1 Nov
2010
1 Nov
'10
9:06 p.m.
My MicroVAX 3400 will not power up. I think it is because the right hand
H7868 PSU has failed, the green LED on it does not light up, while on the
left hand H7868 it the green LED does come on. I have no real electronics
expertise, very limited diagnostic equipment (just a multimeter really) and
I know that fiddling with PSUs is one of the more dangerous things you can
do with a computer. That said, I am willing to have a go at repairing it,
but have no idea what might be wrong. All I can say is that it worked a few
weeks ago when I last powered it on, but when I came to power it on tonight
it was just dead, no pops, bangs or smoke, just silence.
Can anyone give me some idea where to begin?
Thanks
Rob
1 Nov
1 Nov
9:28 p.m.
My MicroVAX 3400 will not power up. I think it is because the right hand
H7868 PSU has failed, the green LED on it does not light up, while on the
left hand H7868 it the green LED does come on. I have no real electronics
expertise, very limited diagnostic equipment (just a multimeter really) and
I know that fiddling with PSUs is one of the more dangerous things you can
do with a computer. That said, I am willing to have a go at repairing it,
but have no idea what might be wrong. All I can say is that it worked a few
weeks ago when I last powered it on, but when I came to power it on tonight
it was just dead, no pops, bangs or smoke, just silence.
Can anyone give me some idea where to begin?
I don't know this supply at all (and at one time DEC made some very
unconventional PSU....) but I can give you some tips for general SMPSUs
Unplug the mains, remove the PSU, and remove any casing from around the
PSU board. Do this with some care, as it's possible that some high
voltage capacitors are still charged.
Try to locate a pair (normally) of capacitors rated at aa fw hundred uF
and about 200V working. Measure the voltage across them (between the
connections of each capacitor on the PCB) with your multimeter. If they are
still charged, you will see about 150V. What I do then is take a 10k
resistor, bend the leads to as to fit between the connections of a
capacitor, hold it in insulated pliers and touch it to the connections
for a few seconds. Then check the voltage again. If they were charged, it
suggests the input rectifier is working, the chopper isn't, and there's
no bleeder resistor (see below)
Ok, it's now safe to to handle Remove the fuse (there is almost certainly
one on the PSU board). Check it with an ohmmeter. If it's open, examine
it. If the glass is blackened orcracked, it suggests a serious
overcurrnet fault, probalby shorted semiconductors on the mains side of
the PSU.
If the fuse is good, there's probably a start-up roblem. Most of the time
thre are resistors of the order of 100k connected from the mains
smoothing capacitors (the ones you checked and discharged). If you can
find such resistors, desolder them and test them with the ohmmeter. Quite
often they have failed open-circuit, in which case replace them and try
again.
-tony
10:16 p.m.
Thank you very much for the advice. I will extract the PSU at the first
opportunity and try your suggestions.
Regards
Rob
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Tony Duell
Sent: 01 November 2010 21:29
To: cctalk at classiccmp.org
Subject: Re: Possible Failed H7868 PSU
>
> My MicroVAX 3400 will not power up. I think it is because the right
> hand
> H7868 PSU has failed, the green LED on it does not light up, while on
> the left hand H7868 it the green LED does come on. I have no real
> electronics expertise, very limited diagnostic equipment (just a
> multimeter really) and I know that fiddling with PSUs is one of the
> more dangerous things you can do with a computer. That said, I am
> willing to have a go at repairing it, but have no idea what might be
> wrong. All I can say is that it worked a few weeks ago when I last
> powered it on, but when I came to power it on tonight it was just dead,
no
pops, bangs or smoke, just silence.
and
Can anyone give me some idea where to begin?
I don't know this supply at all (and at one time DEC made some very
unconventional PSU....) but I can give you some tips for general SMPSUs
Unplug the mains, remove the PSU, and remove any casing from around the
PSU board. Do this with some care, as it's possible that some high voltage
capacitors are still charged.
Try to locate a pair (normally) of capacitors rated at aa fw hundred uF about 200V working. Measure the voltage across them
(between the
connections of each capacitor on the PCB) with your multimeter. If they
are still
charged, you will see about 150V. What I do then is
take a 10k resistor,
bend
the leads to as to fit between the connections of a
capacitor, hold it in
insulated
pliers and touch it to the connections for a few
seconds. Then check the
voltage
again. If they were charged, it suggests the input
rectifier is working,
the
chopper isn't, and there's no bleeder resistor
(see below)
Ok, it's now safe to to handle Remove the fuse (there is almost certainly
one
on
the PSU board). Check it with an ohmmeter. If it's
open, examine it. If
the glass
is blackened orcracked, it suggests a serious
overcurrnet fault, probalby
shorted semiconductors on the mains side of the PSU.
