28 Feb
2007
28 Feb
'07
9:42 a.m.
At 10:12 AM 2/28/2007 -0500, you wrote:
Thats probably good news - nothing harder to fix than a problem
which comes and goes ... If it firmly busted, it shouldn't be too
hard to find.
I sure hope that's true.
I hope some of the many manuals I've downloaded has a good schematic.
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See if it "comes back" after
having been off for a while. Perhaps this is a known condition
of the unit.
I've tried it once a day since Sunday. No go. Some other
folks have mentioned "tantalum time bombs" in the power supply
that die shorted after several decades.
Yes, tants tend to die of old age ... Easy to find and fix. Depending on
how much "ommph" the power supply has, bad tants can often be found by
visual (or olfactory) inspection (If inspecting with power-on, wear safty
glasses - I've had more than one cap blow up in my face). If not, tracing
through the supply path should quickly take you to the problem area. "Ticking" often indicates a switcher trying
to come up into to great a
load (like a shorted cap) - Have you tried pulling all the modules to see
if the DC supplies come up, if they do, it could be a bad module (also
likely to be a bad cap). I'm assuming the internal circuitry of the
mainframe will be enough load to let the supply come up normally - IIRC
it will come up under no load, but I could be mistaken (leaky memory bits).
As far as I remember, you shouldn't run a switcher with no load at all. I
did try to remove all the modules and turn the panel illumination off.
Still does the same thing.
I'm trying
to decide whether to go
after it on that basis or try to get something out of the seller.
Since it hasn't recovered, he probably didn't know it was about to
expire - likely it has been unused for a good long time, he powered
it up long enough to say "it works!" - and you powered it up long
enough for some wheezing component to pack it in. All ya gotta do is
find out what failed - power supply problems are usually pretty
straightforward to diagnose.
Any equipment this age can die at any moment, so even if it made it
past 90 days or whatever, you still might need to fix it sooned than
you would have liked. If I got a good price on it, and it's a piece
I wanted, I'd probably just fix it - but thats your call.
As Tony mentioned - giving the PS a good overhaul could also help
with the twitchy traces you observed when it was working.
>Arrived a couple days ago - inside looked to
be in pretty good
>shape, and I immediately noticed one of the primary line side
>filter capacitors was "humped up" at the top of the can - so I
>removed it and found that it had exploded/leaked ... cleaned up
>the board and investigated a bit more and found a shorted diode
>in the primary line recitifier bridge - replaced both components,
>and the scope works perfectly!
sounds like you had a more positive experience
than me. Good for you!
Well... my expectations were low as it was advertised to be DOA - As the
seller said - he "packed it carefully to make sure that it would arrive
broken". I just got lucky and it was an easy fix - yours could be too!
Best of luck with it.
Dave
--
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
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