Acoustic 136 bass amp scratchy distortion on the low end

[bryandowen]

I just adopted an Acoustic 136 combo amp. The volume was super low and very distorted, and a friend gave it to me for free, so I took it on as a hobby project.

The volume/distortion problem existed in both channels, so I (correctly) assumed it was an issue with the Power Module. I did some noodling to match up the PCB to the schematic from the service manual. (Totally irrelevant side note: I found a discrepancy in how R309 and C306 are actually wired, and also found a shortcut where C307 was soldered directly to a leg on R309, instead of into the intended through-hole on the PCB.) I decided it was a cheap enough bet to replace all the electrolytic caps since the amp is (gulp) 50 years old. And it paid off -- the output coupling capacitor (C311) was damaged, and after replacing it, the amp is NICE AND LOUD.

One residual problem, though: I'm getting scratchy/clicky distortion when I play notes on the low-E string. If I play a low E, I hear a "clicky" / "pulsy" noise at probably 5 - 10Hz (click-click-click-click-click-click....)

I don't have another speaker to run this amp through, but with the amp off I (carefully) push the speaker in and out, and I feel absolutely no resistance, so I'm inclined to say I don't think it's a rubbing voice coil. (Perhaps this is misleading? Folks with more experience will correct me, I'm sure.)

Does anyone know of a common or likely cause for this type of issue? (I saw someone else posting the same problem with the same amp, and it was never resolved, so this is a long shot.)

Thanks,

-Bryan

[J M Fahey]

R309 and C306 are actually wired, and also found a shortcut where C307 was soldered directly to a leg on R309, instead of into the intended through-hole on the PCB.) I decided it was a cheap enough bet to replace all the electrolytic caps since the amp is (gulp) 50 years old. And it paid off -- the output coupling capacitor (C311)

Which mean exactly NOTHING without a schematic.

Please post it.

[Enzo]

It may not exactly match the schematics you have found, but I bet most of it does. One tends to assume that the amp wasn't wired wrong and then just sat for 50 years. So until I had evidence otherwise, I would assume that the amp worked with these modifications. As described, they don't sound like they would cause the reported noises anyway.

You have no other speakers? None of any kind? A PA cab? ANother combo amp? If you had another combo you could disconnect its speaker and use clip wires to it.

[g1]

schematics in this owners manual: http://www.nilssen.us/files/acoustic-136-manual.pdf

[bryandowen]

It may not exactly match the schematics you have found, but I bet most of it does. One tends to assume that the amp wasn't wired wrong and then just sat for 50 years. So until I had evidence otherwise, I would assume that the amp worked with these modifications. As described, they don't sound like they would cause the reported noises anyway.

Sorry, I shouldn't have even mentioned the discrepancies I found between schematic and physical PCB. I don't believe they're responsible for the issue either -- I just thought it was interesting, and tagged it as a "Totally irrelevant side note", but it looks like it's causing confusion nonetheless.

You have no other speakers? None of any kind? A PA cab? ANother combo amp? If you had another combo you could disconnect its speaker and use clip wires to it.

I'll scare something up. I was just hoping someone would say "Oh yeah, common problem with the Acoustic 136 - here's what to check." I'm a wishful thinker...

[bryandowen]

The schematic for the Power Module (from the Acoustic 150 head featured in the Acoustic 136 combo amp):

[bryandowen]

I located another bass amp with a speaker I could borrow to test. The other amp plays nice and clean, so I assume the speaker is good. Caveat: My existing (Acoustic 136) speaker is 4 ohms, while the one I tested with is 8 ohms. I plugged my Acoustic head up to that speaker, and I got the exact same symptom (scratchy distortion at the low end).

Interestingly, I noticed this time that if I lower input volume (i.e., turn the knob down on the bass guitar itself), I hit a range where the distortion gets worse and then the sound cuts out entirely. To my untrained ear, this sounds like it could be crossover distortion. This happens identically through channel 1 and channel 2, so I'm still assuming the issue is with the Power Module.

[g1]

Check that the thermistors R316 and R317 are both around 100 ohms at room temperature.
At idle with no signal, check DC voltages across each of the .5R5W resistors, R318 thru R321, should be around 35mV each.
Check DC voltage at + end of C311, should be around 1/2 the voltage found at Q304 collector.

Full service manual available here: https://www.manualslib.com/manual/89...50.html#manual