23 Aug
2014
23 Aug
'14
4:56 p.m.
While I wait for PROMs to arrive for my 11/05 I've pulled another
"back-burner" project off the shelf.
It's an Interact Model One (http://oldcomputers.net/interact.html) and
it's having power supply issues.
The symptom: At initial startup after being off for any significant
period of time (say, 10 minutes or longer) the +5V comes up for 2-3
seconds then drops to zero. The other supplies (+/- 12V) are fine.
What I've done so far:
- Recapped the electrolytics in the supply (I had the parts on hand,
and it's three capacitors. Sorry for the shotgun approach, Tony :)).
- Removed socketed 2114 RAMs (in case one was shorted).
No change in behavior. The power supply and logic are incorporated into
a single PCB, so it is difficult to separate the two (I guess I'd have
to figure out what traces to cut, and I'd rather not mangle things if I
can avoid it). The DC supply is fed by an external "wall wart" which I
*believe* just contains a transformer -- it outputs several flavors of
AC (16.5V and 19V), no DC.
Any suggestions for where to start with this? Power supplies are voodoo
to me, still. The power supply schematics can be found in here:
http://www.thebattles.net/oddments/interact/InteractSchem.pdf (page 6,
lower right corner).
Thanks!
Josh
23 Aug
23 Aug
7:12 p.m.
Josh,
I'd first try lifting one side of CR22 and see if your +5 returns.
If not, you can remove IC 63 (probably what, a 7805?) and see you can
read anything at the high side of C67. You should be able to--unless
you've got a bad diode.
If the CR22 maneuver doesn't result in any improvement, then you've
probably got a fault somewhere else on the board. Make a quick
resistance check while you have IC 63 out with the power off, checking
the resistance between the +5 lead and ground.
That should get you started on where to look.
--Chuck
11:30 p.m.
On 8/23/2014 7:12 PM, Chuck Guzis wrote:
Josh,
I'd first try lifting one side of CR22 and see if your +5 returns.
If not, you can remove IC 63 (probably what, a 7805?) and see you can
read anything at the high side of C67. You should be able to--unless
you've got a bad diode.
If the CR22 maneuver doesn't result in any improvement, then you've
probably got a fault somewhere else on the board. Make a quick
resistance check while you have IC 63 out with the power off, checking
the resistance between the +5 lead and ground.
That should get you started on where to look.
--Chuck
Thanks! Lifting CR22 (which took awhile to find, it's actually clear on
the other side of the PCB from the rest of the supply) made no
difference in behavior.
Removing IC 63 (which is a 7805, nice guess) and reading voltages on the
positive lead of C67 yields about 15VDC. The resistance between the +5V
lead of the IC63 socket and ground is about 3.5K ohms, so it doesn't
look like there's a dead short here. Would the 7805 be a likely suspect
here? Any easy way to test it?
Thanks,
Josh
24 Aug
24 Aug
1:40 a.m.
On 24 Aug 2014, at 4:30 pm, Josh Dersch <derschjo at gmail.com> wrote:
Thanks! Lifting CR22 (which took awhile to find, it's actually clear on the other
side of the PCB from the rest of the supply) made no difference in behavior.
Removing IC 63 (which is a 7805, nice guess) and reading voltages on the positive lead of
C67 yields about 15VDC. The resistance between the +5V lead of the IC63 socket and ground
is about 3.5K ohms, so it doesn't look like there's a dead short here. Would the
7805 be a likely suspect here? Any easy way to test it?
Thanks,
Josh
The 3 pins on the 7805 are: DC input / Common (ground) / +5V DC regulated output.
All you really need to test it is apply a voltage anywhere from 7.5 to 25V DC between the
input and the common, then apply a small load (and measure the voltage) across common and
the output (should be close to 5V. In a real life circuit you would have capacitors on
both the input and output to filter any ripple, but shouldn?t be required for a simple
test.
In the Interact circuit there are 4 capacitors on the output of the 7805, one of them
could be the problem if the 7805 checks out ok.
I would be inclined to put in a new 7805 anyway (since you already have it out), but I
have a heap of them lying around..
Chris
4:19 a.m.
On Sun, Aug 24, 2014 at 08:40:23AM +0000, Chris Pye wrote:
On 24 Aug 2014, at 4:30 pm, Josh Dersch <derschjo at gmail.com> wrote:
>
> Thanks! Lifting CR22 (which took awhile to find, it's actually clear on the
other side of the PCB from the rest of the supply) made no difference in behavior.
>
> Removing IC 63 (which is a 7805, nice guess) and reading voltages on the positive
lead of C67 yields about 15VDC. The resistance between the +5V lead of the IC63 socket
and ground is about 3.5K ohms, so it doesn't look like there's a dead short here.
Would the 7805 be a likely suspect here? Any easy way to test it?
