Marshall JCM 800 Lead series 2205, help please!

[Appaloosa]

Problem:
I have a 2205 that has no output. I've done some of the really simple stuff like: checked fuses/cables/out/in jacks, replaced bad preamp tube socket, checked all resistors (hopefully got'm all), and replaced all tubes. And replaced filter caps. All of the heaters glow on all tubes and every tube has AC voltage to each pin exept pin 7 of the reverb tube (reverb un-pluged).

Question:
Could I have a bad output transformer? if so what is the best way to check if one is faulty. I checked for continueity from primary to secondary leads and didn't find any.

Any help or advice in my newbie trouble shooting would be greatly appreciated! thank you!

[pontiacpete]

i think you should check the continuity betw the secondary leads and then betw the primary leads.
There a transformer test here : GEO's Effects Project Articles
Did you check for high DC voltage on pins 3 of the el34s?

[Enzo]

Is there signal on that DI output jack?

How about that impedance selector switch as a large suspect? You wanna check continuity? Turn the amp off, plug a cord into the speaker out jack, and measure resuatance from tip to sleeve at the free end of that cord. You are measuring continuity through the cord, the jack, the selector swutch, the internal wiring, and of course the output transformer secondary. If you don't see continuity, the circuit is open. Any one of those parts could be bad, but the transformer is the LEAST likely failure.

Every pin of every tube has AC voltage? What on earth does that mean? Your power tubes should have about 480v of DC on pins 3 and 4 of each socket, and something like -40v of DC on pin 5 of each.

[fireman]

take care working with the scematic as these amps had quite a few changes over the production run. I have a 4210 (switching 50w combo) and the valves are not connected as the sematic shows so double check everything before you start.

[Appaloosa]

Thanks for all the help and tips!

My main problem was that I overlooked a bad connection at C43 and the PC board. (can't believe i missed that!)

I now have the amp running with 460V DC at pins 3 and 4, and -32 V DC at pin 5 on both of the output tubes. I hope this is within acceptable tolerance?

New Problem:
Now the amp seems to function almost like it should without undue noise and hum, but the overall output volume seems to be weak, and the Master Volume Pot. (VR10) seems to act as a Thin/Thick tone changer when rotated and has little effect on the output volume.

So now I assume I have a preamp problem, but where should I begin? I replaced any questionable pots with new ones, and replaced all tubes. Any tips are greatly appreciated, and thanks again for all the help!

[mikeboone]

You should clean the MV pot with some Deoxit or similar cleaner before replacing anymore parts. Its a good suspect in this case and a cheap victory if you strike paydirt.

[Appaloosa]

Wrong pot value?

I will clean it again, thanks. I think I replaced this pot with new one though. Would putting a pot of the wrong resistance in make it act like a EQ?

I have a 1 M audio pot in the master vol. spot (VR 10) could this be the wrong pot.? I know their may be some variations on the schematics from different model years?

thanks again

[Chuck H]

Did this problem start after you installed the pot?

Chuck

[Appaloosa]

I don't know if this problem started after I installed the new pot cause the old one had broken leads. and I just replaced it on visual inspection.

Is their something in the preamp I should look for that may cause the MV pot to just alter the tone?

Could a bad transistor or IC cause a problem like this? I think I have checked all resistors and diodes.

Any advice on troubleshooting the preamp would be great!

[micri5446]

broken leads on a pot...hmmm maube check for cracks on the pcb by the broken pot
peace.....................mike

[azryk]

change the cap c20

[Appaloosa]

Thanks for your tips. The problem with the amp now is that the clean channel seems too dirty to me, and it also seems to be about %15 louder than the overdrive channel. all functions on the amp seem to work reverb, tone controls, etc. and the amp is not overly noisy. It just doesn't sound like it should. Also overall volume seems to be somewhat weak. I went through and checked the values of all the resistors and they seem to be Ok. I checked the voltage on all the tubes and I seem to think I have too high of a plate voltage on V3 pin #6 - I'm getting around 454 V DC, and i have tried different tubes. I'm also noticing 198 V on V4 pin #1 too high? I'm new to trouble shooting amps, what should I look for for tracking down this high voltage? could a bad/failing choke cause this? Or could I have a leaking decoupling cap on V3?


I have attached a voltage chart with highlighted areas that seem to be out of range to me. any help is greatly appreiciated. thank you

ps. I changed tubes and got 6 volts on pin 8 of V3 as well. could this point to a leaking cap? thanks

[Jazz P Bass]

I don't get tube V1.
Pin 6 & Pin 8 are not correct.
Pin 1 at 67 volts is way low.
Pijn 8 at zero volts means the tube is not conducting.
I would suggest testing voltages right at the tube socket pins were the wires are soldered on.
Also check pin 1's plate resitor& that you have continuity between the plate resistor & pin 1.
THe same goes for the cathode resistor. Pin 8.

[Appaloosa]

thanks for your tips. what do you me "you don't get V1" are my reading way off? I did check voltages with amp revmoved and at the solder connections of the sockets. thanks I will check out what you suggested.

[Appaloosa]

I finally got back to working on this amp again.

I went and powered it up and plugged an SG into it and for some reason it seamed to sound good, it had what seamed to be reasonable volume and tone.

But thats were to good news ended. after about 5 min. of playing the amp on the dirty channel the volume seemed to drop by about 30 %, and the tone seemed to suffer too.

the clean channel seemed to remain at its "louder" level though.

I read somewhere that a failing choke could cause lowered output volume is this true? and how would I test my choke?

Does any one have any hints of what my problem would be leading too?

I seemed to have funny tube pin voltage values as well, but I can't seem to find the cause. all resistors seem to be in good order.

Any trouble shooting help is greatly appreciated, thank you.