marshall jcm 2000 dsl 100 phase splitter

[sebi]

Hi,
I have a dsl 100, the power amp board saying jcm2-60-00. It is the circuit were C28 is feeding the audio into the pi.
OK, so far, now the problem: the first half of the phase splitter does not work, I read 390 V at the plate, I have a voltage drop of only 2 or so volts over the 82k res. I have 25V on the grid and 28 at the kathode. I replaced c28, the grid resistor, all other resistors measure fine and I have continuity to the tube socket where there should be. The second half of the tube below the 100k works fine (ok, no signal because of first half :-), but voltages are good. Of course I tried different tubes. With no tube I also have no voltages at the socket at the grid and kathode, so I assume the socket makes contact to the tube and the socket is not conductive, what could maybe cause a problem like this. The tube does not want to draw current, I don't know why.
Anyone an idea ??

[Enzo]

Um... if the voltages on the right triode are good, then you ought to have the same voltage on both cathodes, pins 3 and 8. is there zero ohms between pins 3 and 8? And do both cathodes have the same 28v?

I don;t assume the socket is conductive, I want to verify that. For example I have seen tube socket pins broken off inside the socket. The female portion grips the tube pin firmly, the bottom end is soldered well, but the pin itself is broken in two so the soldered end is not touching the female end. This should be tested by pulling hte tube and checking continuity from the female side of the pin up top and the solder underneath. Or put a tube part way in and measure to the exposed tube leg.

These pc mount sockets have the little slots all around exposing the socket pins like little test points. Verify continuity from the exposed outside vanes in the slots to the female pins in the socket holes. The verify continuity from those side vanes to the solder on the other side. A leg could be broken off on the top side but the remaining part stocking through the board would remain soldered.

Power down, pull the tube, measure resistance to ground from pins 2,3,7,8. Looking for something like 15,170 ohms to either cathode pin, and about 1.0147 meg to the grids. The value is not critical, any reading more or less similar to those is OK.

With no tube is there continuity between pins 4 and 5 of the socket? Do both heaters glow?

[Enzo]

To save anyone the trouble, here is the schematic for that section.

[sebi]

Hi,
I had the socket out and it looked and measured good. Also ohmwise like you said I already checked ok, I'll have to check if both heaters glow. I never thought about... we have 3 heater conn. f f & fm. Is one f to fm one triode, the other f to fm the other? So maybe only one kathode gets heated. Never had a problem like this to date. Thank you for your quick reply.
Sebi

[Albert Kreuzer]

Is one f to fm one triode, the other f to fm the other?

Yes.

HTH
Albert

[sebi]

marshall problem fixed

Hi,
so the problem is fixed. it was the connection between pcb and socket to one of the heaters. the trace is on one side and the solder on the other, but the contact through the board was bad, so taking the socket out and soldering back in didn't solve.
thank you for the tip with the heater, that put me on the right track.