If the fuse is good, there's probably a start-up roblem. Most of the time
thre
are resistors of the order of 100k connected from the
mains smoothing
capacitors (the ones you checked and discharged). If you can find such
resistors,
desolder them and test them with the ohmmeter. Quite
often they have
failed
open-circuit, in which case replace them and try
again.
-tony
2 Nov
2 Nov
12:54 a.m.
Hi Rob,
From my limited experience I would say please check the PCB around the
heaviest items on the board, clearly typically the transformer. The
4000/60 wouldn't power up (with noticeable noise) and I found that the
solder joint on one of the pins had broken. This took extracting the pcb
from the enclosure which mean unsoldering the inlet
wires, but clearly
it was worth it in the end ;)
Regards, Mark.
24 Nov
24 Nov
10:04 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Tony Duell
Sent: 01 November 2010 21:29
To: cctalk at classiccmp.org
Subject: Re: Possible Failed H7868 PSU
>
> My MicroVAX 3400 will not power up. I think it is because the right
> hand
> H7868 PSU has failed, the green LED on it does not light up, while on
> the left hand H7868 it the green LED does come on. I have no real
> electronics expertise, very limited diagnostic equipment (just a
> multimeter really) and I know that fiddling with PSUs is one of the
> more dangerous things you can do with a computer. That said, I am
> willing to have a go at repairing it, but have no idea what might be
> wrong. All I can say is that it worked a few weeks ago when I last
> powered it on, but when I came to power it on tonight it was just dead,
no
pops, bangs or smoke, just silence.
and
Can anyone give me some idea where to begin?
I don't know this supply at all (and at one time DEC made some very
unconventional PSU....) but I can give you some tips for general SMPSUs
Unplug the mains, remove the PSU, and remove any casing from around the
PSU board. Do this with some care, as it's possible that some high voltage
capacitors are still charged.
Try to locate a pair (normally) of capacitors rated at aa fw hundred uF about 200V working. Measure the voltage across them
(between the
connections of each capacitor on the PCB) with your multimeter. If they
are still
charged, you will see about 150V. What I do then is
take a 10k resistor,
bend
the leads to as to fit between the connections of a
capacitor, hold it in
insulated
pliers and touch it to the connections for a few
seconds. Then check the
voltage
again. If they were charged, it suggests the input
rectifier is working,
the
chopper isn't, and there's no bleeder resistor
(see below)
Ok, it's now safe to to handle Remove the fuse (there is almost certainly
one
on
the PSU board). Check it with an ohmmeter. If it's
open, examine it. If
the glass
is blackened orcracked, it suggests a serious
overcurrnet fault, probalby
shorted semiconductors on the mains side of the PSU.
If the fuse is good, there's probably a start-up roblem. Most of the time
thre
are resistors of the order of 100k connected from the
mains smoothing
capacitors (the ones you checked and discharged). If you can find such
resistors,
desolder them and test them with the ohmmeter. Quite
often they have
failed
open-circuit, in which case replace them and try
again.
-tony
I finally found time to take a look at the PSU. This machine has two PSUs
and there seems to be some mains distribution happening before the power
gets to the PSU. The input to the PSU is a 6-pin connector and right next to
that are four capacitors marked 250WV and 680uf. Are these going to be the
first "pair" of capacitors you mention? There are another two identical ones
somewhat further away.
None of the capacitors, or any of the other components, appear to be visibly
damaged.
There is no fuse I can see, but there is a circuit breaker.
Regards
Rob
25 Nov
25 Nov
9:57 p.m.
I finally found time to take a look at the PSU. This
machine has two PSUs
and there seems to be some mains distribution happening before the power
gets to the PSU. The input to the PSU is a 6-pin connector and right next to
that are four capacitors marked 250WV and 680uf. Are these going to be the
Almost certainly yes.
The incoming mains is rectified and smoothed. Look at the connections ot
these capacitors, you may find that pairsof them are connected in
parallel.
Quite often you wil lfind 2 capacitors in srries, +ve of one to -ve of
the otehr. The 'free' +ve end is the main B+ rail, the 'free' -ve end is
the 'common' rail of the maisn side of the PSU (which for obvious reasons
must not be connected to ground). The centre tap (the conection between
the capacitors) is often connected to the input voltage selector swithc
or cirucit, which will convert the input rectifier circuit between a
simple bridge rectifier for 230V mains and a voltage douber fro 115V
mains. In both cases you get aound 350V across the pair of capacitors.
first "pair" of capacitors you mention?
There are another two identical ones
somewhat further away.
None of the capacitors, or any of the other components, appear to be visibly
damaged.
OK. Alas that's ofthen the case. It's always worth carrying out a visual
inspection, but most of the time it reveals nothing.
There is no fuse I can see, but there is a circuit breaker.
Which I assume has not tripped?
-tony
5147
days inactive
5171
days old
5 comments
3 participants
participants (3)
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ard＠p850ug1.demon.co.uk -
mark＠wickensonline.co.uk -
robert.jarratt＠ntlworld.com