Marshall 2205 JMC800 Reverb does not work

[frasen_36]

Hi, I am trying to repair an old Marshall JMC800 mod 2205 for my son. The reverb circuit does not work at all. All other functions seems to works OK. There is no signal through the reverb stage at all. I have tried with a spare reverb unit with same result. I have also tried with new tubes - same result. When signal in there is at V3B:7(grid) appr 0.1Vpp and at V3B:6(anode) appr. 1Vpp.
And at secondary side of the transformer T4 there is appr. 0.5Vpp feeding the reverb unit input. (Is that to low?)
Output signal from reverb unit in to V4B:7 (grid) is hardly readable.
V4B:6 (anode) is appr. 0.01Vpp (far to low..?).
The blocking transistor TR2 collector is loosend.
Resistors and capacitors around V3B and V4B are checked.
V3B:6(anode) is 420V DC and V4B:6(anode) is 200V DC.
DC resistance of transformer T4 primary side is 3500 ohm and secondary side is 300 ohm. (Is there a problem there ??)
The DC resistance of Reverb unit RV1 (Input) is 30 ohm and output is 200 ohm (same resistanse at both reverb units).
I appreciate any help or advise because I have got stucked. What have I missed?

[Enzo]

Isolate the problem, is it drive or recovery?

Turn the amp on and the reverb up part way. Now with the cables unplugged from the reverb pan, touch teh tip of each plug. One should make hum out the speaker. It is the return cable that should plug into the OUTPUT jack on the pan. If neither plug makes hum, your recovery circuit is not running.

likewise test the drive circuit. COnnect the drive cable to another amp. anything coming out the drive cable? Or scope it.

SInce the pan measures OK, it likely is. So connect the INPUT cable to the pan and have the amp trying to drive it. COnnect the OUTPUT of the pan to some other amp. Just plug a cord right from the pan out jack to the input of some amp. IS the pan being driven?

Once we know which half of the system has the problem, it will be easier to start tracking it down.

[Twist]

Measure the cathode voltages on those two sections. The plates (or Anodes) can have B+ but I would like to see what the cathodes have. This is probably a stupid question but have you tried reversing the RCA leads to the tray?

[frasen_36]

Thanks for your attention.
If I touch the tip of (red) cable that (was connected to out at Reverb unit) there is a strong hum.
At the tip of the other cable (black) that was connected to input at Rever unit I have maximum 0.5Vpp measured with a scope (with the cable disconnected from Reverb unit.)
If I reconnect the input cable (black) to input at reverb unit the signal goes down to appr. 0.05 Vpp. Sorry for my poor english: (Is pan the same as reverb unit?)
If I understand it correct it is the first half not working OK.
(If I scratch with a nail at the springs inside the reverb unit I can hear it in the loadspeaker, not so loudly but clear audible.)
V3B:8 (cathode) is 3.0V DC. V4B:8 (cathode) is 1.4V DC.
Yes I have tried to reversed the RCA cables without any changes.

[Enzo]

Reverb pan = reverb tank = reverb unit = spring delay unit all mean the same thing

OK, your recovery circuit works, the drive doesn't. V4B is the recovery tube, so it is fine.

The drive circuit is simple, V3B driving a transformer primary. 3v at the cathode means the tube is conducting at least. With signal to the amp, how much signal is there at the plate of the tube?

With power off and the cable to the reverb pan disconnected, measure resistance through the transformer secondary. It should be low. Could it be open? An open one would still show a little signal to a hi impedance thing like your scope or meter, but would not drive anything. In fact, pull the drive cable out of the INPUT jack on the reverb pan and measure resistance there at the plug. That will not only check the transformer for continuity, but also the cable as well.

COnnecting that secondary to another amp's input would verify if signal is there.

[52 Bill]

Is it possible that the tank is not original?

If the input transducer reads 30 ohms, that makes it a medium impedance coil. Most tube driven tanks use a low impedance coil that will normally read 1 or 2 ohms dc. That mismatch could be enough to kill the reverb.

What brand tank is it, and what are the code numbers?

[frasen_36]

Hi,
V3B:6 (anode) is appr. 1Vpp. The resistanse at transformer secondary side is is 300 Ohm (with reverb unit disconnected). Same resistance if I measure on the cable or direct on the transformer. I have no other unit to connect the scondary side to - but measured with scope there is 0,5Vpp at transformer secondary side. Is that really enough to drive the reverb unit input? (Looks very low to me..) Is 3500 Ohm a normal resistance at the transformer primary side?

I assume the tank is original. At least the reverb has been worked OK for years but suddenly stopped working according my son. It is marked ACCUTRONICS, CARY. IL. Pat 2.982.819 Pat.3.106.610 8DB2C1D.
At V3B:6(anode) there is 0.5-1Vpp with the tank connected and appr 2-3Vpp with the tank disconnected.

[Enzo]

You only have 1v p-p of signal at the plate of the drive tube? Seems really low to me. By the time the transformer steps it down, no wonder nothing is left. Either there is very low drive or the transformer is funky and loading it down.

Go over to RG Keens Geofex site adn check out his transformer test procedure. Apply it to the reverb drive transformer in your amp.

[frasen_36]

Hi, What do you think aboute the resistance readings of the transformer? Prim:3500 ohm and Sec:300 ohm? i sorry but I can't find RG Keens Geofex site? (I have tried with Google without results)

[frasen_36]

I found the RG Keens Geofex site after I had solved my PC-problem - (needed to reinstall my OP due mysterious hangings etc. ..)
Thanks for the advice it was a lot of interesting readings there also.
I build their transformer tester and tested the driver transformer. It seems to be OK according that test. According that site is also the resistanse of input and output of the Reverb Unit within normal readings. I am completly out of ideas. I must have missed some fundamenthal thing here. According my opinion there is low gain/output from V3B:6 (plate) appr 1Vpp. Input to V3B (out from Master Volume when on Max) point G is 0.2Vpp. I appreciate any help and advice.

[frasen_36]

Hi, I am still compleatly out of ideas. I must have missed something fundamenthal.
Question: If I have the output of the tank connected to V4B:7 (gate) and slightly scratch on the springs inside the tank shouldn't I hear that load and clear in the loadspeaker?
I can hear it but very, very weak. I have two idential tanks and both gives the same result

[Twist]

Measure with an ohm meter the output resistance of the reverb tray. I'm guessing it should be around 200 ohms. And did you take TR2 completely out of the circuit? That part number on the tray is a Marshall style. You should also be able to read about 50 ohms on the input side of the tray with an ohm meter.

[frasen_36]

Thanks for your patience. I measured 200 ohm at the output side and 30 ohm at input side. I only lifted the collector side of TR2. I double checked it again for sure and it is the collector. Question: Could the reverb be working poor even if input and output resistances seems correct ?

[Twist]

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.

[Enzo]

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.

[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.