TEKTRONIX 7904 BROKEN-DOWN

[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

[0culus]

The only problem with the U745 theory is... why is the trace defocused?

Separate problem no doubt. U745 is very likely bad as well. 

[0culus]

@OP, It's easy enough to trace the problem with another o-scope. With the calibrator signal fed into the DUT, probe along the vertical amplifier on both sides until you find where one of the differential signals goes away. For me, everything is fine until the outputs of U745 on one side.

[siggi]

  check all power supply V, follow serv manual

Second this. In a scope this old, it's not unreasonable to suspect power supply problems, and until those are fixed (or eliminated), there's no point in trying to diagnose anything eles. When you hear hoofbeats, look for horses before zebras .

[Jose]

before, I want to specify that this problem appears and disappears

all voltages are good  (no high voltage measurement -> no probe to measure)
intensity, focus and astig are normally adjustable when the 7904 starts well

I will follow the lead of Oculus by checking the peripheral components of the U745 because it is differential
and I can only vary on the lower half of the screen when the problem appears

I'll come back to you with the results of the measurements.

[0culus]

Intermittent problems are the most fun!  Good luck!

[jonpaul]

cold start OK, hot de focused....usually failing HV resistance, transformers or multiplier

j

[james_s]

A can of freeze spray (or air duster held upside down) works wonders for tracking down these kind of thermally sensitive intermittent issues. Let it warm up until the problem occurs, then start giving different areas a quick burst of cold spray until you find the spot that affects the symptoms.

[jonpaul]

Freez mist may NOT be used on the HV section as the condensed humidity can short things out

j

[Jose]

This morning, I turned on the 7904 and still the same failure
I turned it off and on again two hours later and it works.
I gently moved the vertical amplifier board hitting the U745
the problem appeared and disappeared
What do you think?

[Jose]

look at the picture

[jonpaul]

ReBonjour cher Monsieur José, j'habite à Paris.

0. from the photos de focus and offset trace, most likely open or intermittent connector pin Vert Output to CRT neck pin.  Open ckt on one side of CRT deflection plate.
ATTENTION! the CRT neck pins are prone to cracking the Kovar glass seals with any stress or bending!
AVEC GRAN PRECAUTIONS

a . apply contact cleaner to neck pins and V out connectors.
b. At CRT neck, gently rotate and slide the neck pin connector up and down the neck pin,
c. possibly the vertical deflection termination resistors assemblée connections.

See 7904 service manual.

1. second suspect harmonica connectors, micro coax, oxidation, corrosion intermittent contact.
Be very careful about the harmonica conn, easy to break the ancient plastic.
Apply contact cleaner sparingly and gently unseat and reseat the connectors.

2. mystery part in 2ond photo left is old mercury or electrolytic hour meter, perhaps application by previous owner.  no relation to the problem.


Bon courage !

Amicalement

Jon

PS how to find French Tektronix vintage group, forum?