Fender Stage 185 Losing Sound, Until I punch It On the Noggin

[GreggM]

Guy brought back a Stage 185 saying that it is cutting out on him but that it will come back when he raps it on top. I have previously replaced the output transistors and filter caps. It's only been 6 months, could the transistors be bad again already? Nothing looks loose or broken. I'm just about to pull the board and check for bad soldering. Any guidance as I go in?

[The Dude]

Have you tried tapping around (chopstick method) first? You may want to try to narrow it down to an area while it's still together so you have a better idea where to look when you take the board out.

[52 Bill]

Welcome to the place.

Nothing looks loose or broken. I'm just about to pull the board and check for bad soldering. Any guidance as I go in?

Looks can be deceiving, try tapping on different sections of the board to try and find the area or the part that is causing the signal to drop out. It could be anything, including a dirty break jack contact.

I usually start by checking the jacks, the pots and any of the larger components for cold or broken solder joints.

[The Dude]

+1 to checking jacks, pots, and break jack contacts. Also, and at the risk of sounding like Mr. Obvious, look for any high heat areas. Often power resistors, regulators, and "things that get hot" will have cracked solder joints.

[Enzo]

Before anything else, just plug a cord from FX send to FX return, any help? Or if there is preamp out next to power amp in jacks, patch them together, any help?

Output transistors don't get high low power when you tap them.

use a wooden chopstick and poke each part, looking only gets you a tiny step along the road to success.

[Richard]

I have an ampeg with bad input jacks that does the same.

[g1]

The good news is if it's cutting in and out, it's very unlikely to be a transistor issue.
May not be the case here, but two areas that I recall being problematic in this model, the thermistor near the fuse, and the big .22ohm 20W resistor in series with the speaker negative. Bad solder or burnt/arcing traces at either or both of these components.

[GreggM]

After taking all of your input, I observed that the level switch in the effects loop seems to be the culprit. I tried cleaning the contacts but it still needs to be wiggled to get the volume back.