1976 Marshall JMP 50W Combo Master Volume Squealing

phatt · Post by **phatt** » 29 May 2022, 22:29

I just re read the first post,,

Where you said, "There is no issue with humbuckers"

Maybe check the wiring of the single coil guitar.
Either way likely have a tecky check the Amp.
Phil.

Manfred · Post by **Manfred** » 29 May 2022, 23:43

phatt wrote: ↑29 May 2022, 22:29 I just re read the first post,,

Where you said, "There is no issue with humbuckers"
Maybe check the wiring of the single coil guitar.
Either way likely have a tecky check the Amp.
Phil.

Check the solder connections of the wires at the output jack of the single coil guitar .

turbonigro · Post by **turbonigro** » 01 Jun 2022, 16:52

Thanks for the tips. With humbuckers is actually more subtle but it is happening and it is still happening without any instrument plugged, just jack into high input.
However i have replaced 2 leaky capacitors and now voltages are more stable but on v1 and v2 there is still oscillation.
I have also isolated the problem to v1. Now i am running the oscilloscope again to spot if the issue is occuring on low jack socket or on something around.

Next week a tech will have it for repair, for sure

turbonigro · Post by **turbonigro** » 02 Jun 2022, 06:05

Solved! With the oscilloscope I have isolated the issue to v1b (high input) grid resistor and the jack itself.
I have installed a shielded wire grounded only on the side of the jack and soldered 68k resistor to it then to grid.

At high volumes and close the amp the squeal comes back so I will install on v1a another shielded wire going to preamp pot pin 2.

From January to June just to probe the circuit and install a shielded wire ahaha.

Lead dress was the answer

bmxguitarsbmx · Post by **bmxguitarsbmx** » 03 Jun 2022, 19:00

Good news!

That still seems suspect to me. JMP's are not very high gain and the HI/Lo inputs are 180 degrees out of phase, so unshielded wiring near each other at that point shouldn't be a problem. You should be able to dime a Marshall and have it remain stable.

CheapPedalCollector · Post by **CheapPedalCollector** » 04 Jun 2022, 05:03

Sus to me too, wonder if a plate resistor is way out of spec and massively raising the gain.

turbonigro · Post by **turbonigro** » 05 Jun 2022, 10:04

CheapPedalCollector wrote: ↑04 Jun 2022, 05:03 Sus to me too, wonder if a plate resistor is way out of spec and massively raising the gain.

I would check the three 100k plate resistors on v1, v2

Would be weird, these resistors are there for over 50yrs

bmxguitarsbmx · Post by **bmxguitarsbmx** » 06 Jun 2022, 22:42

CheapPedalCollector wrote: ↑04 Jun 2022, 05:03 Sus to me too, wonder if a plate resistor is way out of spec and massively raising the gain.

Its still only two stages. You can stick all 3 stages of a jcm800 to 330k and still dime it. I think it is ground, contact, our some other connection issue.

bmxguitarsbmx · Post by **bmxguitarsbmx** » 14 Jun 2022, 18:58

If you haven't already, I would recommend taking this to a tech. Probably cost a bench fee plus parts. A dude over on sloclone was having similar issues smoked his power transformer and is now selling as-is for parts. There is real risk to even turning this thing on. Take it to a tech.