25-40Hz Periodic THUMP - Bad Transistor?

I've got this periodic low-freq thump in one channel of a preamp, and I've traced the source to right around the area of a transistor. It is a bit tedious to replace this transistor so before I begin, I wanted to ask if periodic low frequency "thumping" could typically be caused by a bad transistor (or more likely another problem?)? On the scope it looks like just the top portion of a (slightly deformed) square wave going by every few seconds or so...

I have seen this caused by dried up bypass electrolytics on transistor stages. Usually in parallel with a resistor to ground off the emitter. Every 6 or 8 seconds - Ka Bonk!

Thanks Bill,
It's actually more like a low level "fut-fut-futtt" I've replaced all the lytics so it couldn't be a dried up cap... Any more ideas?

Check for any leaky inter-stage coupling caps. Low freq. oscillations are typically caused by bad bypassing or decoupling. If it has global feedback back to the first stage, it could be caused by a bad cap or open in the feedback network too.

> if periodic low frequency "thumping" could typically be caused by a bad transistor

No.

Since I had already recapped the thing, I went through and re-reflowed the solder joints. Powered up, presto! That did the trick. Thanks guys!

Piss-poor connections produce a prevailing preponderance of problems.

[quote author="bcarso"]Piss-poor connections produce a prevailing preponderance of problems.[/quote]

Smile when you say that, buddy. And use a pop filter.

Peace,
Paul

Paul, I labored some to find a synonym for "connection" that started with a "p" but to no avail ;-) I guess I could try a silent p: "pterminations" mayhap?

[quote author="bcarso"]Paul, I labored some to find a synonym for "connection" that started with a "p" but to no avail ;-) I guess I could try a silent p: "pterminations" mayhap?[/quote]

"Plugs", I suppose, but the context is wrong. "PbSn" is hard to pronounce.

Peace,
Paul