Tek 7904 scope

Tom Peters tpeters at mixcom.com
Wed Feb 28 11:42:40 CST 2007

At 10:12 AM 2/28/2007 -0500, you wrote:
> > >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.
>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.

> > 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.

I hope some of the many manuals I've downloaded has a good schematic.

>"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.
>dave06a (at)    Dave Dunfield
>dunfield (dot)  Firmware development services & tools: www.dunfield.com
>com             Collector of vintage computing equipment:
>                 http://www.classiccmp.org/dunfield/index.html

[Philosophy] Life: a sexually transmitted disease that is fatal.
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