19 Nov
2002
19 Nov
'02
2:49 a.m.
All,
That does it. I'm turning off digest mode. Apologies if I miss an
important message, but this is just too annoying! ... well, that was easy.
Hopefully I'll hear back from myself in just a few minutes...
Toth asked:
...Have you checked the solder joints and connections
between
components with an ohm meter? That might help turn up any problems.
More or less. I tend to probe both on the component and on the pit next to
it for SMD's or on the pad for devices with pins. But I have not been
rigorous about it.
Tony said:
You will _not_ see the oscillations of a 393 comparator
on a VOM. We are
talking about hundreds of kHz / a few MHz...
Oh, right. But I'm assuming the duty cycle of the oscillations can vary,
and I might be able to see that on the (analog) VOM.
RIght. On the other hand, if it _is _ a capacitor, then
it might have
gone leaky, in whiuch case it will have an effect on the DC levels.
Oh my. Well, I don't know of any way to test the capacitor in circuit. Once
the comparator is out, I could measure resistance on the lower leg of the
divider. If it changes while I'm watching it, that'd be a clue, but with
power off the system I don't see what'd make it change. Or if I knew what
the resistance was supposed to be, and it was in the leaky state when I
measured it, that'd be a clue, but at least the first condition isn't met
and maybe not the second.
Are there any component refeneces (Rnnn, Cmmm, etc) on
the PCB?
None that I've seen, other than the "+" markings on one end of the tantalum
caps.
It could be a resistor, sure. And perhaps, then,
there's a capacitor
elsewhere to hunt for...
This comment, waiting for me in my digest as I found the component in
question, was what convinced me to off digest mode.
Tony and Toth agreed:
Get some silver-loaded solder.
Ok. I'm sticking with my plan, to wit replace the comparator and see what
happens. I'll measure resistance on both legs of the divider while it's
off. If that doesn't solve the problem, I'll head back to the electronics
store and try to get replacements for the tantalum cap (and one more for
the backward one, while I'm there) and for the resistors in the voltage
divider - and some silver-loaded solder.
How will I know what values to shop for for the capacitor (maybe not so
critical?) and the resistor? Is there a catalog of 1992 or so surface-mount
devices?
Toth futilely wished:
Erm, I just hope the iron is of fair quality, and is in
the range of
20-25W. Anything larger is likely to damage the board, especially those
awful soldering irons Radio Shack sells (or used to?).
It's an awful Radio Shack pencil-type iron. It's about 20 years old. I
think it's about 20 W, don't remember for sure. I only have one tip for it.
Maybe I should think about a better iron while I'm at the electronics shop,
if it comes to that.
- Mark
19 Nov
19 Nov
3:18 p.m.
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5:34 p.m.
On Wed, 20 Nov 2002, Tony Duell wrote:
I have a working VLC here that I can refer to for component information if
there is anything specific you need to know. The board itself is a 7 layer
board, so it wouldn't be easy to draft a schematic of the 3.3V regulator
and comparator section of the board. I can try taking a digital photo of
that area of the board if that would be helpful.
-Toth
How will I
know what values to shop for for the capacitor (maybe not
so critical?) and the resistor? Is there a catalog of 1992 or so
surface-mount
The capacitor will normally have the value marked on it. Either as
something like '10u' or as a 3 digit number like '226'. The latter you
read like a resistor code (22*10^6 in this case) and the base unit is
_pF_. So 226 is 22uF, etc.
I would be suprised if a resistor had failed, so leave that for the
moment. If you can't decode the markings on it, then perhaps you can
desolder it and measure the resistance out of circuit. Yes, I know it's
faulty, but it works some of the time so you shuld be able to get an
idea of the correct value.
20 Nov
20 Nov
6:36 a.m.
All,
The Good, the Bad, and the Ugly.
The Good: I got the comparator out, visibly intact. No legs lost, no big
divots chipped out of the mainboard, etc. I got its replacement back in.
One cold-looking solder joint, but I can retouch that later and it's
conducting OK for now. The system works like it did before the operation.
The Bad: It works *exactly* like it did before the operation, which means
the comparator was not the problem. Same run-a-little, hang-a-little
behaviour, same drunkenly varying voltage on non-inverting input to side A
of the comparator.
The Ugly:
This problem is vexing me.
Right after I got the old comparator out, I went to work with the
Ohmmeter. (Remember this is an old Archerkit Analog 20kOhms/volt on its R*1
setting, so there's probably some significant internal resistance. I did
zero the needle with the probes crossed before each series of measurements
on the same range, but...) Initially, I read on the voltage divider 24 Ohms
to +5V (across the component marked "1152") and 80 Ohms to Ground (across
"6192" and a capacitor, in parallel).