Page 1 lists the devices at the top. IC63 is listed as a 7805. ;)
The 3 pins on the 7805 are: DC input / Common (ground) / +5V DC regulated output.
All you really need to test it is apply a voltage anywhere from 7.5 to 25V DC between the
input and the common, then apply a small load (and measure the voltage) across common and
the output (should be close to 5V. In a real life circuit you would have capacitors on
both the input and output to filter any ripple, but shouldn?t be required for a simple
test.
Or tack some leads to input and ground and check in circuit checking the
output pin for +5.
I would be inclined to put in a new 7805 anyway (since you already have it out), but I
have a heap of them lying around..
Don't we all? :)
Chris
Diane
--
- db at FreeBSD.org db at db.net http://www.db.net/~db
2:19 a.m.
On 2014-Aug-23, at 11:30 PM, Josh Dersch wrote:
On 8/23/2014 7:12 PM, Chuck Guzis wrote:
Sure, the 7805 could fail after warming up and either go open or short internally. To test
it you could put a load drawing current equivalent to the computer (say 0.75 to 1A) on it
and see if you get the same failure after some minutes.
A scenario that may be at work is a short develops on the +5 line after something warms up
- an IC or a capacitor (are there any tantalums on there?). When the short occurs the
current-limiting in the 7805 regulator kicks in, so the shorted device doesn't see
enough current to go poof and fail completely. You could poke around to see if anything is
getting unusually warm.
A way to distinguish between a short vs the reg going open would be to watch what happens
to the voltage on the input side of the reg when the failure occurs. If it's going
open, the V should goes up due to the removal of the load. If it's a short the V
should go down due to the increased current draw.
I think Dwight had a technique for tracking power rail shorts with a 4-contact ohmmeter.
If you have 800$ to spare you could get one of these and trace current on the PCB:
http://www.aimtti.com/go/iprober/
The gonzo technique is remove the 7805, connect the logic Vcc line to a 20A +5V supply and
find out what component does go poof. Unfortunately it may be a PCB trace burning up
before the bad component does. (All liability disavowed).
Josh,
I'd first try lifting one side of CR22 and see if your +5 returns.
If not, you can remove IC 63 (probably what, a 7805?) and see you can read anything at
the high side of C67. You should be able to--unless you've got a bad diode.
If the CR22 maneuver doesn't result in any improvement, then you've probably got
a fault somewhere else on the board. Make a quick resistance check while you have IC 63
out with the power off, checking the resistance between the +5 lead and ground.
That should get you started on where to look.
--Chuck
Thanks! Lifting CR22 (which took awhile to find, it's actually clear on the other
side of the PCB from the rest of the supply) made no difference in behavior.
Removing IC 63 (which is a 7805, nice guess) and reading voltages on the positive lead of
C67 yields about 15VDC. The resistance between the +5V lead of the IC63 socket and ground
is about 3.5K ohms, so it doesn't look like there's a dead short here. Would the
7805 be a likely suspect here? Any easy way to test it?
9:43 a.m.
On 08/23/2014 11:30 PM, Josh Dersch wrote:
Thanks! Lifting CR22 (which took awhile to find,
it's actually clear on
the other side of the PCB from the rest of the supply) made no
difference in behavior.
Removing IC 63 (which is a 7805, nice guess) and reading voltages on the
positive lead of C67 yields about 15VDC. The resistance between the +5V
lead of the IC63 socket and ground is about 3.5K ohms, so it doesn't
look like there's a dead short here. Would the 7805 be a likely suspect
here? Any easy way to test it?
Just replace the 7805--they should be common as dirt. Since it's a
power IC and can run very warm, it's not uncommon to have a failure
there. particularly when dissipating most of your power input as heat!
I assume that thing doe shave a heat sink. If not, it might be prudent
to install one.
The resistance of the +5 line to ground is certainly well within what
I'd expect.
--Chuck
4:01 p.m.
On 8/24/2014 9:43 AM, Chuck Guzis wrote:
On 08/23/2014 11:30 PM, Josh Dersch wrote:
Success: http://yahozna.dyndns.org/scratch/interact/WP_20140824_003.jpg
Ran out to a local electronics parts place and replaced the 7805 this
afternoon, and it powered right up. Now to rebuild the cassette drive
(needs a new belt or two...)
Thanks for the assistance!
- Josh
Thanks! Lifting CR22 (which took awhile to find,
it's actually clear on
the other side of the PCB from the rest of the supply) made no
difference in behavior.
Removing IC 63 (which is a 7805, nice guess) and reading voltages on the
positive lead of C67 yields about 15VDC. The resistance between the +5V
lead of the IC63 socket and ground is about 3.5K ohms, so it doesn't
look like there's a dead short here. Would the 7805 be a likely suspect
here? Any easy way to test it?
Just replace the 7805--they should be common as dirt. Since it's a
power IC and can run very warm, it's not uncommon to have a failure
there. particularly when dissipating most of your power input as heat!
