Tek 7904 scope

[Tom Peters]

At 08:33 AM 2/27/2007 -0500, you wrote:
> >I think I may know why it was so cheap. The power supply died
> >right after dinner today. Powering it up results only in
> >"tick tick tick tick..."
>
>If this is why it was cheap, the seller would have had to have
>known that it was going to fail.... 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.

>Shouldn't be all that tough to fix... IIRC it uses a switching
>invertor to generate an internal 50v supply which all of the
>lower voltage supplies feed from - Sounds like the regulation
>of the invertor is not working and it is getting shut down.
>I've seen other 7904s which exhibited "ticking" like this, so
>it may be a common problem.

Some other folks have mentioned "tantalum time bombs" in the power supply 
that die shorted after several decades. I'm trying to decide whether to go 
after it on that basis or try to get something out of the seller.

>I recall that the service manual has a pretty good description
>of the power supply - if you don't already have it, you can get
>it in PDF form from:
>    http://bama.edebris.com/manuals/tek/7904

I spent two days before the scope arrived downloading many manuals (low 
serial numbers, high serial numbers, mil-spec, commercial, all the 
plug-ins) from the primary bama site. The mirror worked great for few hours 
but I went back to the primary when the mirror started to act squirrely.

>I had my own positive experience with a dead Ebay scope lately...
>I bought a dead HP54201D (300Mhz DSO) in the hopes of getting
>parts for mine which has been experiencing problems (and HP
>doesn't provide service information) - It was in pretty rough
>shape, power-switch broken, case banged up, screen-burn, no
>probes or state pods, and listed as "immediately blows it's
>line fuse" - but I took a chance on it and picked it up in the
>hope that the analog board would be in better shape than mine.
>
>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! (Much better than my other one).
>
>It also has newer firmware than mine, so I simply swapped all
>boards into the nicer looking chassis... Next step is to use the
>good/working scope as a reference to track down the triggering

sounds like you had a more positive experience than me. Good for you!




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