Then I took the board off the aluminum foil I had it sitting on
while soldering. Both of those resistors started reading much higher
resistances, and their resistances were different depending on which way
round I had the probes and on what range the meter was set at. Here's the
table, board sitting on a towel:
1152->+5V 6192->Gnd
x1 26/ manyKOhm 80/ manyKOhm
x10 18/ many K 32/ many K
x100 1250 / 9000 9500 / 1800
x1000 7800 / 9000 8500 / 10000
Numbers before/after the slashes are with probes forward/reversed
(don't recall which is which, and not necessarily consistent entry to
entry). Table is in near chronological order, and that may be significant.
What makes an SMD resistor start acting something like a diode?
Sigh. What's with the different readings at different scales? Do I just not
know how to use an Ohmmeter, fer cryin' out loud? Is all this the natural
consequences of trying to measure resistances in-circuit, with active
components messing things up?
In any case, it looks like I'm headed for the merry world of
surface-mount soldering. You know, when I got this system I named it
"Sensei" figuring I'd learn something from it. Maybe I'll try to be a
little less clever naming the next system.
Toth wrote:
I have a working VLC here that I can refer to for
component information if
there is anything specific you need to know.
You da man! OK, yes, please!
Sitting facing the rear of the computer, power supply unit to your
right, find the connector on the cable bundle coming from the power supply.
Just beyond it is an 8-pin IC marked LM393.
Less important measurements: With the computer running, I'd like to
know the voltage on pins 2 and 3 (be careful not to short things!). Pins
are counted from Right (pin 1) to left (pin 4) across the far side.
Voltage pin 2 (expect about 3.5 V):
Voltage pin 3 (expect about 4.2 V):
Please watch pin 3 for a while (30 seconds or so) to see how much
it varies (expect none).
With the computer off, look just to the left of the comparator,
where there's a column of 4 surface-mount resistors (?), farther two
brownish, nearer two black. Please measure resistance across each of those.
I've named them from far to near, using the practically invisible numbers
written on their tops. I'm really only interested in the first 3, since I
don't know where the last one goes, but I'm hoping I might learn something
about the naming convention:
Resistance 181 (expect about 180 Ohms) :
Resistance 6192 (expect ... ummm ... ??) :
Resistance 1152 (expect ....ummm ... ??) :
Resistance 4221 (don't know) :
6192 has a capacitor in parallel, so may take a bit to settle. My meter
shows 170 Ohms for "181", except when measuring reversed-probes at X1, when
it shows 75 Ohms.
The board itself is a 7 layer
board, so it wouldn't be easy to draft a schematic of the 3.3V regulator
and comparator section of the board.
No lie. What I (think I) know about the schematic comes from
finding hard shorts between component pins with the Ohmmeter. I've
described it before, but briefly:
Pin 2 on comparator (inverting input) connects to middle pin of the
"88W" device (black, 3 pins, just beyond the inverter) and to one end of
the resistor marked "181". The other end of that resistor goes to +5V. I
think Tony's right, 88W is a 3.5V (or so) Zener diode in a 3-pin package,
one pin of which is not used.
Pin 3 on comparator (non-inverting input) connects to the middle of
a voltage divider formed by 1152 (to +5V) and 6192 (to ground). 6192 has a
capacitor (at the near left corner of the inverter) in parallel with it, so
that it comes up slowly. That's the trouble spot - in operation, that
voltage wobbles around from about 2.5 up to 4.2 on my machine.
Tony guessed (right as usual):
2) The '3 terminal component' is a zener with
one pin not used. Again we
know one pin is grounded, and the other goes to the input of the
comparator. In which case there must be a resistor from that input to a
voltage source (probably a supply rail).
Resistor "181", about 180 or so Ohms.
WRT to capacitor in parallel with 6192:
Aha... That would be my first suspect. Has it gone
leaky?
I do not know, nor how I can find out (other than replace it - along with
(possibly) the two resistors in the divider, which will be my project for
next week,).
If it's had significant reverse voltage, then it
will now be pretty leaky
(or even shorted).
However, it's not unheard-of for the markings on the PCB to be wrong,
just to make life 'interesting'.
Spot on again! I checked voltage on the "reversed" capacitor. Its pip end
is at ground ... but its other end is at -12V (despite the "+" marking on
the PCB near that end). So it's set up correctly after all.
