1 Nov
2010
1 Nov
'10
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