1975 50 watt JMP MK II

[Canada Marshall Fan]

I have a 1975 50 watt jmp mk ll ....it is making a constant rumbling sound....sort of like a distant thunderstorm....I am not tech savvy so I took it to a tech who found a corroded tube socket, replaced that, but did not solve the problem....he had a look under the circuit board and didn't want to dig into it....any ideas on the problem would be much appreciated....i looked at buying a hand wired board for it but id rather keep it somewhat original...really nice sounding amp when its working right.

Thanks, Kurt

 

[FleshOnGear]

I have a 1975 50 watt jmp mk ll ....it is making a constant rumbling sound....sort of like a distant thunderstorm....I am not tech savvy so I took it to a tech who found a corroded tube socket, replaced that, but did not solve the problem....he had a look under the circuit board and didn't want to dig into it....any ideas on the problem would be much appreciated....i looked at buying a hand wired board for it but id rather keep it somewhat original...really nice sounding amp when its working right.

Thanks, Kurt

Hey Kurt. Welcome to the forum!

With a sound like that, the first thing I’d check is the tubes. Did your tech change the tubes? Other possibilities might be leaky coupling caps, maybe dying filter caps, maybe bad potentiometers (especially the presence pot). Do you have access to other techs? I can’t imagine any reason for a tech to say he doesn’t want to fix your amp, other than he’s simply not capable.

Good luck!

 

[Canada Marshall Fan]

I did swap out the tubes and that didn't change it.....I had the caps replaced about ten yrs ago....and the amp sounded real good....admittedly I hadn't fired it up for about 5 years and when I did that's the sound that emerged....you're right I need a new tech...I just think he didn't want to do the work personally but I could be wrong....I live in a small town in saskatchewan Canada and don't have access to many amp techs(if any)...stuff like replacing caps and all that, dang I'm just too chicken to even get in there for fear of making it worse....

 

[Purgasound]

That's shame. Those are the most simple amps to work on. I can't think of an easier one, maybe a Champ... Unbelievable. You don't need another board. Marshall didn't make the greatest PCB's back then but it's fine.

When the caps were changed out, were the 10uF bias capacitors included?

Start with simple troubleshooting.
1) Pull the first preamp tube. Did the noise stop?
Yes, then microphonic preamp tube or failed component on V1.
No, then put it back
2) Pull the second preamp tube
Yes, then microphonic preamp tube or failed component on V2.
No, then put it back
3) Pull the third
Yes, problem in the phase inverter but more likely it's the power section.
No, oh boy...

Check the voltages on all of the tube pins with a multimeter. Write down the information on a chart. V1, then each pin number and the corresponding voltage. Pin 2 on V1 won't really have anything on it. Pins1 and 6 will have anywhere from 170 VDC to 220 VDC. It's important to write it all down. Also check DC voltage before and after each coupling cap. There should be none present. DC in the signal path would indicate a failed coupling capacitor. If they are the lego type square brick caps you probably can't get a meter probe on them but you can still get to the other end as they will terminate on a volume or gain pot. If you don't feel comfortable poking around in there, then don't. At least pull the preamp tubes and check that. It could just be a bad tube. I wouldn't trust anything said or done by someone who couldn't fix one of these.

 

[Canada Marshall Fan]

I will give those things a try......thank you for taking the time to post that for me.

 

[slagg]

Time fir a cap job