JCM 2000 DSL no output

[DRH1958]

Alright I have my test rig set up. Now here is where my lack of skills comes in. Tracing the signal via the schematic, I can find generalized areas with a schematic however tracing the path of the signal is where I get lost staring at it. I have yet to master this. Any assistance would be great. This is the learning curve for me. Thanks

Start at the input and find the plate of the first tube. There will be a capacitor connected to the plate between the plate itself and the plate resistor. The cap is what couples the AC signal to the grid of the next stage. Follow the caps and you will generally be following the signal path. This is just general guidelines for common cathode stages, but the key is to follow the coupling capacitors, usually in the .01 to .1uF range. There might be other tone shaping components between, so sometimes I work from the next stage's grid and go backwards to meet up to where I got lost and meet in the middle. Those caps block the high voltage DC but pass the AC signal. With each stage going towards the power tubes, the signal should be stronger at the output of that stage. Where it is not, this is generally where the problem lies. You will encounter a loss after a tone stack, but keep going and it should be stronger after the next stage. This is by no means all there is but it should get you started.

[Jazz P Bass]

WTH!

121 posts?

[DrGonz78]

Once the posts get into the 50-60 range on a thread I am sure the odds of successful repair diminishes. There were multiple problems with this repair such as an intermittent heater supply, that was fixed. Of course you have to take a step back as Sweatyk made it clear that he is not a pro. Reading those schematics are tough if it the first amp you are going to trace. Many boards with separate pages for each schematic. One problem with many amps like this one is all those cheese ball connections that connect each board together, much like a B52 AT100 or something. So all the tubes have proper voltage and the power amp is working... Time to ask some very specific questions if you are stumped. Sometimes we all bite off more than we can chew, but still somehow we managed to tame the beast.

[sweatyk]

I haven't had a chance to get back on the computer since the other day. The wife has been hogging it for online classes. I'll post soon and attach the schematics for this. Very frustrating

[sweatyk]

Here is where I am with this. Injected the signal and got it at the input jack. Then I went to V1 at pin 7. The signal was there. I checked at pin 6 and it has a volume increase. I then went to R13 where one side had the signal but the other side did not. In the circuit it read 223K somewhat in tolerance. I checked at R19 and there’s no signal there. I’m flying a little blind here so tell me if I’m going it the right direction with this.

JCM2-60-02 (2001) iss7.pdf

[sweatyk]

The grids and plates on both V1 & 2 have signal. There is no such signal on either side of V3. On R39 it is rated at 100K ohms and reads 30K ohms in circuit. Is there a component connected to it the is giving this reading?

[The Dude]

The schematic you posted in #125 won't download for me. If anyone can post a working one, it would make this a lot easier. Without one, I've no idea what points in the circuit you're talking about. Sorry.

[Jazz P Bass]

Good luck tracing out a problem with the 'spaghetti schematic'.
All that Con this on JCM 60 goes to Con that on JCM62.

If you have no signal at R39, I would look at TR1 Jfet.
It may be On.

jcm2-60-02.zip

[sweatyk]

Hope this one comes thru.......

http://www.drtube.com/schematics/mar...%29%20iss7.pdf

[g1]

And here is the next board that Con7 & Con8 at V3 grid go to:
JCM2-61-00 (2001) iss5.pdf

This has probably been covered already, but what are DC voltages at V3 pin 6 & 8 ?

[sweatyk]

And here is the next board that Con7 & Con8 at V3 grid go to:
[ATTACH]34128[/ATTACH]

This has probably been covered already, but what are DC voltages at V3 pin 6 & 8 ?

V3 pin 6 = 245VDC
V3 pin 8 = 2 VDC

[g1]

That sounds reasonable.
How hard is it to get under the board? Maybe try disconnecting TR1 like Jazz mentioned, maybe TR3 also. Or lift one end of R41 & R43.

[sweatyk]

That sounds reasonable.
How hard is it to get under the board? Maybe try disconnecting TR1 like Jazz mentioned, maybe TR3 also. Or lift one end of R41 & R43.

So by lifting either side of R41 & R43 is to disengage TR1 and TR3 from the path correct? Should I do them at the same time or one by one? I can get under the board. I've had that thing out more than a dozen times.

[g1]

I'd do them both at once just to avoid extra flexing etc. If it cures the problem, I'd just replace both TR1 & TR3, as they are probably inexpensive.

[sweatyk]

I'd do them both at once just to avoid extra flexing etc. If it cures the problem, I'd just replace both TR1 & TR3, as they are probably inexpensive.

g1 I believe you have nailed it. I lifted R41 and R43 from the circuit and was able to get the signal loud and clear on V3 and V4. I looked up those jfets and it seems that those J174's are obsolete as far as digi key and mouser goes. Looks like Ebay here I come. I will post once I get them installed.