Tektronix time-base 7B92A dies after 5 seconds

[jonpaul]

Hello all,

I Have a pair of 7B92A and a 7B92, all in various states of disrepair or disassembly.  A fine example of Tek design, a very versatile time-base.

From the three, I hope to get one working to use in 7904, 7104 mainframes.  The remaining junker might yields some tunnel diodes or other spare parts.  The best is a 7B92A, works fine for 5 seconds and then completely dies,

It  seems like a power support is getting loaded or shorted after 5 seconds.  The PCBS are loaded with molded radial tants.  Yet none are visibly darkened or split.  What's the best proceedure to locate a possibility shoted tant or could it be elsewhere?

Or just replace every tant?

Many thanks for your advice!

À Bientôt
,

Jon

[tautech]

Pull some wires from the rails to the outside and monitor the rails for voltage stability. At least that will tell you which rail with tants on it to focus on. 
Pay particular attention to tants with little voltage headroom above the rail its on.

[jonpaul]

Hello again: Many thanks for the fast reply!

NOT A TANT!

1/ all other power is OK! Had assumed the very slow appearance of the bug meant thermally related.

2/ Instead it is related to time/div cam switch and variable: just touching the var knob would cause delaying and/or delayed traces to slowly shrink towards the left.

3/ I tried to clean and exercise time/div using only 100% isopropylene alcohol, injected into slots. No go.

4/ Removed  board on opposite side of time/div cam switch, check for loose screws on switch and injected alcohol thru holes. No go.

5/ Finally realized the var switch in center red knob of time/div may be the culprit.

6/ var Switch is at the back of long shaft, has a plastic actuator moving fingers on PCB.

Used same alcohol/exercise, IT WORKS!

The cause must be a  marginal contact, opening after some time or movement, with  timing Rt open circuit, the trace moves and shrinks slowly  left.

Doing this repair, and removing the board on 6 precision standoffs, with about 30 0.025 long pin interconnects,  I could only marvel at the electronic and mechanical engineering in the plugin! Fortunately I have the original TEK service manulas for both 7B92 and 7B92A.

Really a great plugin especially with 500 pS/div and extensive triggering capabilities. I will compare to 7B92A.

I welcome comments from you TEK 7Bxx mavens!

Cheers,

Jon