Marshall JCM2000 Sluggish / Low Volume

[jagdpanther]

New to the Forum

I have been reading and searching thru what I could find in this forum to repair my amp.
JCM2000 DSL 100w year: 1999
Everything works except it has low volume.
I have gone thru and cleaned contacts and replace pre-amp tubes with known good ones. The bias seems to be stable and have not encountered the thermal issue I have read about.
I have also updated and replaced Resistors (7,10,66,70) from 220k to 5.6k .5 watt resistors. Although with that said R63 does show sign of heat but still checks out with the proper resistance. Caps physically look fine , with out visible damage. I am having trouble getting voltage readings. PIN 3&4 on the power tubes.
What would the V be at connections W9&10 from the PT? That would be the start of the voltage to the main board?

Next step that I will do is pull the main board and touch up all the solder points. I have read these amps are notorious for weak solders?

Please be Gentle, I am a real newbie

[jamesmafyew]

Which speaker jack are you using? If you are using the 8/4-ohm jacks, you need to be aware of the fact that the 16-ohm jack is a switching jack that opens the ground line when it is in use. This is to disable the 8/4 jacks when you're plugged into the 16 jack. Problem is, the contacts have a lot of current through them, heat up, and go high-resistance. There are threads on here about this issue.

If you're using the 8/4 jacks, try a cab in the 16-ohm jack and see if it changes anything. If it does, you need to jumper the ground pins on the 16-ohm jack.

W9 and 10 are the heater wiring. You should see 6.5VAC or so across them with no tubes in the amp.

With no tubes in the amp, here's what you should read (relative to ground) on the power tube sockets with the amp fully on and out of standby:
1 - 0V
2 - 3.4VAC
3 - 490-500V
4 - same as 3 give or take
5 - -35 to -45V or so
6 - 0V
7 - 3.4VAC
8 - 0V

[jagdpanther]

James,
Thanks for the reply and info.
I will check these items

I did try running a speaker thru each out put without any change, but I will double check the grounds. I did use contact cleaner on them as well.
Did try running a guitar into the effects return without change. As well as runnin a guitat in front and sending the signal from the effects send to another amp. Seemed to be stronger with that . That is why I am thinking it is in the power section?
I have been reading thru existing threads that I could find for the JCM before I posted. I appreciate the time anyone can spare to help.

Update:
Pin Tube 1 Tube 2 Tube 3 Tube 4
1 0 0 0 0
2 3.6 V AC 3.6V AC 3.6V AV 3.6V AC
3 509V 509V 509V 509V
4 509V 509V 509V 509V
5 -41.7V -41.7V -42.1V -42.1V
6 This pin shows low voltage but continues to rise in value
7 3.6 V AC 3.6V AC 3.6V AV 3.6V AC
8 0 0 0 0

Power on W9 & W10 reads 3.4V AC

[jamesmafyew]

Yes, correct about the FX loop -- if the signal is strong coming out of the send, but plugging a guitar or a preamp into the return is still problematic, then the issue is most likely in the power amp or the power supply.

Have you tried a different set of known-good power tubes? V4 is also after the FX loop.

Check R6 and R9 as well, although if they were bad you would get meaningless bias readings at the test points.

Check the 1K/5W screen resistors to see if some are open or have gone high-resistance.

Finally, re-check your own solder work on the 5k6 grid stoppers.

[jagdpanther]

Please see above for voltages.

I will check resistors again. I checked before I re-assembled everything.
Unfortunately, I am ashamed to say I have not swapped power tubes. 6L6's I have, Need to find EL34's unless 6L6's will work for root causing the power tubes.

[jamesmafyew]

No, don't try to sub any other power tubes in place of EL34s. That will just muddy the waters. You need to try another set of EL34s.

In the meantime, just to eliminate it as a possible cause, measure the resistance of the output transformer primaries (amp powered off, unplugged, and all caps discharged!). Disconnect the wires from the transformer before you do this. Make sure that each half measures roughly the same. It should be about 16-17 ohms from center tap to each end, about 34 from end to end. The secondary should measure close to zero ohms, and you should get an open circuit (infinite resistance) between the primaries and secondaries and between either winding and the chassis.

[jagdpanther]

Checked the OT today. Attached is a pic (not my current OT) Marked up with the ohms. It showed ope fro LH side to RH side and very continuity between the posts on the RH side.

R78 and R8 are reading 50ohm and I believe they should be 100ohm.

Also touched up a lot of solder joints on the main board and re-checked resistors. Also went and got some new EL34's and plugged in and set the bias at 75.5 mV per side.

Plugged in and tested. Volume was low and then came up to what I think is a normal level. Turned up half way and then beyond it started to cut in and out. bring it back down to about 4 on the dials cleaned up but the harder the pick attack it try and cut out. Cleaned all the jacks and pots in the front with deoxit and tried running the cab in the 16ohm output with the same results.

Ran the guitar back into the effects return not crackling put not much volume. have tried different pre-amp tubes as well.

If I took the OT values correctly and they look as they should. I am thinking it still may be a bad solder joint on mainboard ?

Thanks for the your time

[jamesmafyew]

Well that picture is of a Mercury OT I think -- is that what you have in the amp, or are you using the original Dagnall OT? The stock OT has a different arrangement of the connections. That being said, if the readings you're showing are correct, one half of the OT primary may have a partial/intermittent short. Each half should measure basically the same (not exactly the same, but close) to the center tap, and then across the ends should be the sum of the two readings. 24 is the sum of 8 and 16, so that's as it should be, it's the 8 vs 16 that concerns me. It should be about 16 or 17 on both halves, and all the way across should measure 32-34 ohms. It's not normal for one half to be half the resistance of the other.

