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TEKTRONIX 7904 BROKEN-DOWN

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Jose:
Hello,
recently I bought a TEKTRONIX 7904
but sometimes it doesn't work properly
(see attached picture)
I tried all the settings of focus, beam finder etc... but no change
the position of the vertical amp remains in the lower part of the screen
I replaced the plugins 7A24 and 7B92A by 7A19 and 7B85 but it doesn't change anything
I also changed the location of the plugins
can you tell me on which circuit board to look for the failure

0culus:
Dead, or partially dead, vertical output microcircuit. This was the first mainframe that used an integrated circuit for the final output amp. Good luck finding one. I have a 7904 that I got for free that just needs a new one to be working.

You should also replace all the tantalum caps if you haven't already. Some are very hard to get to (partial disassembly of the plug-in backplane is required).

Jose:

U745, you think it is this component that is causing the problem?
if it is this one, I will take measurements before changing the tantalum cap
it's very hard to take everything apart :-)

0culus:
Yep, that's the one. You'll notice the vertical signal path is fully differential...I have a feeling one side of the U745 is bad. This will cause the trace to be squished to either the top or the bottom in operation (and you won't have enough adjustment range to calibrate it). I would also note that the 7904A has a completely different circuit design that uses the Hypcon packaged microcircuits developed for the 7104. It is far more reliable, and IMO is a superior general purpose mainframe.

jonpaul:
Recommended steps:
 Get serv man for your SN range follow troubleshooting flow charts!

See TEKWIKI, W140, etc. \


 Lube and exercise all controls especially INTEN, FOCUS, ASTIG

  Clean beam finder switch

  check all power supply V, follow serv manual

  Check HV voltage: Failures likey in high meg divider carbon R (go up in vlaue), HV transfgormer , HV multiplier

See Tekscope and Tekscopes2 on grouos.io, post there as well.

BE CAREFUL!

Bon chance,

Jon



Jon

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