Marshall guitar amp schematic /advice please

[zen]

MODEL - LEAD 100 MOSFET

Not producing any audio

Removed the SEND / RETURN link at rear of unit & ran a lead from SEND port to another amp . Audio present therefore at least part of the pre amp is working .

[zen]

Re: Marshall guitar amp schematic /advice please

Model is actually 3210

Slight error on symptom listed .

Does actually produce audio , but it's distorted .

Issue looking like pre amp related .

Removed the SEND / RECEIVE link again & this time inserted an external signal into the receive port , then take out to another amp . Sound is clear .

With the same set up , except link connected , audio distorted .

[redwire]

Re: Marshall guitar amp schematic /advice please

If the DI output is clean, that would leave the mosfet power amp (5 transistors) as having some problem. Op-amp 5A can sometimes get nailed with ESD/overload and damage it, there is no input protection for it,.
Marshall schematics

[SydAmp]

Re: Marshall guitar amp schematic /advice please

Have you tried without the "link"? It shouldn't be in there for normal operation as the jacks are tied together until a cable is inserted. If still having issues, then your diagnosis will be good as the send/return jacks are at the end of pre-amp.

[SydAmp]

Re: Marshall guitar amp schematic /advice please

Actually, you have set it up wrong. Take a line from "send" to another amp. If clean then pre-amp good. Insert a signal to "return" then from speaker jack to speaker cab. If thats distorted then its the power amp section. The speakers do not connect to the output with the "level" pot. In your picture you are running into return then out of the DI/Output which will bypass power amp. The DI is directly after the return jack.

[zen]

Re: Marshall guitar amp schematic /advice please

Thanks very much for your advice .

Will test again & report back

[zen]

Re: Marshall guitar amp schematic /advice please

Seemed to have resolved issue , by replacing the following .


TR3 , TYPE BC184

IC4 , TYPE MC1458

C50 , TYPE 22MF , 25V

[redwire]

Re: Marshall guitar amp schematic /advice please

Oh, those are for the reverb circuit. I don't see a potentiometer for dry/wet so it's always a fixed amount of reverb.

[zen]

Re: Marshall guitar amp schematic /advice please

Hi Redwire

Strange , maybe a coincidence that it corrected itself .

Will run it on a long test before kicking it out .

Very much appreciate your advice , thanks .