My JCM900 4100 is sick.

[Ecopunx]

I have a Jcm 900 4100 from 97. 5881 model. The preamp is Jmp52c. Fresh tubes. I’ve been going the rounds trying to figure out a low volume problem. Both channels affected. Direct input to power amp works fine. Preamp output is suspect. I replaced all of the electrolytics due to the age. I have good voltages from the lt supply at the correct ic pins. I’ve replaced ic 1,8, and 5. I checked them all for heat and dc offset. All ok. Replaced lt supply caps(known to induce oscillation). No help. Cleaned the fx jacks and jumpered them with no luck. I have no oscilloscope for signal tracing, so I’ve been doing what I can with a meter. I’ve finally got volume again, but only by disconnecting the scr from ts1. Something is pulling down the signal with this scn connected. This is the ground for the effects loop I believe. So I rang the resistors in the loop and all seem ok. I apologize for the long post. Any help much appreciated. I guess I should check for voltage on this ground as well that could be affecting the signal.

[Enzo]

Hi
Perhaps if you could post the schematic you are using.

My JCM900 4100 drawing I see no SCRs. SO I am not sure what we are looking at. Also, I can't imagine what a FX loop ground needs an SCR for.

SO help us all get on the same page.

[Ecopunx]

Hello Enzo! I’ve seen many of your posts and you seem like an impressive troubleshooter with these amps! I posted a schematic. The screened wire triple between the preamp and the power amp, blue, red, and a braided scn wire. When I remove it from its tap on the preamp, full power returns. I also attached a pic of its attachment on the preamp board. Thanks again!

[Enzo]

Um, OK, when you said scr you meant the screen (shield) in the cable? To me SCR means silicon controlled rectifier, like a triac sorta. I am guessing you don't mean that.

Then start by checking the cable, is the shield shorted to a conductor, either internally or at an end?

[Ecopunx]

Thank you for the quick response. Yes exactly, just the shield of the wire. I will desolder and test it. Definitely possible. Thank you!

[g1]

In your photo it looks like the screen wire may be touching something underneath it.

[Ecopunx]

I thought so as well, but nothing is touching. I desoldered and isolated and the cable is ok. No continuity between shield and other conductors. As a side note ic5 is warm, and r14 is 150 degrees. It measures at 100k. May be normal as this is a cathode resistor? Thanks for the help guys!

[Enzo]

Wait, you removed the cable and THEN measured it? I would have preferred we measured to see if screen was shorted to conductor while it was still in place. If the cable itslf is not shorted, it is perhaps even likely if we carefully installed it back into the amp, it might work.

[g1]

The blue wire goes to the FX send jack. Likely it has a problem and is giving low resistance to ground. Disconnecting screen wire floats the FX jack board ground and hides the problem.

[Ecopunx]

I tested before and after removal. Blue to screen is 12k. Red is 40k. I tested just the preamp taps isolated and they are both in the megohms to ground. The taps on the power amp show the lower resistance to ground. I reinstalled the cable and the result is the same. Weak audio with screen connected. Disconnected the volume is back, but the sound is still not normal. Very distorted. I also noticed that the screen tap on the power amp has no continuity to the chassis. Shouldn't it be the same potential as the chassis? I tested the other screened wire on the power amp ts2. It is grounded to the chassis. Hmm.... Ive heard of the send and return jacks with intermittent connections but never seen one shorted to ground. Strange. Is it common for the ring and sleeve contacts to short? Maybe I will remove the send jack first and check it out.

[g1]

Not sure when you found no continuity from TS1 scn tab to the chassis whether you had the screen wire disconnected or not. With wire connected you should have connection to chassis.
Flip the PA board and look if there is a trace burnt between the scn connections of TS1 and TS2. They may not be connected. It is not necessarily required. The FX loop circuit gets it's ground from the other end of TS1 cable at the preamp board. That part may not be connected to the other grounds on the PA board.