JCM 600, FX Loop Issue


Hi, I'm wondering if anyone can point me in the right direction with this amp.  I genuinely love this amp, however it's had an issue since purchasing it second hand and clearly had a lot of repairs/bodges through its life.  Originally it would pop fuses so I changed the valves and set the bias and all was working apart from the serial FX loop.  I opened it back up and removed all the connections and then used the schematic to put them back.  I noticed that the serial loop was now working but the parallel loop wasn't so I assume one of the connections was the wrong way round originally.

I would love to get the parallel fx loop working again (even though I don't use it very often) but not sure where to look and what to change first.  I've replaced the sockets for both the serial and parallel loop with no change (serial loop works, parallel doesn't).  Is there any caps, resistors or diodes I should be looking to test or change and if so which ones?  Is anyone familiar with the schematics and can point me to the area I should be looking at to resolve this issue?  Thanks in advance.

Kind regards,


edited 05 Sep 2021 at 08:30 PM

Richard Neill

asked 05 Sep 2021 at 08:29 PM

Richard Neill
Answers: 1


Hi Richard,

Thank you for your comment, we would recommend the amp is looked at by a qualified engineer, if you email servicedesk@marshall.com , they can arrange to have it looked at, at the factory.


Marshall Support

answered 13 Sep 2021 at 07:46 AM

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