Early-mid 70s marshall jmp mkII 50w

[JMMP]

Need some help with this one, it's a 4 input 50 watt non master head. It's been mucked with before i got it and so I'm restoring it to original. Anyone know where i can find a schematic and or component layout layout for this amp. Found some that are close but not exactly this amp. Thanks in advance.

[Enzo]

Look at the top of the page, in the bar that says FORMS BLOGS ARTICLES...

Click on the one : Schematic Hell, and look through the MArshalls there.

[Jon Snell]

Could it be this one perhaps?

[Jon Snell]

Try again ... jmp_pa_50w_1963.pdf

[g1]

Is it this one: https://drtube.com/schematics/marshall/1987mk2u.gif
If not, what is different (other than things that may have been 'mucked' with)

[JMMP]

Ok thank you folks, got it back to original, had to jumper a couple traces as they lifted or were damaged. Making sound but low output (tubes are a test set that i have, all good). Am finding 25ish vdc on both grids of the phase inverter. Remove tube and it's gone, replace with new tube, comes back. I'm not finding the source and am baffled, I'm sure that this voltage is swamping out the PI, anyone ever see this or have any ideas?

[g1]

PI grids can be hard to measure. If the cathode is the proper voltage you can ignore the grid. If the grid (voltage) has a problem, it will show at the cathode.

[JMMP]

Don't recall ever seeing this b4. Looking at the marshall tube chart, its showing v3 PI:
1_ 210v 2_ 0v 3_ 40v 6_ 200v 7_ 0v 8_ 40v

I'm reading:
1_ 170v 2_ 24v 3_ 44v 6_ 159v 7_ 26v 8_ 44v

from what I gather any VDC on grid = bad, why is PI any different? Forgive me if I'm wrong on that.

[Helmholtz]

The easiest way to determine grid to ground voltage in an LTPI is to measure the voltage at the bottom of the cathode bias resistor. Grid to cathode voltage (bias) is the voltage drop across the cathode bias resistor. This method works because there is no voltage drop across the floating grid leak resistors (no grid current).
But if you connect your meter to the grid, the meter's internal resistance will cause a current through grid leak which drops the grid voltage.

[JMMP]

Sure enough, measured this way grid v non existent. Now to find out why all plate v's off. V1a 1=107 2=0 3=.8,
6=86 (chart says 208) 7=0 8=.75
I'm assuming chart is suggested and varies with tube, V2b at 86v probably affecting power output. Thoughts?

[Helmholtz]

Don't recall ever seeing this b4. Looking at the marshall tube chart, its showing v3 PI:
1_ 210v 2_ 0v 3_ 40v 6_ 200v 7_ 0v 8_ 40v

I'm reading:
1_ 170v 2_ 24v 3_ 44v 6_ 159v 7_ 26v 8_ 44v

from what I gather any VDC on grid = bad, why is PI any different? Forgive me if I'm wrong on that.

A "-" or "+" in the voltage chart doesn't mean 0V, only that it's not specified. PI grids must not sit at 0VDC.