Marshall JCM900 help needed!!! (PROBLEM SOLVED)

SaMpLeGoD · May 3, 2011

Hi there,

I was modifying my JCM900 from using 5881 tubes to EL34.
I did all the mods you can see on the schematic.
All doing fine! I adjusted the bias, measured 470V on the plate, adjust the bias around 35mA using a Bias probe, all just perfect! until.. the resistor I changed R35 (now is 100R /25W) that I put with flying wires to the chassi, by mistake shotcircuited one of his leads to the ground, I heard a little carnival bomb that makes a little hole on the chassi also (lot of current passed there at that moment :S)
Well...
After that, the plate voltage rises to 540V (was 470v Before) one of the filter caps has nothing reading (0V) the other has a dangerously (500V) and I can't read any plate current anymore... I thought maybe the Bias probe gone bad, but it's ok... I checked all the resistors, all fine and the 3A Picofuse and all the others fuses are ok!
You can just see all o this in the scchematic on the attachment!
Could anyone help me on this?
Thanks a lot!

spreemusik · May 3, 2011

Can only guess, but perhaps one of the wires is burnt. If there's enough current to burn a hole in the chassis, there's enough to fry a transformer winding too.

lassoharp · May 3, 2011

Check the resistance on R30 (power off) to see if it went open. Also check LD1 and LD2 and doublecheck the two 500ma fuses surrounding the diodes.

Here is the closest I could find to a general 900 series PS schematic for reference. It sounds like the short was only for a second or two. No mains fuse or others blown. Hopefully your PT did not suffer too badly. It usually takes several seconds to start cooking the windings and you won't miss the smell.

http://www.drtube.com/schematics/marshall/2100-60-02.gif

SaMpLeGoD · May 3, 2011

I just reinforde some solder joints and the tubes came back to life, That cap still has no DC voltage and I don't know why

I already adjusted the Bias again, but.... no sound... I'll need to track down with the osciloscope... I'll cross my fongers to have a good OT when star to dig into it.
Cheers!

lassoharp · May 3, 2011

Did you check R30? If it's open you will get no voltage on the caps or anything downstream = no voltage on the preamp tubes = no sound.

SaMpLeGoD · May 3, 2011

lassoharp said:
Did you check R30? If it's open you will get no voltage on the caps or anything downstream = no voltage on the preamp tubes = no sound.

Yes I measured and R30 is fine unfortunately

any other idea?
Thanks

lassoharp · May 3, 2011

First verify that you have B+ at the 3A fuse side of R35(100r). If you do then, recheck the connection at the junction of R35 (100r) and the 50uf cap.

Also, while the amp is on, check the plate voltages of the preamp tubes. If there is an open downstream you should get "0V".

gemini86 · May 3, 2011

SaMpLeGoD said:
lassoharp said:

Did you check R30? If it's open you will get no voltage on the caps or anything downstream = no voltage on the preamp tubes = no sound.

Click to expand...

Yes I measured and R30 is fine unfortunately
any other idea?
Thanks

measured what? voltage drop across r30? ohms? ?? Use your schematic, trace the voltage at each component. If you have no voltage at those caps, then either the 100R is open, or R30 is open. Or maybe you fried a wire/pcb trace between.

Your voltage is climbing because it's unloaded, because the preamp tubes aren't getting voltage. Pull your tubes until you get voltage everywhere it used to be, because those things are getting expensive.

As a side note;

1. You just learned the main downside to using flying leads... Well, things can fly.

2. First rule of amp modding is only do one mod at a time, then if something goes wrong, or you don't like the result, you'll know what what mod is causing it. By doing all this at once, it's going to be hard to troubleshoot faultfind. I know that this isn't your current issue, but I'm just giving you a heads up from the get-go.

3. It looks as if you're trying to 800-ify your 900, replace that 100R with the proper choke, that has quite an affect on the sound.

SaMpLeGoD · May 4, 2011

gemini86 said:
measured what? voltage drop across r30? ohms? ?? Use your schematic, trace the voltage at each component. If you have no voltage at those caps, then either the 100R is open, or R30 is open. Or maybe you fried a wire/pcb trace between.

Your voltage is climbing because it's unloaded, because the preamp tubes aren't getting voltage. Pull your tubes until you get voltage everywhere it used to be, because those things are getting expensive.

As a side note;

1. You just learned the main downside to using flying leads... Well, things can fly.

2. First rule of amp modding is only do one mod at a time, then if something goes wrong, or you don't like the result, you'll know what what mod is causing it. By doing all this at once, it's going to be hard to troubleshoot faultfind. I know that this isn't your current issue, but I'm just giving you a heads up from the get-go.

3. It looks as if you're trying to 800-ify your 900, replace that 100R with the proper choke, that has quite an affect on the sound.