HOWEVER, this does not prove that the OT is bad. Do you have access to a scope? It would help to be able to see the output waveform, and whether it is symmetrical. That would tell us a lot here. With a test sine-wave input, we want to see a symmetrical output wave that looks like the input, just bigger. If the wave is asymmetrical, one half of that wave is bigger than the other, that can indicate a partial short of one half of the OT primary -- especially if the signal on the power tube grids IS symmetrical.

R78 and R8 are reading 50 ohms because the heater winding they're connected across has an almost-zero resistance. That means that measuring across either resistor in circuit, you're actually measuring both of them in parallel. They're fine, so don't worry about them. If they were bad, you would get hum or maybe a blown heater fuse, not low volume.

There may be more than one problem here. A guitar into the FX return would likely not have a strong enough signal to drive the power amp to full output. Do you have a preamp like a POD or multi-FX unit or similar that you can plug into the return? Something like that would be able to put a bunch more signal into the return, which will give you a better sense of what the power amp is actually doing.

[jagdpanther]

The pic was a Merc OT, just to show where I measured and values. I have the stock OT in the amp. Any ways just threw a POD in the return and same thing happens. The more signal I put in the the more it starts cutting out. Unfortunately I don't have a scope, and have not used one before.

[jamesmafyew]

Ok, if it were me at this point, I would be doing two things: trying a spare set of EL34s, and swapping in a spare OT to see if that improved things. Problem is, those are both expensive things that not everybody just has laying around.

Without a scope trace to SEE what's happening on the output, I'm hesitant to say anything definite. But I'm concerned about the OT resistance readings and a partial/soft/intermittent OT short matches your symptoms and is semi-common on late JCM900s and early 2000s. The earlier JCM2000 100W OTs are similar to the JCM900 ones that fail all the time, and are not reliable. Crying "it's the OT" is usually a bad bet -- but if you've eliminated everything else, that or the power tubes are my best guess.

Also, again there may be more than one problem. The low volume could be due to the OT, but the crackling you mentioned could be something else as well. Hard to say without more conclusive testing and test equipment.

[jagdpanther]

Put a new set of Power tubes in it. I was thinking after looking at the schematic, could there be an issue with A) the caps (C34,35,38,39) and B) Resistors(64,72,73,75)? It looks to me from the schematic that these may be in "line" to W5.?. Seeing that the leads W3 and W4 to the OT are directly from the power tubes(pin 3?). Before I spend the money on and OT, I think I will look at these components. I took some notes when I started and it looks like I may have had trouble getting resistance values from the these resistors. I will pull the main board and either check on the board or R&R the resistors. What do you think? At this point it is only my time and a couple of resistors.The caps I am not sure how to check other than visually. discharge and pull from the board?


The volume has come back and is not weak any more. But when I start pushing the volume up past 12 o'clock it starts to cut out. Same for any combination of raising the volume / gain.

[Jazz P Bass]

If this was mine to fix, i would look at the output tube grid signal voltages.
If the signal does not squash when the output tubes do (and they are known good tubes) then it sure points to the output transformer.

[jamesmafyew]

I agree, being able to compare the signal voltage on pin 5 of the output tubes with the output waveform is the sure way to know what's happening here. A scope/dummy load/sig gen is really needed for this. I suppose it could be done without a scope (using two AC volt meters), but you would still need a dummy load and a constant signal on the input, so you could watch the meters as you varied the volume of the input signal.

The resistors you mentioned are bleeder resistors, they discharge the supply caps when the amp is turned off (so you don't need to discharge them, just turn the amp off and wait 5 minutes). You won't get consistent readings across them because your ohmmeter is charging the caps, so the the reading will vary until the caps are charged up, then will stabilize at something OTHER than the schematic value, since there are other resistances in parallel with what you're measuring. It is extremely unlikely that there is anything wrong with those resistors.

As far as the supply caps themselves, it's also unlikely that there's anything wrong with them, if the socket voltages are correct (they are) and the idle current is correct (you said that it was). Bad supply caps would cause blown fuses, hum, or strange other things, not likely what you've described. If you want to touch up the solder through the power supply, then by all means do so -- it could be helpful to check the bottom of the mainboard with a magnifier and a strong light, to look for cracked or otherwise compromised solder connections. But it's probably a waste of money just to shotgun the power supply.

W5 is the center tap of the output transformer. It's connected to the output of the bridge rectifier for the B+ supply. It's filtered by C38/39. There's nothing in series with it except for the actual rectifier, which I'm certain is fine. R71 is a series dropping resistor from the B+ supply to the screen node, which is filtered by C34/35, and then fed to all the pin 4's through individual 1K screen resistors. If there were bad solder joints on R71 or any of the screen resistors, that's a possibility for cutting out (though R71 is in series with the rest of the supply, including the entire preamp, so it's also unlikely). But I still think that the OT is where the focus should be.

[Tage]

Your symptoms match very closely to what I experienced with a 900SLX recently, that is volume loss especially once the volume is turned up. Mine was an intermittently shorting Output Transformer.

It took me a lot of testing to get the OT to hard fail, i had to run the amp hard and finaly I got a super low resistance on one half of the primary. there was a lot of reading here...

http://music-electronics-forum.com/t27566/

[jagdpanther]

Happy Update:
Thanks to all who helped me with diagnosing and fixing this amp.

In the end it turned out to be the OT. I took a chance based your inputs and some more reading on the forums. Since I don't have all the proper tools, I thought the risk was low, cost wise and put a ClassicTone OT in it. She's a rockin' now. One more palette of sound in the stable of our studio.

Again thanks jamesmafyew, Tage and Jazz P Bass for the input it has been greatly appreciated! I hope I can help someone out in the future as well.