Crate CA-15 loud hum/buzzing

[CrateOfDispair]

Hi everyone, I have a Crate CA-15 that I've been trying to troubleshoot for a little while now. When its turned on it is quiet but within 5-10 seconds a sustained buzzing ramps up. If I plug headphones into the headphone jack, there is a buzz but with lower intensity, if I plug the headphones into the external speaker jack, the buzz is loud.

One interesting note, if I plug a live source into J9 (CD Input - RCA jacks) the buzzing seemingly goes away and will play music cleanly (with or without a guitar input as well.)

I found another thread with a similar issue and in it it mentioned that the IC chip TDA2040 is bad and there might be voltage at the speaker connector if so.
https://music-electronics-forum.com/...n-a-crate-gx15

However, I do not get high voltage at my speaker connector (0.1V) but I do see in the schematic that the J9 is on one side of U2 (TDA2030) so it still may be an issue(?)

I've checked all the grounds and they seem to have good continuity. The only other thing I can report is that the voltage is lower by 3VDC than spec (I get 12VDC past D3 and 10VDC past R7, instead of 15VDC and 13VDC).

Is there anything else I should be looking at before I condemn the IC TD2030?

Thanks in advance.

[g1]

If it works good through the CD input, the TDA2030 is good.
What do you mean 'with or without guitar input' when it is working good with CD input. Does the guitar signal still have the buzzing?
Is the buzzing a constant frequency, or is it distortion?

[CrateOfDispair]

If it works good through the CD input, the TDA2030 is good.
What do you mean 'with or without guitar input' when it is working good with CD input. Does the guitar signal still have the buzzing?
Is the buzzing a constant frequency, or is it distortion?

Thanks for the response. I should have been more detailed. What I meant was I can play some external music via the CD Input cleanly. Right when the RCA jacks are connected, the buzzing stops. At this time I can also plug a guitar into Input 1 or 2 and the play simultaneously with the CD input and everything sounds clean. Once the RCA connectors are removed from the CD input the buzzing resumes immediately.

It is a constant frequency, not distortion.

[Mick Bailey]

As there's no contact switching with an RCA jack I'm inclined to think the amp may be being grounded via the RCA connection. As an experiment, plug in your guitar and jumper the RCA ground connection with a length of wire (or clip lead) to the audio source ground. If the buzz stops, try something else that's grounded. If that works there's a grounding or earthing issue.

[Jon Snell]

With P1 & P3 at minimum, does the hum stop?
If it does, check/replace the input sockets or grounding to them.

[Helmholtz]

Is C24 good?

If C24 is open, NFB will increase to 100%.

This is likely to cause oscillation manifesting as audible buzz.

Connecting a CD player restores an NFB voltage divider and stabilizes the power amp again.

[Mick Bailey]

That's an excellent observation.

If C24 is good, check R28 and its continuity to ground.

[Helmholtz]

Yes, any open connection between pin 2 and ground would have the same effect.

The instability theory is further supported by the 2030 datasheet stating that the closed loop gain must be larger the 24dB.
With open ground path CL gain would be 1 or 0dB.

[CrateOfDispair]

Yes, any open connection between pin 2 and ground would have the same effect.

The instability theory is further supported by the 2030 datasheet stating that the closed loop gain must be larger the 24dB.
With open ground path CL gain would be 1 or 0dB.

Happy to report we can mark this as solved. I found a bad solder joint that pulled up the pcb contact with the solder. This was on the C24 to R28 trace. I was able to repair it with a short piece of wire and now everything is working great!

I should have been more diligent while looking at all the solder joints.

Thanks to everyone for taking the time to help troubleshoot. I really appreciate it!