Question about replacement 100 watt Marshall PT

[acorkos]

i'm having a low output issue with a 2203 JMP. this amp came to me with a dead PT. I bought the 100 watt marshall replacement PT from CE Distribution, and i'm not sure i have it hooked up correctly. there are 2 bias secondaries.. a white wire and a green wire. i connected the white wire to the bias circuit, and the green to ground. is this right?

all the voltages check out, 461 volts dc on the power tube plates, 458 on the screens and -50 on the grids. bias current is 50mA.

a couple board components look to have been changed previously....so i'm not sure of the quality of the previous repair.

i did find 3 bad 100k plate resistors...the ones on pins 1 and 6 of V1, and the 100K in the PI. two were open, and one was giving erratic readings.

the wire from the 68K input resistor to pin 7 of V1 is shielded, but the shield is connected to pin 6 and its 190 volts. is this normal?

the voltages tell me this thing should be cranking, but the output is still low. one other thing.....there is 11vdc on all lugs of the presence pot. is this normal?

i'm thinking i need to get to the underside of the PCB and check/resolder every joint. maybe swap out a few of the coupling caps in the preamp. any other suggestions?

thanks

[Amp Kat]

the wire from the 68K input resistor to pin 7 of V1 is shielded, but the shield is connected to pin 6 and its 190 volts. is this normal?

No that definitely is not right. You have the shield ground going to the plate voltage or maybe a direct coupled stage but still not right as that would kill the signal. It should be grounded to the closest point to the jack and as short as possible.

[acorkos]

No that definitely is not right. You have the shield ground going to the plate voltage...

i disconnected the shield where it connects to pin 6.....now it's noiser (hissy). there must be something incorrectly wired somewhere else causing the hiss in the first place (scratches head...)

[Amp Kat]

Did you reconnect the shiel to ground or just disconnect it. If you don't tie it back to ground (chassis) you are creating a ground loop. I would get a schematic and go over the whole thing and make sure it's wired correctly. I don't think this amp has loop jacks so it's more difficult to isolate but I can't imagine why that was wired like that. Where did you get this amp ? What is that a master lead 100W ?

[acorkos]

2203 master mkII 100 watter. when i ground the shield on the input wire and turn up the volume controls, i get a high pitched squeal....something's oscillating. i need to disconnect the wires on the tube sockets and flip this board over to have a look. thanks

[acorkos]

i solved it today....here's the final score:

4 x 6550 power tubes shot
a bad power transformer
3 bad 100k plate resistors
3 leaking preamp coupling caps
both input jacks wired incorrectly (sorta master volume/sorta 4-input wiring, instead of correct master volume wiring - designed to bypass a gain stage when using the low gain input)

and the shielded input wire w/shield connected to plate voltage was actually necessary to keep the oscillation squeal under control....
http://music-electronics-forum.com/showthread.php?t=937

BTW....this thing sounds great, but it is very loud

[1971SuperLead]

My 79 JMP 50 watter used to squeel with nothing in the input and just cranking the volume all the way up. it turned out to be the effects loop ( a simple mod) jacks were poorly insulated and arcing to the ground ( the back panel ) at high voumes. The tech added some more insulation to the jacks and it's been good since.

Good luck