The capacitor will normally have the value marked on
it. Either as
something like '10u' or as a 3 digit number like '226'. The latter you
read like a resistor code (22*10^6 in this case) and the base unit is
_pF_. So 226 is 22uF, etc.
No markings on any of the capacitors that I can see. There are 3 sizes:
large, with rectangular cross section, medium, with square cross section,
small, with rectangular cross section. All just have bright orange bands,
no writing.
I would be suprised if a resistor had failed, so leave
that for the
moment. If you can't decode the markings on it, then perhaps you can
desolder it and measure the resistance out of circuit. Yes, I know it's
faulty, but it works some of the time so you shuld be able to get an idea
of the correct value.
I've had a thought. Would the following values make sense in the voltage
divider?
1152 - 1152 Ohms, to +5V
6192 - 6192 Ohms, to Ground, parallel with capacitor.
Or is it more likely:
1152 - 11,500 Ohms
6192 - 61,900 Ohms
The tap is spec'ed to have an "input biasing current" of 25 nA. Either set
would fit with the other resistor in that row having a value of around 180
and marking of "181". Current should be enough that the tap drain is
negligible, but low enough not to toast anything. Ratios are just about
right to generate the voltage I'm seeing, when the system is running.
Given either set of values, and that the machine is 25 MHz clock, what's a
sensible range of values for the tantalum cap. parallel with 6192? Is it
critical to get that value just right, or is bigger better?
I'll be on travel Thurs-Sun, so anybody not reading this gets a break from
discarding my (probably by now tiresome) messages. For the rest of you, my
thanks again!
- Mark
2:33 p.m.
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22 Nov
22 Nov
6:18 p.m.
Not the resistor, but I bet that darn tantalum
capacitor could. I still
suspect that.
You were probably right all along. I found someone at work who deals with
SMD devices on a regular basis. He has a small-nozzle heat gun, and agreed
(next week, when he has some time) to try to pull and replace the cap. for
me. After his and Toth's warnings about my current soldering iron (and your
own recommendation on what would be better to use), I think I may wait for
him and watch over his shoulder when it happens.
[1} Remove all the solder you can with a hot iron and
braid. Then melt
the remaining solder on one end and _carefully_ lift that end of the
component a fraction -- don't damage the pad on the other end of the
component. Then do the same at the other end. Repeat until it comes free.
I'll save this advice in case I get very impatient, though.
Hmmm. Maybe the value is printed on the bottom side of
the cap for some
strange reason.... Anyway, it's probably not that critical -- see above.
It'd be true for *all* of the caps on that board. I'll see and report when
that cap. comes off, but I don't see why the square cross-section ones
would be that way.
If those values are correct, then you'd expect
5*61900/(11500+61900)
volts on the tap. Or around 4.2V. That's what you're getting when the
machine works, isn't it? So the values might will be right.
That's what I see. But with components of resistance same order as my
voltmeter, I'm surprised to see that with any accuracy.
Oh, try something between 1uF and 10uF to start with.
OK, will do. I may even ask friend at work to pull one of the other caps
and test it on a bridge, just so I know what the proper value is.
- Mark
23 Nov
23 Nov
1:35 p.m.
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25 Nov
25 Nov
3:33 a.m.
Excellent. If you can use the right tools, do so. But
it is _possible_ to
do SMD rework using a normal (good) soldering iron.
I'll ask. But now I'm convinced the board has some hope of living, I
hesitate to address it myself...
Are there any marking at all. You mentioned an orange
band(s). How many?
One. I shouldn't have said "band", it's really the whole outside of the
component except the contacts at either end. It looks way too wide to be a
color-code band.
Wait a second. The meter is 20kOhm/V right. I have no
idea what range
you're using, but lets say it's a 10V range. So the meter's resistance is
200k.
Yep, 0-15V range.
Now, it's a consequence of Thevenin's (?spel)
theorem that the output
impedance of a potential divider is the given by the 2 resistors in
parallel. Here you'vr got 11500 Ohms and 61900 ohms. The equivalent
resistance of those in paralle is 9.7k. So the meter is 20 times as high
an impedance. That would give approximately a 5% error. Not that bad.
Well, true if I'm right about the resistor values being 11.5k/61.9k. If
they are 10 times higher, then I'm sunk.
I wouldn't. For one thing, desoldering SMD parts
risks damaging the PCB
and even more so the component (most manufacturers recomend against
reusing desoldered SMD parts). After desoldering the value might not he
right anyway. For another, you don't know that all the capacitors on that
board should be the same value.
