Marshall JCM2000 DSL

[JC@]

Hello I have a problem here.
The only schems I can find for this amp shows the power supply and main board as JCM2-60-02 but the one I have is JCM2-60-00

Maybe because it is a version to use on 230 volts mains.
I'm affraid to follow this schems because they vary between years and versions and where one component is called R12 on the other schem it may be R45...

The blown components are marked as R61, R62 and R63. Comparing between schems, I believe these are 4K7/1W, 1K/1W and 4K7/1W respectively.

Maybe someone has the correct schem, and that would help for sure.

[Amp Kat]

http://www.drtube.com/schematics/mar...60-02-iss7.pdf

Is the right schematic. If I'm not mistaken this board is really a 100 watter that they use for the 50 watt. If you look to the right & left of the two power tubes there should also be spaces for two more output tubes. It should also have pins coming out of the back for biasing and two bias pots labeled pr1&2. For some reason this amp is very proned to damage caused by tube shorts and the plate supply and screen is where it gets damaged although yours is worst than the last one I did where it blew the whole trace off of the bottom of the board between pin3 and the other nonused socket trace. Your problem is it took out all of the resistors along the path of the screen and preamp plate supply. R-61-63 are all tied together in series from the screen supply to the plates of V3. I would be real careful that when you replace all the blown resistors that you also check the 1k 5 watt screen resistors and I'd just go ahead and replace the coupling caps from the peak inverter to the bias supply also as they may cause quick redplating on one side. This amp is actually easier to take apart than most. Lots of gray ribbon type two conductor cables. I mark and match those to the connector numbers on the board and pull all of them out on the power board side only. Theres a green ground wire in the back underneath the top board also. Then pull the screws out of the plate and the four power tube socket screws and that whole plate drops out. Then you just remove the PCB board screws and the whole pcb pulls away from the mounting plate. Inspect that board real good and ohm out everything you can and look for other damaged traces and resistors and get those shorted tubes out of there and your good to go !

[JC@]

Actually, the amp I'm working in has four power tubes.
That's all I need.Thank you very much for the detailed explanation and the tips about pulling the board out , I really appreciate that.

[Enzo]

I remove the control board at the front panel, and the upper rear jack board. Clip the two tie wraps across the back holding the OT wires. Now remove the power tube socket screws and the tubes. There aer 6-8 screws holding the main board to its subchassis. There is enough slack in the wires that you can flip the entire pile over and solder on the main board. I usually disconnect the two red wires from the PT. Oh yeah, unplug the littel bias board too.

I have that schematic, but it does n't entirely match the 60-00 board. I believe all teh numbers are the same, but for example there is one optocoupler in the schematic, but there are three in teh corner of this board. Also the drawing shows JFETs TR2,3, but the amp also has TR1, which switches a cap on and out of V2 cathode. It goes in teh open space under pin 3. The TSL100 drawing shows it.

I have one of those amps in here now.

I don't think the mains matters. Coming out of the power transformer, the voltages are the same.

[JC@]

still big problem.

The current draw from the whole amp is ok, but
even with the power tubes (new ones) out of their sockets,
both 4k7 1watt resistors (R61 and R63) now replaced, get so hot that they melt the solder points. R62, 1K 1watt gets very hot also.

[Amp Kat]

Looks like your screen supply has a problem. What is the voltage at pin 4 of the power tube socket. if ok remove V3 and lift R61 on the 100k plate resistor side and measure the voltage at the plate of V3 at that junction. If it's ok your problem is from the 10k dropping resistor back. A bad tube could cause this or it is posible that C34/35 could be shorted. Are those 1k screen resistors ok ?

[JC@]

Looks like your screen supply has a problem. What is the voltage at pin 4 of the power tube socket. if ok remove V3 and lift R61 on the 100k plate resistor side and measure the voltage at the plate of V3 at that junction. If it's ok your problem is from the 10k dropping resistor back. A bad tube could cause this or it is posible that C34/35 could be shorted. Are those 1k screen resistors ok ?

voltage is about 15 volts less on the screens that on the plates.
I lifted R61 and I measured 450 volts on the plates of V3 (Thats a lot isn't it?),
but the problem seams to go away, that is no melting on R62 and R63.

