59 Bassman RI low end fizzle distortion

[nateflanigan]

A friend dropped off a 59 Bassman RI, it's making what I'd call a low end fizzle distortion.  Like the highs are clear, but the lows, well they fizzle.  Here's a sound clip...
https://soundcloud.com/acrzuvdemos/bassman-noise


So far, all the standard voltage points tested OK, tube swaps yielded nothing, bias read low and would only adjust to 28 ma.  So I adjusted the bias voltage to 54.5 volts (as per the manual) Mild improvement, but still totally unusable.  The problem is much less severe on the bright channel.  I don't want to jump to conclusions, maybe a bad coupling cap?


It seems like a major pain to pull out that circuit board, so any insights that could help narrow the field would be great.

[tubeswell]

Have you tested all the filter caps to make sure they are all pulling their weight (and that none of them are shorted, nor have their ground sides disconnected)? Quite often in these poxy PCB amps, the traces become corrupted, and even if it looks like all the wires are connected to the right connection points, you can still have broken traces and or broken jumper wires, which mean that the bits are not connected. I recently diagnosed this in DRRI that a mate of mine brought in for me to fix. Took me three hours to find the broken trace. A real PITA

[nateflanigan]

ughhhhhhhhhhhhhh


That's good advice though.

[nateflanigan]

I made one of these audio probes http://billmaudio.com/wp/?page_id=1254 and traced the signal through the entire amp without finding any problems. Amp was running into a dummy load.  So, maybe it's a blown speaker, or bad connection.  I connected it to a different cab, seemed much better, but I could still hear the fizzle a little bit.  Reconnected the 4x10's one by one, found that the top right (viewed from the back) is making the most noise.  I need to set up some more tests to eliminate variables, confirm suspicions etc, but right now my hypothesis is that the speakers are not bad, they are just vibrating something in the chassis. 

[nateflanigan]

Finally got around to testing out the theory that the proximity of the chassis to the speakers is vibrating something on the circuit board, and it seems to be true.  I bought some RCA extension cables and placed the chassis about 6' away on a pillow, no buzzing.  I think I'm going to try and sell the owner on the idea of dropping in a new hoffman board.  I'm just not interested in futzing around with the fender PCB.