JCM 2000 DSL no output

[g1]

I don't think the amp could possibly work in any manner with those kind of voltages.
Maybe circuit ground is not connected to where you have your black probe?
Can you measure DC across (one probe to each end) R65, then R69 ?

[sweatyk]

I don't think the amp could possibly work in any manner with those kind of voltages.
Maybe circuit ground is not connected to where you have your black probe?
Can you measure DC across (one probe to each end) R65, then R69 ?

I double checked the ground and it was rock solid.

across R65 = 0V
across R69 = .002V

I have doubled checked these to make sure.

[TimmyP1955]

I checked V1 thru 4 in a known working amp. All tubes are good.

Maybe they are. I recently had a problem with a 6505. All of the tubes tested OK. One of them worked fine in one position, was a little down in gain in another position, and non-functional in another position. This is where a scope comes in handy.

[The Dude]

Presumably, you have B+ or the power amp wouldn't work correctly. This makes no sense??????? The "topside" of R69 is where B+ comes from, yet you say there's nothing there? Did you maybe blow the HT fuse while measuring or something of that sort?

[sweatyk]

Presumably, you have B+ or the power amp wouldn't work correctly. This makes no sense??????? The "topside" of R69 is where B+ comes from, yet you say there's nothing there? Did you maybe blow the HT fuse while measuring or something of that sort?

I'm going to give it a rest tonight and come back to it tomorrow going over the last couple of posts and get those voltage readings again. I just verified the power amp again. Plugging directly into the "return" on the loop. Plenty clean and fair level of volume. I will ground it at a different spot this time even though I checked it out before. I was careful not to cross anything and the HT fuse is good. R69 was a bit of a challenge to get my test leads in there given it's location slightly tucked under another PCB but I managed.

[g1]

Have you verified that your meter is good on the DC range?

[sweatyk]

Yes the meter is functioning properly however I own 2 meters so tomorrow I will use the other one. Thanks for suggesting that.

[sweatyk]

OK retested with different meter. Use different ground location than last. Readings are relative to ground.
R9 = -.1mv, -.1mv
R20 = 0V, jumped around from 2mv to .1mv
R21 = 0v, 0v
R45 = voltage jumping around on either side
R46 - .71V, 26.1V
R51 = voltage jumping around on either side
R52 = 1.35V, 19.24V

Measuring across:
R65 = Started at -32mv and dropped to near 0
R69 = Started at high number and dropped to -1.1mv

The numbers were jumping around so quickly it was difficult to determine a start or ending reading.

Here are the conditions:
amp plugged directly into the wall outlet
both power and standby switches in the "on" position
All power tubes removed
connected to speaker (probably unnecessary but can't hurt)
solid connection to a ground source
previously tested the power section by plugging into the ex loop return getting decent level of volume ( I can video tape it and post it here for proof if you deem it necessary)

Please tell me if I am testing this incorrectly. I may not be the brightest tech but I have enough basic skills to perform these simple voltage readings.

[g1]

Find the ground for the main filter caps and use that for your black probe connection. (Negative end of C27 or C31).

[sweatyk]

If I cannot connect to the negative side of C31 because of where it is on the pcb could I use the one side of R40? Would that bring the same results? It does have continuity to ground from there. Or are you thinking there is a break around C31?

[g1]

Are you using the schematic from post #2? If not can you post the one you are using?
R40 in the schematic I have does not go to ground. It is in the phase splitter circuit.
Rather than C31, you could use the end of R65 that connects to C31 negative end.

[sweatyk]

What I have found is that there are several schematic versions for this model amp depending on what year it was made. It took me a while to find the one where the values matched compared to what I have. The number that is stamped on the main board is JCM2-60-00. However the schematic for that one does not match what is actually on the board. I found that the one for JCM2-60-02 matches on some things. This has got to be one of the most confusing models for schematics that I have ever seen. It has given me a whole new level of dislike for Marshalls from a tech point of view. It’s looking like I need to step back and see what I can find.

[sweatyk]

I am truly at a loss for the right schematic.....................

I guess I can contact U S Music Corp this week and see if they can help me get the correct one.

[g1]

Ok, that's a lot better. At least we can talk about the same components now .
The issue5 is also close to the issue7, but they are both quite different from the issue1 from post #2.
Issue5 & 7 are both here: Marshall Schematics.
If you can look at them both and see which is closest, we can probably work from those.
One side of R73 should have continuity to W8 and W12. Try using that side of R73 for your black probe.
As the amp is somewhat working, there must be DC at the main filter caps. You have tried another meter, so it is likely some kind of measurement issue. Once you can get some voltage readings in the supply progress can be made.

[sweatyk]

It appears that issue7 is the one. And yes there is continuity from one side of R73 to W8 & W12. So where do we go from here?