Issues with 76 jmp

Guest · Post by **Guest** » Sat Apr 09, 2005 10:58 pm

[quote="Anonymous"]Hey guys, I just got back from the Dallas guitar show all excited to try out the 76 JMP I just bought. I couldn't try it out because the boot didn't have power, but the guy said the usual; new tubes, recently serviced, great shape, yadda yadda..... Anyway, I check the resistance on the old resistor and the value keeps fluctuating. I then switch it out. Same thin happens. When I try and bias the tubes the current wont sit tight. It starts out really high and then comes down without me touching anything. Power it of and back on and the current is different again and still fluctuates. THis is with different tubes also. I just got of the phone with Clay Finley(great guy btw) and he is pretty sure the 10uf bias caps are suspect. Makes sense to me. I gonna order some. Does anyone else have some insight on the problem?

Thanks,
Scott

Flames1950 · Post by **Flames1950** » Sun Apr 10, 2005 12:01 am

Checked the resistance on what old resistor? Depending on where in the circuit a resistor is you may or may not be able to measure it without removing it.
If Clay thought you should take a lookie at the bias caps then I'd do it. He's been around a few Marshalls before!

Guest · Post by **Guest** » Sun Apr 10, 2005 12:15 am

Flames1950 wrote:Checked the resistance on what old resistor? Depending on where in the circuit a resistor is you may or may not be able to measure it without removing it.
If Clay thought you should take a lookie at the bias caps then I'd do it. He's been around a few Marshalls before!

He told me the way to check would be to pull the tubes and check the -DC bias voltage. He also said if it was the bias caps, the voltage would be unstable. I did that and it looked fine. I am going to meet him tommorow at the guitar show, so I'll let him know what I've checked so far.

Thanks,
Scott

Guest · Post by **Guest** » Sun Apr 10, 2005 10:01 am

Problem solved!!! Rewind.....

The first problem I had yesterday when I got it home was there was no sound. I imediately thought the worst(OT). I checked the mains fuse, good..I went to check the HT fuse, as soon as I barely touched it, I got sound. I pulled the chassis out and say that the Power Tranny was replaced and someone didn't solder the wire to the fuse holder, they just fed the wire through the hole and twisted it. I soldered this and fired it up. Got sound.. Sound good. Then we move to the above problem.

It only took me until 3:30 in the morning:mallet I kept trying to trouble shoot the bias circuit. After checking continuity on every joint and component, and value, I noticed that there was a cold solder joint on pins 1 an 8 going to ground. Now the bias reading is steady. I just need to get a 56k resistor so I can get the curent down a bit more(El34s). currently steady at 46ma@477v.

Thanks guys!!

Guest · Post by **Guest** » Mon Apr 11, 2005 10:55 pm

Well guys, i just got done with the remaining repairs (except mid pot) I put in the 56k bias resistor and switched the bias feeders from 150 to 220k. The -DC bias range was from -34 to -40, but the current would still start out high and slowly work its way down. I decided to go ahead and replace the bias caps with some sprauge atom 8uf 150v caps. Tried again and finally stable at last. I then jmmed out for a bit but the amp was a bit bright to my ears so I clipped the bright cap of the volume pot. Tried again and perfect the amp actually starts to break up pretty early(or maybe that was eardrum distortion:D Threw some pedal in front of it and I'm smiling ear to ear. No need for a hotplate for me!!:D