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Author Topic: Marshall VS232 Oscillating Noise Issue  (Read 3327 times)

bgott

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Marshall VS232 Oscillating Noise Issue
« on: July 17, 2014, 12:49:28 AM »
I have seen a post on this forum before about a problem with these amps with an oscillation bleeding through into the audio path. The last post it seems was in 2009 with no resolution of the issue seemingly. I recently purchased a VS232 that needed the audio jack repaired for $50 off of craigslist. After fixing the audio jack and plugging in for a test jam, I noticed a lot of static low volume popping noises etc along with an oscillating noise. As a method of trouble shooting I simply turned down all of the settings and adjusting every knob one by one I determined that as you increase the speed of the chorus channel, the oscillating noise speeds up and slows down in the background regardless of whether or not the chorus is on. The post before seemed to be more focused on the reverb circuit and I just want to see if anyone has advice on what this could be or possibly a better method to troubleshoot. Thanks in advance for any and all guidance.

Roly

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Re: Marshall VS232 Oscillating Noise Issue
« Reply #1 on: July 17, 2014, 12:11:25 PM »
Hi bgott, welcome.


Nice inexpensive amp, and yes these do seem to suffer a few generic noise problems.

Quote from: bgott
a lot of static low volume popping noises etc along with an oscillating noise

The fact that the oscillating noise tracks the chorus speed strongly suggests that this is the chorus clock generator getting into the signal path somehow, but is it likely that this is an indirect result of the same fault that is causing your popping noise.

Try using headphones into the headphone socket and see if the fault persists/changes?

You could also try firmly tapping the front panel and chassis in various places to see if the fault has any mechanical sensitivity.
 
Circuit for the VS230 here (not the 232 but should be pretty close);
http://schems.com/manu/marshall/vs230r_2x30w_stereochorusrev.pdf
If you say theory and practice don't agree you haven't applied enough theory.

esmith

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Re: Marshall VS232 Oscillating Noise Issue
« Reply #2 on: September 15, 2014, 03:20:00 AM »
Hi bgott,

I just went through this exact issue with a VS232 I bought on CL.

I had to do three things to fix the problem.

First, one of the power supply capacitors for the +15 volts for the chorus circuit had a broken solder joint. Check the solder joints on C17 and C18. A bad solder joint here caused a TON of noise to get coupled into the amplifier section, (whether the chorus was turned on or off).

Second, the high frequency noise had caused two resisters to get burned out in the the power amp section.  These are 1/2 watt 10 ohm resisters, R10 and R18. Check those and replace them if they are burned.

That solved 95% of the noise from the chorus circuit.

The last thing I did was to add two 0.1uF ceramic capacitors to decouple the power supply pins of the two chips that are part of the analog delay for the chorus, IC4 and IC7.  For IC4, I soldered the 0.1uF capacitor between pins  1 and 5, for IC7 between pins 1 and 3.  I was using a oscilloscope, and I could see a lot of voltage spike noise being generated from the clock signals between these two chips.  Adding the 0.1uF decoupling caps got rid of most of the remaining noise.

Please let us know if any of this helped and if you have had any further luck repairing your amp!
« Last Edit: September 15, 2014, 03:21:54 AM by esmith »

Roly

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Re: Marshall VS232 Oscillating Noise Issue
« Reply #3 on: September 15, 2014, 05:14:17 AM »
Welcome esmith


Thanks for your helpful input on what looks like a design shortcoming on this model.
If you say theory and practice don't agree you haven't applied enough theory.

bgott

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Re: Marshall VS232 Oscillating Noise Issue
« Reply #4 on: January 15, 2015, 07:48:49 PM »
Hi bgott,

I just went through this exact issue with a VS232 I bought on CL.

I had to do three things to fix the problem.

First, one of the power supply capacitors for the +15 volts for the chorus circuit had a broken solder joint. Check the solder joints on C17 and C18. A bad solder joint here caused a TON of noise to get coupled into the amplifier section, (whether the chorus was turned on or off).

Second, the high frequency noise had caused two resisters to get burned out in the the power amp section.  These are 1/2 watt 10 ohm resisters, R10 and R18. Check those and replace them if they are burned.

That solved 95% of the noise from the chorus circuit.

The last thing I did was to add two 0.1uF ceramic capacitors to decouple the power supply pins of the two chips that are part of the analog delay for the chorus, IC4 and IC7.  For IC4, I soldered the 0.1uF capacitor between pins  1 and 5, for IC7 between pins 1 and 3.  I was using a oscilloscope, and I could see a lot of voltage spike noise being generated from the clock signals between these two chips.  Adding the 0.1uF decoupling caps got rid of most of the remaining noise.

Please let us know if any of this helped and if you have had any further luck repairing your amp!

Thanks for your feedback esmith. Unfortunately I never got notification when this was first posted. I am curious if you ever had any luck getting the noise to go away the rest of the way. I am going to try what you suggested first and hopefully that will do the trick. The amp has kind of been on the backburner for a while but now that I've seen this awesome feedback I'm excited to get on it!