The 1K 5w screens resistors are ok

With all the tubes pulled out, preamp, PI and power tubes, the problem persists when I solder R61 back in it's place. So maybe there's something very wrong in the circuit from the input to that 10K dropping resistor...

I haven't checked C34 and C35 yet.

[Amp Kat]

voltage is about 15 volts less on the screens that on the plates.
I lifted R61 and I measured 450 volts on the plates of V3 (Thats a lot isn't it?),
but the problem seams to go away, that is no melting on R62 and R63.

.

Screen voltage should be only about 5 volts less than the plate voltage if that much. 450 at V3 is to much even with R61 disconnected. Coming off the screen to R-63 the 4.7k should drop there to around 280 for V4 (PI.) then drop again at R62 for oh around 200 then drop again at R61 for V2. The reason R61 is frying is because 450/1k is more than the rated wattage for that resistor as opposed to 180/1k. It is also possible that C34/35 network is bad and ohm out that resistor between the plate and screen supply what is it 270 ohms 7 watts could be bad also but it seems like that connection on R63 4.7k may not still be making good contact because the voltage should drop after that resistor and note when you put the tubes back in it will drop further than without them in as they tug on it a little also.

[Enzo]

Uh, I didn't follow that at all.

The only thing that would burn up R61 is something down stream of it drawing too much current - like a short to ground,

If my R61 were burning up, the first thing I would check would be C31 and C14 just left of R61. Lift them from the circuit and see if the burning stops. C10,11 could be bad, but then R19 would be getting hot and R61 less hot. Same deal with C20,22, Then R24 would get hot. But check them anyway. Again, a simple lift from circuit will tell you if it is the problem. The tubes all have high resustance plate resistors, so they are not the problem. And there is nothing else on the downhill side of R61. Other than a hardware short or solder bridge, that is.

You cannot push exces current down through R61 from upstream, something downstream has to provice a current path for the excess. I'd be betting on C31.


If C34,35 are shorted, then no B+ even gets to R61.

Or lets look at it another way, pull the preamp tubes. Now there should be zero current through R61. No paths to ground except caps. If the R61 still warms up, then some cap is leaky or short.

I don't see how the screens can affect R61.

[JC@]

C14 was dead short.

[Amp Kat]

Uh, I didn't follow that at all.

.

Yeah I kinda miss that one Enzo and stand corrected. I did think there was a cap shorted but had the wrong one. The reason I thought screen was that's where the voltage dropping resistors are tied to. Glad the amp is back to normal.

[JC@]

Well, the trouth is that I had C31 already desoldered when I read Enzo's post, but it was testing ok! I couldn't imagine that that little high voltage ceramic C14 in a corner of the board, all shiny and good looking, was in fact a dead short to ground.

I couldn't find the right words to express my personal admiration on how Enzo can dive in an amp schem, that's why I just said, C14 is bad.

The amp is now waiting for the next encounter, as I'm taking my time because the little ceramic cap really stressed me a lot I hope there's no other faulty component there...

[Amp Kat]

Man I tell you what. When tubes short they cause some freaky things and I can only guess what or where it all goes and what it takes out as they are usually somewhat different. It is awesome to have someone like Enzo who has great skills and experience with the ability to understand exactly what's happening in the circuit and I'm sure these are a lot easier than those pinballs he used to work on. I've stared down many nice shiney components not knowing that little bastard is the one giving me all the problems. Nice job fellows !

[Enzo]

I used to do field repair on the pinballs, and the onlookers often would say, "Look at all them wires, how can you tell what you are doing?" I always looked at them and said, "The wires are not what goes bad."

[JC@]

done

It's definitely fixed.
Thank you!