Marshall JMP 77 MV 100W amplifier HT issues

[henter2017]

Hi folks,

Im trying to fix an old Marshall JMP MV and getting trouble to find the problem with it, it was popping HT fuses, I have totally replaced all filter big can caps for new TADs, hooked it to the current bulb limiter and is fine when on standby but as soon as I go to play mode the bulb from the current limiter lights bright, I have as well checked the rectifier diodes and they tested fine...

(all tested were done without any power or preamp tubes on it and hooked to a resistive load)

Following Webers troubleshooting tips, it all indicates that it should be the filter caps, but they are all new...

[nsubulysses]

If the light lights bright with the current limiter on you should test power supply voltages to see what is off/low. Since it is current limited it is in a safe state, or if you're worried and have a variac turn down the wall voltage. You need high voltage on pins 3 and 4 of the power tubes and pins 1 and 6 of the preamp tubes. Since the amp is current limited the voltages will be lower than what they read on the schematic, but all voltages should still be relative to each other.

Also it is good practice to turn down the AC wall voltage when running an amp without tubes becauase with no current draw to pull down the supply voltage, the high voltages may exceed the maximum ratings of hte power supply capacitors.

Check polarity of all power supply caps you replaced because one may be installed backwards by accident.

[henter2017]

Thanks I have double check all caps and they look fine polarity wise.

I have took some voltages while on Play mode:

AC IN 8.88V
AC at standby switch 6.27V

DC HT in circuit 4V

EL34s PIN 3 3.92V
EL34S PIN 4 3.94V

ECC83S PIN 1 3.89V
ECC83S PIN6 3.85V

Voltages at power supply Caps

Lower left cap corner + leg = 3V

4 caps together:
top caps bridged +s= 4V
low caps bridged +s= 2V


Voltages on standy mode:

233V AC IN

ecc83s filaments 6.34V
el34s filaments 6.39V


Unfortunately I dont own a variac to lower the voltage while unloaded

[52 Bill]

Voltages on standy mode:

233V AC IN

ecc83s filaments 6.34V
el34s filaments 6.39V

What dc voltages do you get in standby mode? What happens if you pull the power tubes out and switch the standby off?

[henter2017]

What dc voltages do you get in standby mode? What happens if you pull the power tubes out and switch the standby off?

DC Voltages while on standby:

4 caps together:
left side top and bottom:
0.80V on the top ground pole.
0.80V on the yellow/red poles (bridged) and connected with the top ground pole.
0.0V on the rest of the contacts from the other 2 caps, as well as the bottom left of the chassis single 47uF cap.

Then I get on the negative side of the PCB mounted 10uF caps I have:

-54.3V on the left one.
- 43.6V on the right one.

then HT 0V too.
and 0V on the pin3 and 4 of the 34s as well as 1 and 6 of the ecc83s.


I have the power tubes pulled out already, when I take the standby off the bulb from the limitator goes totally bright.

[nsubulysses]

with DC voltages that low and all capacitors 100% confirmed oriented in the correct polarity, no tubes, and rectifier diodes confirmed to not be shorted, the PT or OT may be shorted.

You could disconnect the OT by lifting the center tap connection at the B+ node and turn the amp on with current limiter. If it does not light the lamp when standby is switched on and the power supply voltages are there then the OT is shorted. If it still lights the lamp then the PT may be shorted in the high voltage winding.

If you could post a schematic that would help. I don't really understand where your power supply voltages are being taken when it says 4 caps together, but I do understand tehy are very low. Maybe I am overlooking something, but a schematic would help

[henter2017]

Thanks a lot!

Is very close to a 2203:

http://ampspecs.com/wp-content/uploa...203-layout.jpg

I will try lifting the OT CT and report back thanks!

[henter2017]

Ok Folks,

I have lifted the OT CT to air, took off standby, the limiter bulb flashed bright and came back dimed immediately and stayed dimmed.
tried again, and no more brightness...
I guess we could consider this as a fried 70s marshall OT?

[52 Bill]

I guess we could consider this as a fried 70s marshall OT?

Probably, but now you should test the transformer primary windings with your ohm meter to see.

[nsubulysses]

Yes as 52 Bill is saying you should be able to test the OT from center tap or plate lead (pin 3) to chassis ground and get a 0 ohm reading if the OT is internally shorted

Also now with center tap of OT lifted and amp no longer lighting the lamp brightly, all of your other power supply voltages at pin 4 of the power tubes and pins 1 and 6 of the preamp tubes should be reading high voltage again. They will not read the full supply voltage since the amp is current limited but will still read a few hundred volts DC.

[The Dude]

Agree with the above. Also, look to see that someone didn't install flyback diodes in the thing. If so, they could be shorted.