrote:
Anyway, I think we can assume for the mom,ent that the
head possitioner
in ytour drive is working peoperly.
YEs, but what is it failing to do? Can it find the sctory (== read the
header)?
No. The test is specifically to test whether it can read the headers and
can't read any of them.
More when I get time later this weekend to put the scope on it.
Steve
--
mass involved!
What
error do you get if you try to read them? Does it manage to find
anything?
No, it fails 64 times to read every sector on every track.
12:47 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
On Sat, 24 Apr 2010, Tony Duell wrote:
Is this a hard sectored dirve? In any case does the
cotnroller use the
index pulse fro anyhting? Is that presnet and correct?
Sorry, didn't answer this part of your post: From my read of the
technical manual, it is hard sectored. There's a dedicated index track on
the bottom of the lowest platter and a dedicated, fixed head to read it.
It's more of a synchronous counter than a sector pulse. There are DIP
switches on the board to set up a divider circuit that turns this pulse
train into discrete, properly-spaced index pulses. Thus, the sector size
is configurable.
Steve
--
26 Apr
26 Apr
10:28 a.m.
New subject: Memorex 102 20MB Hard disk - Found It!
On Sat, 24 Apr 2010, Tony Duell wrote:
Well, it _may_ be... I've had another look at the manual on Bitsavers.
There's a clock track on the bottom platter with a fixed head (as you
say). This seems ot be used as the feedback of the motor speed control,
it also can be divided down to produce the sector pulse or a bit clock.
Of course the controller doesn't have to use that signal.
Unfortuately much of this circuitry seems to be in one of the gate
arrays. This will make it difficult to debug and even more diffuclt to
repair. I hope that's not the problem.
But I woul still check it. See if tyhe motor speed feedback signal looks
sensible too while you're in there..
And also, of course ,look at the read data signal. Does it look senesible?
Could this be a controller problem? Do you know whether either the
controller of drive are definitely OK?
-tony
Is this a hard sectored dirve? In any case does
the cotnroller use the
index pulse fro anyhting? Is that presnet and correct?
Sorry, didn't answer this part of your post: From my read of the
technical manual, it is hard sectored. There's a dedicated index track on
27 Apr
27 Apr
4:08 a.m.
New subject: Memorex 102 20MB Hard disk - Found It!
On Mon, 26 Apr 2010, Tony Duell wrote:
I believe that's fine. By my reading, the drive would not come ready if
there were a servo lock problem.
On Sat, 24 Apr 2010, Tony Duell wrote:
Well, it _may_ be... I've had another look at the manual on Bitsavers.
There's a clock track on the bottom platter with a fixed head (as you
say). This seems ot be used as the feedback of the motor speed control,
it also can be divided down to produce the sector pulse or a bit clock.
Of course the controller doesn't have to use that signal.
Unfortuately much of this circuitry seems to be in one of the gate
arrays. This will make it difficult to debug and even more diffuclt to
repair. I hope that's not the problem.
But I woul still check it. See if tyhe motor speed feedback signal looks
sensible too while you're in there.. Is this a hard sectored dirve? In any case does
the cotnroller use the
index pulse fro anyhting? Is that presnet and correct?
Sorry, didn't answer this part of your post: From my read of the
technical manual, it is hard sectored. There's a dedicated index track on And also, of course ,look at the read data signal.
Does it look senesible?
I've poked around a bit with a scope. The base clock signals mentioned in
the troubleshooting section of the manual are present, shaped correctly,
but the periodicity is exactly 2x what's printed there. This is a tough
call, though. The manual is for a different (and I believe later) drive
model, so that difference might be as expected. It's also possible that
it's a typo. If the period was too small, I'd suspect a bad latch in the
clock divider. But, _slower_? It's hard to see what type of failure mode
would cause that, but perhaps I'm missing the obvious.
The index and sector pulses appear to be right on the money.
If I look at the output of the analog section, I can see what looks to be
reasonable read data. Unfortunately, from this point on there are
significant differences between the bitsavers doc and this drive. The
board layout is very different on mine and it has fewer gate array chips.
The manual suggests that the clock separator PLL is in the gate array,
while mine has a DIP MC4044 PLL serving that function. I admit to being a
bit fuzzy on how this stage operates (not an EE by background).
