6 Jan
2011
6 Jan
'11
10:20 a.m.
A while back I mentioned that I have a MicroVAX 3400 with one of the H7868
PSUs not wanting to work (green light fails to come on). I have just tried
the "faulty" PSU in a MicroVAX 3500 and it works just fine. I took a PSU
from the 3500 and put it in the 3400 and it does not
work in the 3400
either, only in the 3500. So clearly there is something else that
is causing
the problem and I am looking for suggestions, especially as the machine is
in an awkward location and hard to dismantle speculatively. As a reminder
this machine was working fine and I had not done anything at all to it prior
to its failure, I had not moved it, changed any components or anything.
Regards
Rob
6 Jan
6 Jan
12:31 p.m.
A while back I mentioned that I have a MicroVAX 3400 with one of the H7868
PSUs not wanting to work (green light fails to come on). I have just tried
the "faulty" PSU in a MicroVAX 3500 and it works just fine. I took a PSU
from the 3500 and put it in the 3400 and it does not work in the 3400
either, only in the 3500. So clearly there is something else that is causing
the problem and I am looking for suggestions, especially as the machine is
in an awkward location and hard to dismantle speculatively. As a reminder
this machine was working fine and I had not done anything at all to it prior
to its failure, I had not moved it, changed any components or anything.
My first thoguht is the obvious one, taht something in the 3400 is
shorted and overloading one of the outputs of the PSU that isn't coming on.
Could it be something as trivial (but not trivial to find!) as a
decoupling capacitor that's shorted?
-tony
30 Jan
30 Jan
2:12 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 06 January 2011 20:31
To: cctalk at classiccmp.org
Subject: Re: PSU for MicroVAX 3400 Not Faulty But Not Working
>
> A while back I mentioned that I have a MicroVAX 3400 with one of the
> H7868 PSUs not wanting to work (green light fails to come on). I have
> just tried the "faulty" PSU in a MicroVAX 3500 and it works just fine.
> I took a PSU from the 3500 and put it in the 3400 and it does not work
> in the 3400 either, only in the 3500. So clearly there is something
> else that is causing the problem and I am looking for suggestions,
> especially as the machine is in an awkward location and hard to
> dismantle speculatively. As a reminder this machine was working fine
> and I had not done anything at all to it prior to its failure, I had not
moved
it, changed any components or anything.
My first thoguht is the obvious one, taht something in the 3400 is shorted
and overloading one of the outputs of the PSU that isn't coming on.
Could it be something as trivial (but not trivial to find!) as a
decoupling
capacitor that's shorted?
-tony
I found the problem was indeed the CPU board. I tested the capacitors with a
multimeter and found one to be shorted as the resistance measured zero.
While measuring other similar capacitors I found the resistance to be about
20ohms on the rest, but one seems to be open circuit. The replacement
capacitor I bought (but have not yet fitted) also seems to be open circuit.
I tested the capacitors on a spare CPU board I have and they too were
generally about 20ohms. What values should I expect for resistance? Have I
got a load of capacitors which are all about to fail?
Regards
Rob
31 Jan
31 Jan
12:01 p.m.
I found the problem was indeed the CPU board. I tested
the capacitors with a
multimeter and found one to be shorted as the resistance measured zero.
While measuring other similar capacitors I found the resistance to be about
20ohms on the rest, but one seems to be open circuit. The replacement
capacitor I bought (but have not yet fitted) also seems to be open circuit.
I tested the capacitors on a spare CPU board I have and they too were
generally about 20ohms. What values should I expect for resistance? Have I
got a load of capacitors which are all about to fail?
A capacitor should, in theory, test as open-circuit. It has no DC path
through it.
Some cpacitors are polarised, and may test lower in the 'wrong'
direction. On most digital metes, the red probe is, indeed, positive on
the resistance ranges, on most analogue meters the red probe is -ve.
Hwoever, any capacitor which tests as low as 20Ohms is defective.
I assume you were testign them out of circuit. If they're on the PCB,
there are all sorts of other things in parallel with them (like the
chips, if they;re supply line decoupling capacitors) so you can't measure
just the resistance of one capacitor.
