Jcm 2000 tsl 100

[gbono]

Working on a TSL 100 with very low output (all channels). When checking the signal into V4 I see a clean channel signal which is very low and not symmetric and overdrive channel looks like a square wave at the same point (between C28 and R53). I assume that the issue is on the overdrive channel PCB TL10-61-02. Any ideas on how to diagnose the signal flow on these amps? If anyone has a "block" diagram of the signal flow it would be much appreciated - I have a list of all the connectors and how the route to the various boards.

Schematics are here: Marshall Schematics

BTW this amp is a 2004 model with an issue 10 output board - so the bias is very stable.

[km6xz]

There is not enough information to suggest if the overdrive signal at C28 is wrong, what signal input and gain settings? Does it clean up if you set the gain mid-way and a small signal of 1mv rms? It sounds as if the stages before the PI are doing their job so you have proven that the problem is in or after the Phase Inverter. Are you getting drive signal to the control grids of the power output tubes? What are the static DC voltages on all the pins of the power tubes? When driving the amp that hard can you hear a faint buzz from the output transformer at the frequency of your test signal? If so, but very low speak output jack level, the jack board could be the problem.
When you say output is very low, how low? With the signal at C28 just at clipping, what voltage do you get across your 8 ohm dummy load?

[Jazz P Bass]

Don't overlook the 16 ohm output jack.
It is a switching jack.
Try going in that jack with your load, simply as a test to see if the volume comes up at all.

[gbono]

There is not enough information to suggest if the overdrive signal at C28 is wrong, what signal input and gain settings? Does it clean up if you set the gain mid-way and a small signal of 1mv rms? It sounds as if the stages before the PI are doing their job so you have proven that the problem is in or after the Phase Inverter. Are you getting drive signal to the control grids of the power output tubes? What are the static DC voltages on all the pins of the power tubes? When driving the amp that hard can you hear a faint buzz from the output transformer at the frequency of your test signal? If so, but very low speak output jack level, the jack board could be the problem.
When you say output is very low, how low? With the signal at C28 just at clipping, what voltage do you get across your 8 ohm dummy load?

Hello Stan,
I injected a 70mV 1000Hz signal at C28/R53 node on V4 and observed a nice strong output (V5-8 seem to be working) into an 8 ohm resistive load. I'm concluding that the issue is on the TL10-61-02 board (overdrive channel) - or C28 is bad - a polyester film capacitor bad - really? I hate taking the power amp board out of this box.......another thought - I should be able to send the same signal through the FX loop and get signal at the output of the amp. Next experiment.

I also noticed that Con11 on the TL10-60 board comes from Con12 on the overdrive board. It looks like the output of IC3 passes through relay RL3b and terminates at Con12. I looked for a signal at C26/R22 but there is nothing coming out of IC3. I'm slammed for time, of course, and didn't a chance to check out VCC+- on IC3. Funny feeling it is the Fx loops on this box that is causing the issue. Thanks for any help.... George

[Alex R]

+1 on the 16 ohm jack, the speaker output goes through this to the 8 ohm jacks and it can get carbonised across the contacts.

[gbono]

+1 on the 16 ohm jack, the speaker output goes through this to the 8 ohm jacks and it can get carbonised across the contacts.

THX but with an external signal at the input of the PI - I get a nice clean/large signal from the amp into an 8 ohm resistive load. My question is: what is the signal path that feeds the PI on this design? ........ 3 channels and several FX loops(?)

[g1]

It would sure be nice if the newer marshall stuff included block diagrams and connector routing, but they don't. So it makes things really hard and most people don't want to spend the time required to figure out the lousy schematics if they don't have to fix the thing themselves. I'm one of those, sorry.
As your problem is common to all channels, I would suggest you check all power supplies first. Preamp tube supplies and solid state stuff supplies.
If you are suspecting the FX loop, are you getting a good signal to the FX send?

[gbono]

Try this again at the right thread......

Neat flow diagram for the DSL - helpful for TSL as well thanks L. Sparbar...I'll keep posting here until I find the cause of the low output.

[g1]

Just to verify, it's good if you inject signal at the PI, but not good if you connect into the FX return?

[gbono]

Okay I found the issue with the low output - the plate resistor on V4a was open. Not sure what happened before when I was getting a signal out of the amp with an injected signal on the grid of the PI - working too late into the morning or just getting old.

So the amp has another issue - with 100mV at the input and the clean channel engaged I cannot get the gain knob on the clean channel to shut off - I still get a signal from the amp (2-5 VRMS) at the output into a resistive load? With the volume on zero I still get signal out with gain knob at zero. Also odd is that the gain knob at 5 has a lower output than at say 2 - but turning past position 5 up to 10 the signal increases as it would normally, weird?

The lead and crunch channels work just as they should - and gain and volume do not have the problem - only the clean channel.

I tried checking the resistance value of the clean gain potentiometer (VR1a) on the clean board but didn't get far. I did notice that R6 measures .1 ohm instead of 10 ohms. I'm going to get at it when I get back in town...

[km6xz]

You have an intermittent problem, the plate resistor does not fit the original symptom and the 70mv injection into the grid of V4. Have you double checked the effects loop normalized contacts? If they are oxidized, low or no output would be expected but sometimes work. Inserting a jumper cable from send to return could tell if that is the problem. It is in 25% of all amp repairs with low or distorted weak tinny output. The weird clean gain control function will be fairly simple since there is not much that can control that except the pot itself. It sounds like a high resistance ground return connection on the pot.

I think when this is done you will have found that there were more than one problem, possibly before, but definitely after troubleshooting it.

[Mark Black]

In addition to the great advice by those above I ALWAYS found numerous cracked solder joints in the JCM2000's and eventually took to retouching at all jacks, pots, switches, multipin headers, fuseholders, AC inlet, tube sockets, large and/or heat-producing components, and anything else I found during inspection with a magnifier lamp just right up front if there was an intermittent problem. Good thing to do even if you have found/cured a problem if you want the amp to work for a long time.

The power amp board isn't usually as big a problem with the joint cracking (being double-sided), but all the other single-sided boards are just about a sure bet for it.

Just my experience...