True even with the proper (hot-air) desoldering tools? But suits my
inclination anyway, except that I *would* like to know what that cap. is
supposed to be.
- Mark
4:44 a.m.
On Mon, 25 Nov 2002, Mark Tapley wrote:
Actually a modified soldering iron is less harmful to PCBs (for small
parts) than a hot air tool (which may melt adjacant connectors etc) What I
have done for removing SMD caps is to take a Weller controlled temp iron tip,
hacksaw off the tip, leaving a blunt copper end. Next I tap the end of the tip
with a 4-40 tap. Now I take some 1/32 copper sheet, and bend it into a 'U'
shaped piece that fits loosely over the SMD part, contacting both solder
lands. With this tip in the Weller iron, and adequate flux and solder to make
good thermal contact with the part and card, its easy to remove the part.
The most important thing to remember is to put _0_ force on the
circuit card when hot, this is what lifts pads (causing bad words to be said).
Both ends of the solder holding the part should be _completely_ melted before
attempting to remove the part...
A (good) tantalum capacitor will withstand soldering temperatures
(the SMD ones can even be wave soldered) but I wouldn't reuse SMD parts on
general principles.
BTW of the millions of SMD parts that we have used in the last few
years, I dont think we have ever seen a field failure of a resistor ( other
than mechanical damage from rough cicuit card handling or elecrical overload
Excellent. If
you can use the right tools, do so. But it is _possible_ to
do SMD rework using a normal (good) soldering iron.
I'll ask. But now I'm convinced the board has some hope of living, I
hesitate to address it myself...
Are there any marking at all. You mentioned an
orange band(s). How many?
One. I shouldn't have said "band", it's really the whole outside of
the
component except the contacts at either end. It looks way too wide to be a
color-code band.
Wait a second. The meter is 20kOhm/V right. I have
no idea what range
you're using, but lets say it's a 10V range. So the meter's resistance is
200k.
Yep, 0-15V range.
Now, it's a consequence of Thevenin's
(?spel) theorem that the output
impedance of a potential divider is the given by the 2 resistors in
parallel. Here you'vr got 11500 Ohms and 61900 ohms. The equivalent
resistance of those in paralle is 9.7k. So the meter is 20 times as high
an impedance. That would give approximately a 5% error. Not that bad.
Well, true if I'm right about the resistor values being 11.5k/61.9k. If
they are 10 times higher, then I'm sunk.
I wouldn't. For one thing, desoldering SMD
parts risks damaging the PCB
and even more so the component (most manufacturers recomend against
reusing desoldered SMD parts). After desoldering the value might not he
right anyway. For another, you don't know that all the capacitors on that
board should be the same value.
True even with the proper (hot-air) desoldering tools? But suits my
inclination anyway, except that I *would* like to know what that cap. is
supposed to be.
- Mark
from abuse or other component failure) Passives much
more likely to fail are:
1. Aluminum electrolytic capacitors (in your power supply...)
2. Tantalum electrolytic capacitors
3. Ceramic high value capacitors (bypass)
Peter Wallace
26 Nov
26 Nov
3:36 a.m.
Peter Wallace wrote:
Now I take some 1/32 copper sheet, and bend it into a
'U'
shaped piece that fits loosely over the SMD part, contacting both solder
lands. With this tip in the Weller iron, and adequate flux and solder to make
good thermal contact with the part and card, its easy to remove the part.
This sounds neat. But I don't understand yet - you screw the copper "U"
into the end of a tip which itself then screws into the end of the iron?
- Mark
5:29 a.m.
On Mon, 25 Nov 2002, Mark Tapley wrote:
Peter Wallace wrote:
Yes, the 'U' shaped copper part is attached with a screw to the modified
(blunted) tip which is then installed as usual into the iron.
Weller temp controlled (TCPN?) tips dont screw in though, they slide into the
iron and are held in with a separate sleeve and threaded ring.
Peter Wallace
Now I take some 1/32 copper sheet, and bend it
into a 'U'
shaped piece that fits loosely over the SMD part, contacting both solder
lands. With this tip in the Weller iron, and adequate flux and solder to make
good thermal contact with the part and card, its easy to remove the part.
This sounds neat. But I don't understand yet - you screw the copper "U"
into the end of a tip which itself then screws into the end of the iron?
- Mark
25 Nov
25 Nov
1:41 p.m.
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7915
days inactive
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11 comments
4 participants
participants (4)
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ard＠p850ug1.demon.co.uk -
mtapley＠swri.edu -
pcw＠mesanet.com -
tothwolf＠concentric.net