Mg100hcfx issue - my.marshall.com

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Mg100hcfx issue

Hi, I bought the above amp as a standby should my JMD1 bow out. I've had the HCFX about 5 years and never had to use it. However, the issue is the gain/eq/volume controls do nothing in either preset or manual mode. All else (FX/buttons and master) controls are fine. I've reset the amp several times with no change. I'm an electronics engineer and don't mind getting my hands dirty and prod about with a scope or meter. I've had a look around, but nothing obvious. Any help you could give would be appreciated.

asked 21 Jul 2023 at 03:51 PM

Answers: 1

Hello John,

If the controls are not working in either mode there may be a fault with the actual front board, we would be happy to look at the amplifier for you, if you would like to make any arrangements please contact our returns department on 01908 375411.

Kind Regards

Marshall Support

edited 24 Jul 2023 at 09:34 AM

answered 24 Jul 2023 at 09:34 AM

Hi, further investigation has revealed the front panel PCB is OK. Each of the 8 pots vary a 5v signal into1 of 8 inputs of a CD4051 multiplexer chip. The common output from the 4051 feeds back to the main control PCB. A scope shows the 8x voltages varying on the output of the chip. This also means the 3 channel select pins are functional. My first thought was that the "C" select signal (bit 2) was not working, as the Gain, Treble, Middle & Bass controls occupy the top 4 channels (channels 4-7) accessed by this. However, the multiplexed voltage output shows all are present and are presented to the main control PCB. This leads me to believe the problem lies there.

As I said, I can wield a scope and am happy to prod around at signals.

John - 26 Jul 2023 at 11:18 AM

Hello John,
We only sell parts to qualified engineers that are registered with us, but as I said we would be happy to look at the amplifier for you.

Kind Regards
Marshall Support - 26 Jul 2023 at 12:35 PM

I can wield a scope and am happy to prod around at signals. - 11 Mar 2024 at 01:47 PM