JCM 2000 Super Lead Bias Issue

[plexi50]

I have pretty good experience with these amps. They never fail to surprise me with odd part values that vary between different years and revisions made. I believe i have the correct schematic for this one. This amp has followed the schematic perfectly until i got to the bias resistor. It is factory installed and not done later. The schematic says 68K for the bias resistor. It reads 22K out of circuit. The color coding on these new resistos throw me off and so i need to ask. Going by the pic i have provided is this resistor 22k?

The amp has no bias and the EL34 tube plates look like they got pretty hot.  The 220K coupling resisitors are fine and read 220K. The two 47uf bias caps will be replaced even though the read good on an analog meter

The last thing to check right now is the small bias trimmer pots on the panel. Im going to laugh if it is one of them. Seeing as the trimmer pots work against each other in setting the bias

I dont mind working on these amps but i still hate them for the way they were designed. Customers laugh when you tell them that they have a RATS Nest for an amp. They dont understand. Come to think of it niether do i

[sluckey]

Going by the pic i have provided is this resistor 22k?

yes

[plexi50]

Well theres no easy answer to the bias resistor value difference from the schematic but to assume it may be one of those amps that got some wrong values loaded when they were made. I cant say thats the case here. I cant assume anything and do my job right. Should i replace it with the same 22K value? The trimmer pots are each reading from zero to 9K.

Last thing i can think of is to change the coulping caps. When i first got the amp in i turned it on and powered up. Immediate LOUD HUMM!!! Shut down and pulled all the power tubes. Probed bias point and found none at all. This is where im at now. Doing this-

MY MISTAKE. Bias resistor is 33K. I was looking at the part number on the board-

[sluckey]

Nothing wrong with that resistor. Put it back in.

[Ritchie200]

No bias, but it hums?  That's odd..  Output tx?  Any untold history from the owner?

Jim

[plexi50]

No history of problems with this amp. I put in new bias caps and 22K resistor. Am going to fire it up again without the tubes and see if i have bias in 30 minutes. The hum was with all tubes in the amp. I shut it down and pulled all the tubes. I should have started pulling preamp tubes but with no bias i knew i had to get to that issue first

[plexi50]

Where is the bias voltage supply on this schematic?

[sluckey]

page 2, lower right corner. D6, D7, D8, D9.

[plexi50]

Yes i found it. The bias supply at D9 is -56VDC. So it may look like the bias supply voltage is not getting where it needs to go. No grid voltage (Zero) on pin #5

[sluckey]

There are several connectors between those diodes and the tube grids.

Doug has a clearer schematic, at least it's easier for me to read...
http://www.el34world.com/charts/Schematics/files/marshall/jcm2000_tsl100_100w.pdf

[plexi50]

Yeah i looked at that schematic and its much clearer.  Its going to be one of those days. D7 & D8 show -21.50 VDC across from the anode band. D6 shows 1 volt and D9 shows -26.50 VDC at the anode and -56.31 on the other side of this diode

I had to pull the power supply earlier to lift the board but those connections are back on and tight now

This is a clear schematic and is the one that is following most of my part values. The bias voltage is there on the diodes where they should be. This schematic shows D7-D8-D9-D10 and is the one im using. The bias trimmers do respond when test point is at  D9 & D10. Both those diodes show -56VDC. But no voltage on pin #5 at all.

[jojokeo]

can you adjust the bias pots? I put it at 39mA for bias but you'd want 39mV if doing it w/out a bias meter. Check all of the solder joints carefully on all of those components. That's usually the issue, not the values. You'd see burnt parts if so which would be obvious to see.

[sluckey]

can you adjust the bias pots?

He has ZERO volts on the grids at this point.

[plexi50]

I dont remember if this amp will show grid voltage with the standby off. Shouldnt it? The bias voltage is there on the diodes so i would think i could get a reading at the grid without full on

BREAKING NEWS! R77 is a 10k. Lifted one end off the board. It reads 3.90K. Could this be the PITA?

[jojokeo]

I think you should have it on with tubes running to test and set but then I don't have the shop manual either. That's just how I've sucessfully done recently.

[sluckey]

I don't think you are looking for a resistor that has changed value. I think you're looking for an open circuit such as an open resistor, bad connector pin, broken pcb trace, etc., or maybe a shorted bias filter cap.

*** DON'T PUT THE TUBES IN UNTIL YOU CAN MEASURE APPROX -50VDC ON PIN 5 OF EACH OUTPUT TUBE.***

I doubt that the STBY switch needs to be on for this troubleshooting, but go ahead and turn it on. With tubes out, you ain't gonna hurt anything, except maybe yourself. Measure the bias voltage on each side of R77 (with both ends of R77 in circuit). If you measure the same thing on both sides, then there is an open circuit somewhere around CON1, pin 1. I have not seen the mating connector for CON1 so I don't know what the missing circuit looks like. But for sure, the bias voltage goes thru CON1 pin 1 to some circuitry, then returns to one set of tube grids thru CON1 pin 2, and to the other set of tube grids thru CON1 pin 6. Do you have bias voltage on CON1 pins 2 and 6? If not, find out why.

[plexi50]

Yes i am looking for an open resistor or bad cap. Con 1 is hiding on the schematic and so far i cant find it on the power board. There are other Con 1 plugs on the other 2 boards. Im tracing from that 10K now on to C42 47uf.

R77 10K is showing -26.70VDC in and -56.05VDC  out and on to C42. I have to remove the board again to get to C42 and remove it to check it out

Con 1 is the ribbon cable that goes from the power board and plugs into the bias trimmer board with the 2 pots on it

I'll remove it and check all the terminal connection points. Im hateing this amp

[jojokeo]

is page 5 the "con 1" you're looking for?

[plexi50]

Con 1 is just to the left of the EL34 power tubes. Man you can go blind and crazy with this schematic. It's all right in front of me but still it makes you work to find it. I have checked the board and traced every thing and part from R77 10k and through the 47uf cap. I have traced and confirmed Con 1 ribbon cable is connected and going through all bias,grid,coupling caps parts

I have found 7 resistors on the power board that were very barely soldered to the board

I have removed and checked the trimmer pot board again and all pins are connecting to thier traces and through the trimmer pots themselves

Im not sure i trust these tiny pots. My max reading is 8+K. At no time before have i been able to get any grid voltage even when rotating the trimmer pots back and forth. I thought they might be wore out or have a dead spot on them but not to be

Im putting the board back in the amp now. Better dam work now! 

[plexi50]

We have Grid voltage Houston! The amp is on and i am biasing in a few seconds /

[punkykatt]

Here are the bias pots for this amp if you still need it

[plexi50]

Thanks punkykatt. I can always use diagrams. I have the amp with EL34 tubes biased at -82 mv

Clean channel sounds wonderful / Dirty channel has loud hum. Thats what i heard this morning when i flipped the amp on and then off

I found a cracked trace that was pretty evident before scraping away the green coating and soldering a small wire across it

This is the connection to pin#1 for R77 10K. Customer said the volume would come and go and get real distorted

Now i have another problem to solve with the dirty channel. The hum is so loud i am looking at PS caps before i do anything else

Maybe be a HV cap for that node of the amplifier. The clean channel is stone quiet and sounds great