-tony
1:38 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 31 January 2011 20:02
To: cctalk at classiccmp.org
Subject: Re: PSU for MicroVAX 3400 Not Faulty But Not Working
be
I found the problem was indeed the CPU board. I
tested the capacitors
with a multimeter and found one to be shorted as the resistance
measured zero.
> While measuring other similar capacitors I found the resistance to be
> about 20ohms on the rest, but one seems to be open circuit. The
> replacement capacitor I bought (but have not yet fitted) also seems to open circuit.
through
I tested the capacitors on a spare CPU board I
have and they too were
generally about 20ohms. What values should I expect for resistance?
Have I got a load of capacitors which are all about to fail?
A capacitor should, in theory, test as open-circuit. It has no DC path it.
Some cpacitors are polarised, and may test lower in the 'wrong'
direction. On most digital metes, the red probe is, indeed, positive on
the
resistance ranges, on most analogue meters the red
probe is -ve.
Hwoever, any capacitor which tests as low as 20Ohms is defective.
I assume you were testign them out of circuit. If they're on the PCB,
there
are all sorts of other things in parallel with them
(like the chips, if
they;re
supply line decoupling capacitors) so you can't
measure just the
resistance
of one capacitor.
-tony
I did not test out of circuit so I will desolder the suspect capacitor and
test it again as it certainly measures differently to the same one on a
working board.
Regards
Rob
5 Feb
5 Feb
4:50 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Rob Jarratt
Sent: 30 January 2011 22:12
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: PSU for MicroVAX 3400 Not Faulty But Not Working
about
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 06 January 2011 20:31
To: cctalk at classiccmp.org
Subject: Re: PSU for MicroVAX 3400 Not Faulty But Not Working
>
> A while back I mentioned that I have a MicroVAX 3400 with one of the
> H7868 PSUs not wanting to work (green light fails to come on). I
> have just tried the "faulty" PSU in a MicroVAX 3500 and it works just
fine.
> I took a PSU from the 3500 and put it in the
3400 and it does not
> work in the 3400 either, only in the 3500. So clearly there is
> something else that is causing the problem and I am looking for
> suggestions, especially as the machine is in an awkward location and
> hard to dismantle speculatively. As a reminder this machine was
> working fine and I had not done anything at all to it prior to its
> failure, I had not
moved
it, changed any components or anything.
My first thoguht is the obvious one, taht something in the 3400 is
shorted and overloading one of the outputs of the PSU that isn't coming
on.
Could it be something as trivial (but not trivial to find!) as a
decoupling
capacitor that's shorted?
-tony
I found the problem was indeed the CPU board. I tested the capacitors with
a multimeter and found one to be shorted as the resistance measured zero.
While measuring other similar capacitors I found the resistance to be 20ohms on the rest, but one seems to be open circuit.
The replacement
capacitor I bought (but have not yet fitted) also seems to be open
circuit.
I tested the capacitors on a spare CPU board I have
and they too were
generally about 20ohms. What values should I expect for resistance? Have I
got a load of capacitors which are all about to fail?
Regards
Rob
I was checking the capacitors in-circuit and understand that this is not
correct.
However, I would have thought that a 0 ohm measurement in-circuit would
indicate a shorted capacitor because there would surely not be any point
designing a circuit where both ends of the capacitor are connected together?
That capacitor also measured differently on a working board so it was
clearly suspect. However when I de-soldered it, it turned out to be open
circuit. The resistance in-circuit was not absolutely 0 though, although it
had seemed negligibly close, so it must have been the surrounding load as
has been suggested to me.
Is there a simple way to find shorted caps in-circuit? Would an absolutely 0
ohm resistance while in-circuit indicate a shorted cap for the reason I
suggest above?
Thanks
Rob
6:39 a.m.
From: robert.jarratt at ntlworld.com
---SNIP---
I was checking the capacitors in-circuit and understand that this is not
correct.
