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Acoustic 230 head weak output in power amp section

Started by periscopeblues, December 01, 2022, 11:39:18 PM

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periscopeblues

I recently acquired an Acoustic 230 guitar head, sold "as is" for quite cheap. It did not include foot switch to initiate the EQ, reverb, or master volume circuits, so I rigged one up myself after finding a Jones 6-pin connector online. Upon testing the amp with my 2x12" cab (4 ohms/2x 8 ohm V30s), the preamp section appears to be find and sounds good, but the output level is noticeably quieter than it should be with an amp like this and the cab (when I run the cab with my Silvertone 1464, I can't play it above ~1 without it being too loud for my apartment). When the Master Volume channel is turned on, the entire output is dramatically reduced; like not much louder than iPhone speakers even when everything's maxed out. Patching the pre-amp out and power amp in around back on the unit with a 1/4" cable seems to make a barely noticeable improvement but nothing significant. I have tested the preamp with another power amp and it seems to be fine, so it's looking like the issue is in the power amp section somewhere. Not sure if it's worth mentioning, but the only other issue with this head is the on/off switch, which really needs a good punch sometimes to turn over, and sometimes doesn't for a few tries. Anyways, if anyone might have some insight to what is causing this issue of weak output in the power amp section and what components I should focus on testing right away as potential culprits, please let me know. Myself and a friend who is an amateur tech are in the process of doing some troubleshooting. Also to clarify, the output level does not increase or decrease over time or fluctuate; it remains steadily weak. Thanks in advance.

Tassieviking

First check the voltage on C102 in both AC and DC.
Should be approx 98V DC.
Check the voltage on the speaker out with no signal both AC and DC.
If you have a large DC on speaker out then don't leave a speaker plugged in.
There are no stupid questions.
There are only stupid mistakes.

phatt

And after that I'd be looking into the limiter wrapped around the Power amp section, as it maybe faulty.
The master volume switch might be connected to that as well ,, hard to tell.
Also there is a feedback setup in the power amp but not connected to the output?? maybe a drawing omission.

Limiter is Q1&2, &Q11
Feedback is at R15

Phil

periscopeblues

Quote from: phatt on December 02, 2022, 10:02:52 PMAnd after that I'd be looking into the limiter wrapped around the Power amp section, as it maybe faulty.
The master volume switch might be connected to that as well ,, hard to tell.
Also there is a feedback setup in the power amp but not connected to the output?? maybe a drawing omission.

Limiter is Q1&2, &Q11
Feedback is at R15

Phil

Thanks Phil, gonna check all of this out and report back asap. Appreciate it.

periscopeblues

Quote from: Tassieviking on December 02, 2022, 03:20:45 AMFirst check the voltage on C102 in both AC and DC.t
Should be approx 98V DC.
Check the voltage on the speaker out with no signal both AC and DC.
If you have a large DC on speaker out then don't leave a speaker plugged in.

Checking on this too. Thanks