Acoustic 136 bass amp possesed!

[J M Fahey]

Hi Tom.
Doing quite well so far.
Don't worry that much about those 66V loaded you get, Power Amp PSUs do fall when loaded, way too expensive to build them otherwise.
More important than that is the ripple voltage you get on the +B line, which indicates a sub-par (dry?) electrolytic.
Anyway you are replacing it, so ...

[TomCarlos]

Ok.... one more thing about the power supply. Last night, it looks like the bridge rectifier died. Maybe that too had been stressed previously. There was a dead short between one of the diodes.

Does anyone have a recommended replacement?

I see the following at Mouser: GBPC4001 Taiwan Semiconductor Bridge Rectifiers

I don't see a real part number for the Green Monster (I guess it is a GE Rectifier).

[gbono]

I use a 15A 200-400V version of the same - 40A is way over design. I'm assuming the mains switch is still functional (they are riveted onto the front panel and then ACC glued the aluminum cosmetic strip over that) - don't forget to disable the "polarity" circuit.

[TomCarlos]

Disable the polarity circuit? Is that a temp thing or do you disable permanently? And the best way to do that (rewire)?

[gbono]

I remove the capacitor and remove/reroute wiring from the switch. Change the mains cable if it's original to a proper grounded one.

[TomCarlos]

In trying to finish up a long term project, I have one last thing to mull over. On the Acoustic 136-140-150 amps, would it make sense to (physically) connect the case to the negative terminal on filter cap (the main negative ground). It seems that there is a low signal ground on the pre-amp board, a high output ground on the Power Module boards, other grounds, etc. I also see where the 1/4 phone jacks use the case as ground. But in the original design, I do not see a direct connection between the Negative side of the supply to the case.

Anyone have ideas on this?

[timeman]

Hello Tom, I just found this thread from a few years back. The reason I'm commenting is that I have an Acoustic Control 140 bass amp. I had it altered back in the mid 80's to have an effect loop put in. They did it by changing the input jacks to have a send and return. The input jacks on the left (as you look down on the chassis) have been made to both be inputs etc. I've been trying to get this wired back to original. I was wondering if you could post any pictures you have of the chassis, and mainly the wiring for the input jacks. I'm a bass player/composer/band director and not an electrician so any help would be very much appreciated. Thanks Tom! Brian Derek

[gbono]

I would suggest starting a new thread since this one is 8 years old. Also upload a picture of what you have. The wiring from the jacks to the two preamp PCBs is very straight forward but I don't know how your amp was modified.

[TomCarlos]

Hello Tom, I just found this thread from a few years back. The reason I'm commenting is that I have an Acoustic Control 140 bass amp. I had it altered back in the mid 80's to have an effect loop put in. They did it by changing the input jacks to have a send and return. The input jacks on the left (as you look down on the chassis) have been made to both be inputs etc. I've been trying to get this wired back to original. I was wondering if you could post any pictures you have of the chassis, and mainly the wiring for the input jacks. I'm a bass player/composer/band director and not an electrician so any help would be very much appreciated. Thanks Tom! Brian Derek

Agree.... Please start a new thred and send me the link (via Private Message) to that thread (as I am not here everyday). This should be an easy fix (I hope) !!

[BStone63]

I am new here and recently purchased a 136 that is very distorted at low volume. Does anyone know of a modern equivalent of Q302?

[TomCarlos]

Take a look at this thread - Acoustic 450 missing 40409HV and 40410HV

There are probably other threads with similar recommendations.

And you are sure that Q302 is bad?

If you need help, please start a new post.

[Enzo]

Those 40409 and 40410 are still available, I know I have some.

[The Dude]

......And you are sure that Q302 is bad?......

^^^^^^^^That. There are other defects that can cause those symptoms. Have you tried another speaker? A slightly rubbing voice coil will be more noticeable at lower volumes.