However, I would have thought that a 0 ohm measurement in-circuit would
indicate a shorted capacitor because there would surely not be any point
designing a circuit where both ends of the capacitor are connected together?
That capacitor also measured differently on a working board so it was
clearly suspect. However when I de-soldered it, it turned out to be open
circuit. The resistance in-circuit was not absolutely 0 though, although it
had seemed negligibly close, so it must have been the surrounding load as
has been suggested to me.
Is there a simple way to find shorted caps in-circuit? Would an absolutely 0
ohm resistance while in-circuit indicate a shorted cap for the reason I
suggest above?
Thanks
Rob
Hi Rob
The 0 ohms indicates a short but doesn't tell you what is shorted.
It could be the cap you are measuring at or one half way
across the board. It may not even be a cap since there are
many ICs also across the power lines.
I have a method that works quite well at finding such shorts
but requires a uV meter or a good 5 digit meter and a bench
power supply that can be set on current limit.
Alternate methods use a bench supply set on current limit
and hunt around with ones funger or take a picture in
the dark without the flash.
Also, there is that thermal paper.
The hunt and peck method of lifting one lead of each
part until you find the shorted one.
Some have success with a two lead ohm meter but
I've not seen this work well. A 4 lead has a better
chance.
Dwight
6 Feb
6 Feb
10:06 a.m.
dwight elvey wrote:
I once saw an instrument that measured the potential between two points
while the card was powered by a current limited supply. The twist is
that it didn't presented the voltage, it made a tone which pitch got
higher the lower the voltage difference between the probes was. That way
you never needed to look at an instrument.
This instrument was used by a contract manufacturer on a lot of faulty
cards and it didn't damage the circuits. It was very quick.
/G?ran
From: robert.jarratt at ntlworld.com
---SNIP---
I was checking the capacitors in-circuit and
understand that this is not
correct.
However, I would have thought that a 0 ohm measurement in-circuit would
indicate a shorted capacitor because there would surely not be any point
designing a circuit where both ends of the capacitor are connected together?
That capacitor also measured differently on a working board so it was
clearly suspect. However when I de-soldered it, it turned out to be open
circuit. The resistance in-circuit was not absolutely 0 though, although it
had seemed negligibly close, so it must have been the surrounding load as
has been suggested to me.
Is there a simple way to find shorted caps in-circuit? Would an absolutely 0
ohm resistance while in-circuit indicate a shorted cap for the reason I
suggest above?
Thanks
Rob
Hi Rob
The 0 ohms indicates a short but doesn't tell you what is shorted.
It could be the cap you are measuring at or one half way
across the board. It may not even be a cap since there are
many ICs also across the power lines.
I have a method that works quite well at finding such shorts
but requires a uV meter or a good 5 digit meter and a bench
power supply that can be set on current limit.
Alternate methods use a bench supply set on current limit
and hunt around with ones funger or take a picture in
the dark without the flash.
Also, there is that thermal paper.
The hunt and peck method of lifting one lead of each
part until you find the shorted one.
Some have success with a two lead ohm meter but
I've not seen this work well. A 4 lead has a better
chance.
Dwight
5 Feb
5 Feb
7:53 a.m.
I was checking the capacitors in-circuit and
understand that this is not
correct.
However, I would have thought that a 0 ohm measurement in-circuit would
indicate a shorted capacitor because there would surely not be any point
designing a circuit where both ends of the capacitor are connected together?
Yes and no...
Firstly, there is one very common circuit (common in the world, not in
classic computers) which effectively shorts out a capacitor, at least for
DC tests. And that, of course, is a parallel LC (inductor/capacitor)
tuned circuit. An inductor should have a very low DC resistance, of course.
Secondly, the fact that 2 poitns on a PCB test as being shorted together
meands that _something_ is connecting them. That something doesn't have
to be the obvious compomnent between them. That capacitor is presumably
conencted to other components. A short in one of those could be the problem.
Is there a simple way to find shorted caps in-circuit?
Would an absolutely 0
ohm resistance while in-circuit indicate a shorted cap for the reason I
suggest above?
