power section pops like frying bacon
 

I am attempting to repair a guitar amp that belongs to a friend of mine. I isolated the power section and power supply from the preamp and tone controls. Its making random pops and crackles as if I am wiggling a loose connection. The power supply is a simple unregulated bridge rectifier and 2 caps. The amp is a TIP35 and 36.

I have re-soldered all of the component connections and changed all of the electrolytic caps with no success. What you recommend next?

Hi Pbassred,

this could be a cold solder joint.

.....Or a deep-fried semi, assuming you have no sound. Did you have some output before dismantling?. If not, run a DVM over the base emitter junctions and verify there is ~0.6V across each. junction.

Highest on my suspect list would be the input transistor(s) of the power section. That is, the one that follows the tone/volume control section. That's where noise from a bad transistor or cold solder joint would bother it the most and be noticeable.

It works properly as an amplifier. I had already reflowed the solder joints so it can't be a cold joint. I'll swap the input transistors and see where that gets us.

Although the other causes are more likely I am sure, I had a frying bacon noise (low) on both channels in a power amp from some tantalum input caps (5.0 uf) I installed. Caps were purchased at RK distributing, a local parts store. Had it from the day I activated the purchased dead amp until 20 years later when I replace the caps again. Wasn't any internet in those days, didn't know what to expect from a transistor amp. This replacement time I used 4.7 uf 50V CPO ceramic caps. Much better! Still have the OEM (original equipment manufacturer's) 1970 transistors on one channel.

Freezer spray is good for helping to find such faults. Apart from joints and caps, don't forget resistors. I've seen lots of usually high value (>47k) resistors go dodgy after several years if the leads are bent close to the body.

Reflowing joints can make them worse, while at the same time hiding the evidence by giving them all a nice shiny appearance. Bad joints should be redone, the rest left untouched.

Perhaps this amp suffers from popcorn noise. If that is indeed the case, I would suspect a high gain circuit.

I think that popcorn noise is intrinsic to the device. i.e it would have sounded that way from the time of manufacture.