Tek 2901 Time Mark Powr Supply Problem

[KK6IL]

Recently acquired Tek 2901 has a power supply problem. 120 Hz ripple on the -30V supply, which causes a ripple on the +3.6V supply also.  Voltage drop on R430 is 0.155V, so 15.5mA current drain.  Voltage on C430 drops from about -50V to -27V, which is more of a swing than I would expect with a 15mA load, but I'm mostly a digital engineer, so not up to calculating capacitor discharge rates.

Power supply section of PCB is badly discolored. Photo hopefully attached.

I would appreciate it if someone with a 2901 could measure the voltage across R430 and R411.

[pbarton]

We all know that an ink drop on blotting paper spreads uniformly in all directions with time.
The center of the PCB discoloration appears to be that axial capacitor which is adjacent to the bridge rectifier marked as 152-0199.
The right hand lead of that cap appears to be discolored.
You could try replacing the cap.

[tggzzz]

I found the rectifier became excessively hot. The black connector behind it became very fragile.

What are the ripple voltages? If they are too high, replace the relevant decoupling capacitor.

I gave mine away since I found it boring and there are other ways of achieving the same objectives, so I can't measure anything for you. I wouldn't give my Tek 184 away, though, since the internal metalwork is to interesting

[jonpaul]

First get Tektronix service manual off BAMA or tekwiki


Next check electrolytics for  C, esr, ripple, these 1970s  vintage electronics usually neeed psu recapping.

Several tantalum axial caps can go shorted on either top or bottom PCB.



Tektronix harmonica connecters deplasticize over tge decades, DO NOT TOUCH they crumble

Burned PCB not unusual and unlikely to be the fault.
The rectifier and series power resistance are underrated.
G10 deplastocizes and after decades of high heat burns

If you still need assistance go to groups.io tektronix2 or tektronix,  many more tek experts.

 I found   my 2901, works perfectly,  PCBs clean and perfect no brown, rectifier normally hot but not excessive, all PSU caps ripple and V fine.

We had replaced  large tant next to rectifier. Voltage across 0.5 Ohm power R was ~ 0.5V so ~ 1A load.

Comparing my unit and your pix, I think yours was left on 24/7 for years in a lab.

Bon courage

Jon

[KK6IL]

The problem was indeed C430, 550 uF/50V. Voltage across the cap is 50V, so replacement with a higher voltage cap is desirable.  Replaced with  a 820 uF/200V Rubycon cap from the junkbox which was a very tight fit in the capacitor holder, so additional mounting hardware not necessary.

C453 was slightly warm to the touch, but that seems to be heat from the bridge rectifier which is much hotter.