There's no simple way, it's easier just ot desolder at least one end and
test them effectively out-of-circuit.
The fact that you've found something that's shorted that shouldn't be is
a clue I think. What does that capacitor connect to (it may be easier to
trace this o nthe good PCB where the 2 ends don't test as being shorted
together).
-tony
5:40 p.m.
From: ard at p850ug1.demon.co.uk
---snip---
Is there a simple way to find shorted caps
in-circuit? Would an absolutely 0
ohm resistance while in-circuit indicate a shorted cap for the reason I
suggest above?
There's no simple way, it's easier just ot desolder at least one end and
test them effectively out-of-circuit.
The fact that you've found something that's shorted that shouldn't be is
a clue I think. What does that capacitor connect to (it may be easier to
trace this o nthe good PCB where the 2 ends don't test as being shorted
together).
-tony
6 Feb
6 Feb
1:04 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of dwight elvey
Sent: 06 February 2011 01:40
To: General Discussion: On-Topic and Off-Topic Posts
Subject: RE: PSU for MicroVAX 3400 Not Faulty But Not Working
Hi
With the correct equipment, the method I spoke of works quite well. I've
never failed to locate the short and within
5 minutes. No damage to the board or components that could occur by
unsoldering parts until the shorted one is found.
Dwight
=
I have to say I didn't really understand the described method .
Unfortunately I don't have a bench power supply of any description and can't
really justify the cost at the moment, so I will just have to go round
de-soldering components until I find the bad one. Still, it would be useful
to understand the method. Would you be able to elaborate?
Regards
Rob
From: ard at p850ug1.demon.co.uk
---snip---
Is there a simple way to find shorted caps
in-circuit? Would an
absolutely 0 ohm resistance while in-circuit indicate a shorted cap
for the reason I suggest above?
There's no simple way, it's easier just ot desolder at least one end
and test them effectively out-of-circuit.
The fact that you've found something that's shorted that shouldn't be
is a clue I think. What does that capacitor connect to (it may be
easier to trace this o nthe good PCB where the 2 ends don't test as
being shorted together).
-tony 
10 Feb
10 Feb
1:21 p.m.
Rob Jarratt [robert.jarratt at ntlworld.com] wrote:
However, I would have thought that a 0 ohm
measurement
in-circuit would indicate a shorted capacitor because there
would surely not be any point designing a circuit where both
ends of the capacitor are connected together?
All you know is that *something* is presenting zero ohms between
those two points. Tony gave an example earlier of where that can
happen while not being a fault (an LC tuned circuit). That's
not likely to be the case here. However, any thing between those
two points could be the culprit. So if that capacitor is sitting
between ground and an IC pin and the IC is shorted internally,
then the cap could easily be fine.
That
capacitor
also measured differently on a working board so it was
clearly suspect. However when I de-soldered it, it turned out
to be open circuit. The resistance in-circuit was not
absolutely 0 though, although it had seemed negligibly close,
so it must have been the surrounding load as has been suggested to me.
Or in removing the cap you removed an unintended solder short or
your original measurement was off or ...
Is there a simple way to find shorted caps in-circuit?
Would
an absolutely 0 ohm resistance while in-circuit indicate a
shorted cap for the reason I suggest above?
Unless you have the tools to try the method that's been suggested a
few times here, then probably not.
You've removed the capacitor so now you can test it out of circuit.
Many multimeters these days can measure capacitance. (The cheaper
ones may well have some limitations re: accuracy and repeatability
but if that's all you have they'll at least give you an indication
of functionality). If you have an ESR meter, now's the time to check
that too.
If the cap is a cheap standard one you could try dropping in a new
replacement. However, I'd be tempted to trace the path between the
two points and seeing what else might be a problem.
Antonio
5109
days inactive
5144
days old
11 comments
5 participants
participants (5)
-
arcarlini＠iee.org -
ard＠p850ug1.demon.co.uk -
axelsson＠acc.umu.se -
dkelvey＠hotmail.com -
robert.jarratt＠ntlworld.com