Acoustic 470 fuse blower

[randomair]

Without looking back through the whole thread to recap where we have been, the first thing I'd suspect would be small electrolytics in the signal path. Little caps like 1uf/50v or 4.7uf/50v. If only loud signal can press through for example, some DC level is probably squashing the signal.

This is actually the first time I have even addressed the pre-amp boards, I have been so focused on the power supply and power amp, but now that is taken care of, I should have suspected these other little caps would need to be addressed. By looking at the pre-amp boards, they have never been touched by a tech, which is good in a sense, no modifications or the like. Now its tracking this issue and we should be good, for now anyways. Ill probably just recap the boards for good measure.

By the way Enzo, thanks for all your help along the way, greatly appreciated!

[randomair]

So I replaced all of the electrolytics in pre-amp board A, as well, I replaced the all of the pnp 2n4248's, but the problem is still persisting. The problem again, is when a low note is played on the guitar, there is a slight delay then the signal is present, then there is an almost delay as the signal fades out. The signal is very harsh sounding overall, again channel B sounds great. Any other tips on this?

[Enzo]

The caps probably needed replacing anyway, no harm. I can't figure out why you might think the PNP transistors might be bad but not the NPNs? I am not a fan of wholesale replacing all the transistors in something anyway. I would be far more suspicious of one of the JFETs than the bipolars.

You have the schematic, chanel A is the 0043 board. Not exactly sure what delay means here, but it still sounds to me like you have some voltage situation the signal must overcome. The drawings have DC voltages at every stage, go down the channel and compare yours to the drawing.

[randomair]

Yeah the delay thing is a bit weird, i dont quite understand it myself. As far as the pnp's? I had them,so I just replaced them, I was in overhaul mode. I will reference the schematic and voltages to see the comparisons. You have hit the nail on the head with the signal trying to overcome the voltage, it sounds like it is starving or indulging. Whichever the case may be, there is indeed a mean unbalance somewhere. Thanks for the help Enzo! Ill report back with what I find.

[randomair]

Found the problem with channel A, there was an open 1.5 meg resistor tied to the base of an npn and the power rail. I found it by doing the testpoint tests and there was no voltage getting to the base of the NPN, replaced the resistor, now it sounds great! Thanks for the help on this everyone!