I assume that thing doe shave a heat sink. If not, it might be
prudent to install one.
The resistance of the +5 line to ground is certainly well within what
I'd expect.
--Chuck 
4:27 p.m.
It would seem my guess was wrong. It seems to be the regulator.
Dwight
Date: Sun, 24 Aug 2014 16:01:38 -0700
From: derschjo at gmail.com
To:
Subject: Re: Power supply debugging advice (Interact Model One)
On 8/24/2014 9:43 AM, Chuck Guzis wrote:
On 08/23/2014 11:30 PM, Josh Dersch wrote:
Success: http://yahozna.dyndns.org/scratch/interact/WP_20140824_003.jpg
Ran out to a local electronics parts place and replaced the 7805 this
afternoon, and it powered right up. Now to rebuild the cassette drive
(needs a new belt or two...)
Thanks for the assistance!
- Josh
Thanks! Lifting CR22 (which took awhile to find,
it's actually clear on
the other side of the PCB from the rest of the supply) made no
difference in behavior.
Removing IC 63 (which is a 7805, nice guess) and reading voltages on the
positive lead of C67 yields about 15VDC. The resistance between the +5V
lead of the IC63 socket and ground is about 3.5K ohms, so it doesn't
look like there's a dead short here. Would the 7805 be a likely suspect
here? Any easy way to test it?
Just replace the 7805--they should be common as dirt. Since it's a
power IC and can run very warm, it's not uncommon to have a failure
there. particularly when dissipating most of your power input as heat!
I assume that thing doe shave a heat sink. If not, it might be
prudent to install one.
The resistance of the +5 line to ground is certainly well within what
I'd expect.
--Chuck
5:11 p.m.
On 08/24/2014 04:27 PM, dwight wrote:
It would seem my guess was wrong. It seems to be the
regulator.
Herb seems to think it's a good idea to replace old linear regulators on
S100 boards as a matter of course as well. Given that the "knee" in the
7805 (according the Fairchild data sheet) occurs at about 2.2A and then
*slowly* drops back down (e.g. limits current to 2A at a 15V
differential, which has the regulator dumping 30W in heat), it's
probably not a bad idea. Many vendors either didn't heatsink those
TO220 packages or exceeded the ratings quite routinely.
Makes me grateful for switching regulators like the LM2678 that makes
for a cool 5A design. I think someone even makes retrofits for the 78xx
and 79xx series.
--Chuck
1:15 p.m.
While I wait for PROMs to arrive for my 11/05 I've pulled another
"back-burner" project off the shelf.
It's an Interact Model One (http://oldcomputers.net/interact.html) and
it's having power supply issues.
The symptom: At initial startup after being off for any significant
period of time (say, 10 minutes or longer) the +5V comes up for 2-3
seconds then drops to zero. The other supplies (+/- 12V) are fine.
I am goign to assume that the service manual on that site applies to your
amchine, nd that the schematics in said manual are therefore helpful.
The poser supply is quite simple. The wallwart is, as you suggest, almost
certainly jsut a transformer 9Well, maybe fuses too, we can always
hope...). The outp[uts are bridge rectifierd, smoothed and fed to 3
terminal regulators to get the regualted votlages for the rest of the
machine.
When you say the 5V soems up fro a few seconds if the machine has been
off for some time, does that mena if you then power cycle it the 5V lien
comes up and stays up? Or waht?
I would start by checkign the input to the 5V regualtor (probably a
7805). Yoiu need abotu 8V there to get the 5V output. Does that come up
and stay up?
Then try lifitng the output lead of the 7805 (look at a data sheet to see
whic hthat is, and whatever you do make sure the common lead -- the one
in the middle -- si correctly ocnencted . If not the machien will get the
unregualted inptu votlage -- OUCH!), and connect an ammeter in sereis
with it -- +ve side of the ammeter to the 7805 output lead and -ve side
ot the track on the PCB. Power up, What sort of current do you see? Is it
excessive (more than 1A)? If so, you have a short somwehre.
Otherwise the 7805 iteslfe is a liekly suspect. They are cheap and easy
to find. But those 3 terminal regulators ahve an interesting property that
cuaught me once. If at switch-on the output pin is -ve wrt the common pin
(it can happen), the thign will shut down. Is it possible the -12V supply
is coming up first and due to some problem the 5V rail is going
momentarily -ve?
-tony
3:06 p.m.
Hi It doesn't sound like a supply problem, it sound like a excessload.The regulator is
most likely shutting down on current or thermal.I looked at your schematic, it is using
the full wave bridges astwo half output but there must be a center tap wire from the
transformer.You might check that the connection from your wall wart is goodor the circuit
won't work ( ohm meter ).Is the regulator getting hot ( I mean really hot 212F + ).If
so, it is most likely a load issue.When the supply shuts down, what is the voltage to the
inputlead of the regulator to the ground lead?Dwight
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