Marshall 2205 JMC800 Reverb does not work

[frasen_36]

Hi, GREAT!! Thanks to your advise I have probably found somthing!! The Reverb pot seems to have interruption in the hot end. I cant understand how I could have missed that. That area around transistor TR2 was the first I suspected. I thought Ive measuer all components there. I have not disconnected it yet. It is late around 2 in the night here in Sweden - need to get some sleep before work tomorrow. I came back and let you know. Thanks.

[frasen_36]

Have you tested the cable to and from the pan?

Disconnect the two cables at both ends and check them for both contiuity through teh cable AND for short between inner conductor and shield.

Yes I did checked the cables and they were OK. Thanks.

[frasen_36]

It would only not work if there was something mechanically wrong with the springs. If you have disconnected the collector of TR2 then you should be able to turn the reverb level all the way up and read 100K across the two outside post of that pot.

Hi, GREAT!! Thanks to your advise I have probably found somthing!! The Reverb pot seems to have interruption in the hot end. I cant understand how I could have missed that. That area around transistor TR2 was the first I suspected. I thought Ive measuer all components there. I have not disconnected it yet. It is late around 2 in the night here in Sweden - need to get some sleep before work tomorrow. I came back and let you know. Thanks.

[frasen_36]

Yes, it was the Reverb pot that caused the whole thing. Thank you very much to lead me back in to the right way again. Afterwards it seems so easy and fundamenthal but now I understand why I missed it and was out of ideas.. When I tipped with my finger at the output cable from the tank I heard a load and clear hum in the loadspeaker. That's why I assumed the receiver stage V4b and signal path from it through the Reverb pot to PA-stage was correct. Tonight I see that this hum-signal does NOT go this way at all!! I disconnected C29, R31 and C25. Still the hum from my finger at the mid leader at Reverb Output tank cable can be seen at V5:1 and 6 (anodes) and sounds load and clear in the speaker. Somewhat mysterious - or?? I think it comes through capacitive coupling some way. There are no shielding caps on the tubes in this amplifier at all - I don't know if it should be..? Or if the hum signal is feeded via earth or the anode voltage paths?? I have stopped that investigation - even if I am little curious...
Anyhow I have replaced the Reverb pot temporary with a resistor and now it works perfektly. Remaining job is to find an 100k pot that fits mecanically... I have rechecked all signal levels I have posted earlier here and they are the same. So they are correct and valid for this amplifier even if they seemed very low at some positions. Thank you all for your great support when I was "blocked".

[frasen_36]

Hi again,
Just an additional question:
How is the instruction to check the setting of the biasing pot RV1 ?

[pdf64]

There is a lot of info around on how to set bias, see
http://www.duncanamps.com/technical/lvbias.html
or
http://www.fenderforum.com/forum.htm..._number=658009
Peter

[frasen_36]

Thanks for the biasing advice - I have checked and it was almost perfect just a slightly adjustment needed.

I need to correct myself for all signal-level readings I have posted earlier.
The scope I used had a very, very good hidden attenuator selector at the input channel. All measures I made(and posted) were 1/10 of the real level.
I missed to do a calibration before start measuring - clumsy me.
The correct signal-levels were:
V3B:7 (screen) 1.0Vpp
V3B:6 (plate) 10Vpp
Tank input: 5Vpp
V4B:6(plate) 0.1Vpp

Anyhow now it works thanks to all support I got here.