Could this be a controller problem? Do you know
whether either the
controller of drive are definitely OK?
I have no assurance that either is functional. They were in storage for
25 years and the previous owner isn't clear on whether the drive was
functional (he bought the system from the original purchaser and did not
use it).
Steve
--
28 Apr
28 Apr
11:28 a.m.
New subject: Memorex 102 20MB Hard disk - Found It!
But I woul
still check it. See if tyhe motor speed feedback signal looks
sensible too while you're in there..
I believe that's fine. By my reading, the drive would not come ready if
there were a servo lock problem. And also, of course ,look at the read data
signal. Does it look senesible?
I've poked around a bit with a scope. The base clock signals mentioned in
the troubleshooting section of the manual are present, shaped correctly,
but the periodicity is exactly 2x what's printed there. This is a tough call, though. The manual is for a different (and I
believe later) drive
model, so that difference might be as expected. It's also possible that
it's a typo. If the period was too small, I'd suspect a bad latch in the
clock divider. But, _slower_? It's hard to see what type of failure mode
would cause that, but perhaps I'm missing the obvious.
For the moment I'd not worry about it. But again, rememebr the oddity.
The index and sector pulses appear to be right on the money.
OK, so the problem is not there.
If I look at the output of the analog section, I can see what looks to be
reasonable read data. Unfortunately, from this point on there are
significant differences between the bitsavers doc and this drive. The
board layout is very different on mine and it has fewer gate array chips.
The manual suggests that the clock separator PLL is in the gate array,
while mine has a DIP MC4044 PLL serving that function. I admit to being a
bit fuzzy on how this stage operates (not an EE by background).
What do you get at the output of this stage? Do you get
reasonable-looking data and clock signals?
If you have fewer gate arrays and more standard chips, it actually makes
life easier because you can probably get data sheets on them. The MC4044
is a well-known device that somebody must have the data sheets for.
Oh, and for the record I'm not an EE either :-)
I really do wonder if the problem is in the controller. Do you have any
documetation on that? Scheamtics?
-tony
14 Apr
14 Apr
6:52 p.m.
New subject: Memorex 102 20MB Hard disk - Found It!
On Tue, 13 Apr 2010, Steven Hirsch wrote:
On Tue, 13 Apr 2010, Tony Duell wrote:
As you surmised, the problem was on the 12V rail, which measured 0.5 ohms to
ground. It runs to quite a number of chips, but was surprising easy to
follow around and through the various via holes. I dragged out the dremel
tool and did a binary search by slicing traces. Took the full logN tries,
but on the last cut I nailed a 4.7ufd / 16V tantalum cap with a dead short
:-).
Well, there might have been an easier way of finding the short, but this
worked fine. I replaced the cap, patched everything up and fired it up.
Purrs like a kitten :-). All the voltages look correct, nothing is
excessively hot (although a few of the 74LS parts are a bit warmer than I
would have expected).
Will be a few days before I can hook it up to the controller and see if
it's functional.
Steve
--
The
downstream leg of the 10-ohm power resistor goes to a 2SD768 power
device and I'm having a lot of trouble following it from there. I think
your guess about a regulator for the 12V circuitry is probably correct.
THis does sound very like the schematic that I was looking at in the 112
manual. The relay there eneables the spindle motor (and maybe the
positioner drivers too), the resistor feeds a power transistor that's the
pass tranasitor for the 12V regualtor.
The fact that the drive fails totally if the resistor is removed would
seem to indicate that the otuput of that 12V regulator is somewhat
present.
I have suggested in my least message that iy would eb worth measuring the
votlages on the leads of that transistor.
What if the transistor was shorted? The regualtor would then become a
simple resistor + zner circuit. The reissotr would ahve to drop 10 or
12V, and woul;d get hot and bothered!. The zener diode wouldn't like it
much either (are you sure _nothing_ else is getting hot?). Anyway, it's
worth checking this.
15 Apr
15 Apr
11:47 a.m.
New subject: Memorex 102 20MB Hard disk - Found It!
Well, there might have been an easier way of finding
the short, but this
worked fine. I replaced the cap, patched everything up and fired it up.
Purrs like a kitten :-). All the voltages look correct, nothing is
Excellant... It's when it meows like an angry tomcat that you have to
worry ;-)
-tony
5352
days inactive
5369
days old
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