Hi Rodney! thanks for you reply!
Well, I just measured the resistance of the resistors, and they are ok for the values...
You right in all you said... I made the mods from a Marshall's service manual, looked for the schematic, all seemed to be all right, is not such a big deal change some screen resistors, bias resistors to match the EL34 specs and get another 100R with more power (to make the choke's place) but unfortunately I just don't fixed the 100R / 25W to the chassi at that time because I was made that at 1AM and I couldn't drill at that hour... so things happens, in a distraction the lead touch the chassi and... well... here I am

I have no voltage across the R30 even no voltage at the 50 + 50 cap marked, no voltage at the 100R resitor... seems to me that the problem is somewhere else before in the last 12AX7...

Thanks a lot, I'll post the results!

Eddie

lassoharp · May 4, 2011

I have no voltage across the R30 even no voltage at the 50 + 50 cap marked, no voltage at the 100R resitor... seems to me that the problem is somewhere else before in the last 12AX7...

That would seem to point at the 3A picofuse or connections surrounding it. . .

gemini86 · May 4, 2011

I would flip the PCB and look closely around the 100r. Marshall is cheap and so are their pcbs. Reflow the solder joint for the mods you did, using a tiny dab of fresh solder to flux it up nice and clean. I did this swap for a friend not more than a couple years ago. The el34 sound good biased down around 17watts each, IMO. Good luck and consider uploading pics so we can help.

lassoharp · May 4, 2011

Ditto for checking the pcb connection. Easy to damage the land or get poor flow.

I've recently repaired 2 Peavey amps where the hot side of the main filtering caps had pulled completely loose. Looked to be normal tension on the other caps mounting but it shows how even that small amount of mounting tension can, over time and vibration overcome the joint strength. I just don't like pcbs in tube amps, esp in the PS section.

SaMpLeGoD · May 4, 2011

lassoharp said:
Ditto for checking the pcb connection. Easy to damage the land or get poor flow.

I've recently repaired 2 Peavey amps where the hot side of the main filtering caps had pulled completely loose. Looked to be normal tension on the other caps mounting but it shows how even that small amount of mounting tension can, over time and vibration overcome the joint strength. I just don't like pcbs in tube amps, esp in the PS section.

Nothing like a good PTP wiring! turret's board and stuff! I agree totaly!

gemini86 · May 4, 2011

You gotta remember that unlike studio gear, guitar amps get all sorts of road abuse and even when they're sitting on stage, they're sitting on top of a rumbling 4x12. Combined with heat, most PCBs fail. A solid 2oz copper clad would be better, but that costs $$$.

SaMpLeGoD · May 10, 2011

lassoharp said:
First verify that you have B+ at the 3A fuse side of R35(100r). If you do then, recheck the connection at the junction of R35 (100r) and the 50uf cap.

Also, while the amp is on, check the plate voltages of the preamp tubes. If there is an open downstream you should get "0V".

Hi!

Yes, I read 0v on the preamp tubes plates... 0v across R30, and 0v across the 100R (R35)... what you mean for "open downstream"?
Thanks for the help guys

Cheers

gemini86 · May 10, 2011

so, with the power off, check continuity between each component, then you will find your open, which will cause no voltage "downstream" or further down the chain, like as you found, 0v on the preamp tube plates.

lassoharp · May 10, 2011

Assuming the solder joints are good you could have an open on the pc trace or land. I just repaired an old Ampeg this weekend that had this problem. It's shown in the attached diagram. These can be tricky because you can't physically see the open from the surface. I had to jumper a new wire to another open spot on the Ampeg. Reflowing the solder joint didn't help in this case.

You can check continuity between the relevant points and traces to verify whether you have a problem there.

SaMpLeGoD · May 10, 2011

lassoharp said:
Assuming the solder joints are good you could have an open on the pc trace or land. I just repaired an old Ampeg this weekend that had this problem. It's shown in the attached diagram. These can be tricky because you can't physically see the open from the surface. I had to jumper a new wire to another open spot on the Ampeg. Reflowing the solder joint didn't help in this case.

You can check continuity between the relevant points and traces to verify whether you have a problem there.

Hi guys!

Problem solved! it was a bad soldering and a bad resistor in the path! the amp is working, all the voltages are normal! and it is sounding good

thanks a lot for all of you!!!!!
Best wishes!

Eddie

SaMpLeGoD · May 13, 2011

Talking about replacing the 100R by a choke... wich choke could fit in that circuit? I found a 3H to 4H choke that is used in the early marshalls... is that right? could this choke be used here? I'm building a JPM50 plexi and I'm thinking in using it for the project.
thanks

SaMpLeGoD · May 13, 2011

lassoharp said:
Assuming the solder joints are good you could have an open on the pc trace or land. I just repaired an old Ampeg this weekend that had this problem. It's shown in the attached diagram. These can be tricky because you can't physically see the open from the surface. I had to jumper a new wire to another open spot on the Ampeg. Reflowing the solder joint didn't help in this case.

You can check continuity between the relevant points and traces to verify whether you have a problem there.

This kind of problems are hard to solve

you just look at the things and they appear be ok... but they're not. It's hard to check each solder point in a complex path... well... life isn't easy at all.
Thanks a lot for all of you that helped me to solve the Marshall issue! I really appreciate that!
Cheers!

Marshall JCM900 help needed!!! (PROBLEM SOLVED)

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