Help Needed: Troubleshooting Volume Loss in Guitar Amplifier

[cruzer]

Hello everyone,

I'm looking for some help with my guitar amplifier. Recently, it suddenly lost its volume, and sometimes the sound drops out completely. I took it to a local technician who replaced the TDA2050 (which I believe is used as the power amp), but the issue wasn't fixed.

I'm an electronics engineering student, so I figured this would be a great project to work on myself. I've made a schematic of the circuit (attached below). I apologize if it’s a bit messy—this is my first time creating one, and I just followed the circuit as best as I could. Please also disregard the component values for now.

Could anyone help me identify what might be causing the problem? Any advice or guidance would be greatly appreciated.

[Brad1]

Obvious questions, first.
"...it suddenly lost its volume, and sometimes the sound drops out completely."
Does it run full volume at all, anymore, then drop out some, then completely? If so, does it start off fine cold, then start futzing as it warms up?

The first scenario could be a loose connection, or flaky solder joint, etc. CAREFULLY tapping on components with a wooden dowel or chopstick might find something. ( May not need to tell you, but USE CAUTION WHEN WORKING ON ANY LIVE CIRCUIT!)
The second scenario could be heat-related and one of the components flaking out as it warms up, or the heat of the leads or pins of a component causing intermittent.
It could be a dirty pot, or dirty or loose jack?
That's an admirable job of a schematic, but nobody knows for sure what the amp is, or if that schematic is correct, as you admitted it might not be be accurate.
May help to reveal what the amp actually is? You're calling it a GA-30, which I assume the GA is "guitar amp", but not sure about the "30"? Just so you know, GA-30 have been popular Gibson tube guitar amps since the 1940's, so you may want to designate that as something a bit different, to avoid confusion?
Explain in more detail exactly what is happening, and identifying the amp may be helpful.
Just some suggestions.
Good luck.

Brad1

[Chuck H]

Like Brad said.

In which case knowing the actual circuit isn't that important since the fault is likely a physical rather than an electronics issue.

I would start by cleaning all jack switches. These are possibly sealed switching jacks so may be impossible. But clean any unsoldered contacts you can, Plug and unpug listening for glitches, etc. Wiggle the cord at the input jack checking for intermittent faults or shorts. And...

I didn't peruse the schematic for errors but at least one did jump out at me. I don't see any path to 0V for your negative speaker terminal.

[cruzer]

Thank you for your detailed response!

To answer your questions:

• The amplifier doesn't reach full volume at any point; it stays consistently low and then the sound intermittently cuts out entirely.
• The sound doesn't crackle or something, it is just low constantly, I think it lost 70% of its original volume.

Regarding the amp's identification: GA-30 is written on the circuit board, the amp is cheap solid state amplifier.

Thanks again for your suggestions Brad

[g1]

I would start by cleaning all jack switches. These are possibly sealed switching jacks so may be impossible. But clean any unsoldered contacts you can, Plug and unpug listening for glitches, etc. Wiggle the cord at the input jack checking for intermittent faults or shorts. And...

I didn't peruse the schematic for errors but at least one did jump out at me. I don't see any path to 0V for your negative speaker terminal.

I think the speaker neg. gets routed through the headphones jack, and that jack is a prime suspect as it is a switching jack that mutes the speaker. So agree that your suggestion to clean the switching jacks is the best place to start.

cruzer , have you tried the amp through headphones, and do you still hear the problem?