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Solid State Amplifiers => Amplifier Discussion => Topic started by: boghymnal on October 31, 2019, 04:48:06 PM

Title: Acoustic 220 quieter volume than expected
Post by: boghymnal on October 31, 2019, 04:48:06 PM
Looking for some repair/troubleshooting tips. I purchased a vintage Acoustic 220 bass amp a while back (without being able to test it) and finally got a cab to pair it with.

Based on several posts I've seen about this amplifier on different forums, I expected it to be LOUD (comments like "can't go past 9 o'clock without drowning out my band!"). However, even with the volume cranked I can speak over the sound of the amp. The actual tone seems fine and every knob and slider (aside from the bass pot which I assume is just a bad pot I can replace) affects the tone -- it's just the volume is quieter than expected. I've tried another head with the cab I'm using so I can confidently rule out the cab. I've tried both speaker outputs on the head and both have the same output volume.

Based on the symptoms, I'm assuming maybe something with the power amp section is bad? Any idea of what this might be? I'm having trouble finding a readable schematic.
Title: Re: Acoustic 220 quieter volume than expected
Post by: DrGonz78 on November 01, 2019, 03:49:22 PM
First thing to check is the preamp output and poweramp in mono jacks. The main instruments signal path passes right through those jacks. Often those jacks get dirty and the signal starts to fade as heads through to the output section of the amp. Easiest way to test the jacks is to just simply connect the two jacks together using an instrument cable. Does plugging this patch cord from preamp out to poweramp in make any difference the volume?

Later you can connect the preamp out (Acoustic 220) to the poweramp of another working amp to test the preamp section. Then take the preamp out of another amp (Guitar signal, iphone preamp works too) and input it into the power amp input of the Acoustic 220 amp. That way you can find out if there is a problem with the preamp or poweramp.
Title: Re: Acoustic 220 quieter volume than expected
Post by: boghymnal on November 07, 2019, 10:42:42 AM
Thanks for this response -- I plugged a patch cable between the preamp output and power amp input jacks. Unfortunately it didn't result in any changes to the behavior/volume. This amp is currently at a practice space so I think at a minimum I need to take it home, open it up, and put it through a good cleaning and at least a visual inspection of the board.

Now I need to find another preamp and poweramp to move on to your next suggestion to try and narrow down where the volume loss might be happening. Also considering building an audio probe after seeing a couple youtube videos about using them to troubleshoot amps. This is my first amp repair after doing a lot of small scale electronics work (pedals, etc.).

Also, thanks for the schematic -- it's much clearer (particularly the power amp section) than the one I'd found elsewhere.

You seem to be familiar with this model -- any other typical issues that might be good to hone in on?

Thanks again!
Title: Re: Acoustic 220 quieter volume than expected
Post by: DrGonz78 on November 07, 2019, 03:23:10 PM
I am actually not familiar with this model and have not worked on one before. Basically we just need to break the amp up into sections, Preamp & Power amp. If the preamp is putting out a good healthy signal then the problem is pointing to the power amp and vice versa. Conquer and divide is the term for such an exercise and we want to know where in the circuit to focus our attention.

Edit: Typical issues are sometimes good to look at but can easily get us looking the wrong direction for the fault in the amp. Best to troubleshoot in a logical way first. Once we determine if the amp has a preamp problem or power amp problem then the next steps to to troubleshooting will follow. For example, we might have a power supply issue where signal is not passed through either the preamp or power amp. So honestly testing for power supply voltages might actually be the very first step in the process. Since testing preamp and power amp is simple we sometimes just jump right in to check that first.