Tektronix 2440 repair

[jrabeau@gmail.com]

Hi all,

I have a relatively old Tek 2440 oscilloscope that I was given. I switched it on and the display was odd, unfocused, not seeming to respond to any knob turning (I had not signal input). After a few minutes an enormous amount of smoke started to billow out.
I put it aside for several weeks with other things to do, but today pulled it out again to see about fixing it. I found the culprits: there are 4 tantalum caps, 2.7uF, I believe 20V. From the digging I did online, they appear to be Vishay, axial and are marked "275" and "8938".
The problem is, I can't seem to easily find replacements or alternatives. They also appear to be very expensive! Can anyone give some advice on alternatives?

I've attached a snip of the circuit.

Thanks for any help!
Best, Jim

[capt bullshot]

Some ordinary electrolytics with higher value (10uF ... 22uF ballpark) should work. Maybe with a parallel 100n ceramic. I don't think this is too critical here, since they are "just" power supply decoupling caps, and often were placed into the circuit by the dozen.

Anyway, if these four are all toast at the same time, I'd suspect some problem with the supply rails, check them thouroughly. Sometimes tantalums fail for no obvious reason, but not four in a row.

[Testtech]

Recap the power supply! Replace ALL the small electrolytic caps on the power supply board. Just do it, don't test them, etc. They are all suspect, and some are definitely bad! I have replaced hundreds of caps on the 24xx digital scopes, the very first thing to be done!

When replaced, check the voltages, if good, then see what other problems the scope may have.

It is unlikely that the PSU would go out of regulation with a high rail, but it is possible. All test points are on the PSU, or on the side board.

[jrabeau@gmail.com]

Thanks for the tips. I'll get some re-capping on the PSU. Agree there must be something giving rise to the blown tantallums.