Pro Reverb Low Volume

[Shawnobi]

Right I think I've gotten somewhere and I think the motorboating sound is possibly coming from a classic bit of fender board moisture. However I popped my meter across the board and couldn't seem to read any conductance. I noticed the sound started to die out as the amp warmed up. Plugged a guitar in and everything sounded as it should. I then started chop sticking around and found that I could bring it back with some pressure on the board. I reflowed the solder joints but still the problem was there.
Here are some videos I took. One has a higher frequency than the classic motorboat sound.

Motorboating (15) Share - YouTube

(15) Share - YouTube Higher Frequency

So it sounds like the broken plate resistor was the initial problem. This motorboating sound is either part of the original fault, or a coincidence.

[pdf64]

I wonder if a resistor has a near invisible fracture? The 100ohm at the bottom of the LTP is a favourite for that happening, and the board seems very sensitive in that area.

[Shawnobi]

I wonder if a resistor has a near invisible fracture? The 100ohm at the bottom of the LTP is a favourite for that happening, and the board seems very sensitive in that area.

Sorry what is the LPT?

I decided to measure for voltage on the board and I'm seeing around 30V DC that quickly runs away through the meter. This is on both the main board and the diode/rectifier board for the power section.


Ah! Long Tail Pair. I see it. I'll swap it out. But the voltage on the board seems bad.

[Shawnobi]

I wonder if a resistor has a near invisible fracture?.

Well low and behold I changed the 100k and there are no more strange noises. Thank you pdf64, Leo and Everyone else who chipped in.

I've still got voltage on the fibre board which is confusing me. Especially as the board is screwed into the chassis with self tappers. But amp sounds absolutely fine.

[g1]

I wonder if a resistor has a near invisible fracture? The 100ohm at the bottom of the LTP is a favourite for that happening

Well low and behold I changed the 100k and there are no more strange noises.

Did you mean the 100ohm that pdf64 mentioned?

[Shawnobi]

Did you mean the 100ohm that pdf64 mentioned?

Yes, sorry, Typo on my part.

Started it from cold this morning and there was briefly some motorboating as it warmed up. Prodding the board gives some squealing, mostly around the normal channel. Def' improvement. Really feels like it's a board issue in someway.