Marshall MG15CDR - high pitch noise

[David26]

Hello everyone,

Reaching out to the community for help with this little amp. I got it from someone that sold me a guitar and threw this amp in because he did not want to bother with it any more.
I am comfortable opening it up and replacing components but I do not know how to troubleshoot the problem. Here are the symptoms:
1. There is a loud, high-pitched noise that does seem to get louder/quiter with the volume pot but it is always present. I do not know how to acurately describe it in tech jargon so I am attaching a short video.
2. The guitar signal does not come through when I plug it into the dedicated input jack (1MΩ), just the noise. The cable is not the issue.
3. The CD input (3.3kΩ) does work and I am able to get the signal through, but the noise is still present.
4. A thump sound when turning it on. I do not know if this is normal for this amp or related. Just mentioning it, in case it's relevant.

I am attaching the schematic.

I have to say I would not bother with it but, when ignoring the noise, the sound that comes out of this little amp is promising. I would love to be able to fix it for my 10-yr old. If anyone is able to help me diagnose it, you will make a father and son very happy!
Much appreciated!

[David26]

I am sorry, the site does not allow me to upload the video. I can describe the noise as high-pitched, almost like a hiss but more like a whistle. It is constant, there is no fluctuation in pitch or length. There is also some occasional crackle when turning the volume up or down but that seems to be the potentiometer.

[Jon Snell]

If R34 is damaged, incorrect / dodgy mains earth can do it, the output IC will oscillate with the possitive feedback.

Check the components circled.

[Chuck H]

And I'll add that "dodgy" could also mean the resistor is ok but the connection is bad. I haven't been in one of these amps but it's possible where R34 connects is a stress point on the board (cracked or cold solder joint) or it may connect through a flying lead like a ribbon cable (possible bad friction contact).

[David26]

Thank you both! I'll open it up today to check and get back with what I found.

[The Dude]

Also check solder connections on the input jack itself.

[Chuck H]

Yes. This ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Pretty much any ground in the signal path common to both channels. But the input jack, as with most amps that have them board mounted, has a high risk of abuse and failure. I almost mentioned it except that Jon came off with such confidence and HE didn't mention it. Since I have no experience with these amps I didn't want to speak out of turn.

[x-pro]

1. There is a loud, high-pitched noise that does seem to get louder/quiter with the volume pot but it is always present.
2. The guitar signal does not come through when I plug it into the dedicated input jack (1MΩ), just the noise. The cable is not the issue.
3. The CD input (3.3kΩ) does work and I am able to get the signal through,
4. A thump sound when turning it on. I do not know if this is normal for this amp or related. Just mentioning it, in case it's relevant.

Check the soldering quality of all controls and test them on an open circuit board by jiggling them slightly and adjusting them with the amplifier on. It is better to use headphones to avoid overloading the speaker.
It is also possible that SW1 pins are oxidized or C48 is defective.
Answer to question 4.
A clicking sound may be heard when the amplifier is turned on.
If you have a tester, be sure to check the +15V and -15V supply voltages relative to the case. It may differ within 1V.
The rest as you proceed.

[David26]

Also check solder connections on the input jack itself.

Thank you all for the prompt and knowledgeable input - this is a great community!

Problem solved.

I apologize for having forgotten to follow up on it.

As it turns out, I got away easy with this one after all: while the jack pins themselves were properly soldered, the circuit layer of the board had become detached around the jack... Hardly noticeable but enough to create the problem... I ended up having to jump a pin connection to a nearby point on the board, where the integrity of the circuit was not compromised. It was a simple enough job so I didn't think to take photos at the time but if anyone else runs into this, I will open up the amp again to do so and share.

On a side note, this is a decent amp for its size and I find that it was worth the time. I enjoy listening to my son jamming on it!

Thanks again!