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After pulling the rear panel PCB off this Marshall JCM 2000 to inspect, find and repair the traditional solder joint fractures of the Impedance Select Switch, along with those on the output jacks, I put that board back into place to see if I had a working amp. Prior to powering up, I noticed R63 4.7k/1W decoupling resistor in the power supply string had overheated, and then seeing R44, a 22k resistor from the Drain of TR1 having been burnt beyond recognition, I looked at the circuit, and just briefly powered up the amp, with my probes sitting at the junction of R37, R39 and the jumper leading to R44, it now being open circuit. The voltage was over 200VDC quickly, as I switched the amp off.
I clipped out C23, thinking that was the fault, and also removed V3, just in case. Tried again, and I still had the same voltage present, and checking the cap I had just removed, found no short across it. So, at this point, having already replaced R44 from the top side of the board, I was forced to pull the main PCB out of the chassis.
With this board removed, and having a damaged spare board intact in this circuit area, I got my Kikisui 7372A 0-300VDC/0-150mA power supply out, so I could apply DCV to the junction of R63/R62 and ground, with no tubes installed. The spare board had no voltage present at R44, while this amps’ PCB still had DC Voltage leaking from someplace.
Looking at what was subject to this high voltage, I replaced TR1 and TR3 JFET’s (J174’s). Not finding any obvious path to provide this voltage, and knowing it wasn’t from the plate of V2, that led me to the top side PCB voltage trace from R61 to R24, which ran underneath R30, R31, R32, C23, R37, R38, R39, R40, R41 and R42 in this immediate region on the board. I unsoldered the resistors from the bottom side of the board, since all the resistor leads are folded over, preventing simple heating of the solder pad and raising the resistor body up off the board. Now with that obstacle out of the way, I one by one heated the solder pad and lifted each end of these resistors up off the surface until I had all lifted off. Re-soldered each into their new position, and re-applied the external DC voltage. Now, no voltage was present at R44, which had been present when all were flush on the board.
I was able to get some Corona Dope applied to the top side of the board in that location to insulate the PCB trace as well as get coating onto the resistor bodies. I never got as far as finding exactly where the fault was, other than it was from that trace making inadvertent contact to cause all this.
The Reverb circuit was also damaged, as the input cap to the LM348 IC was only a 47nF/63V part. I replaced that, along with the IC.
This morning, I put the PCB back into place, had to also go thru some power tubes to get proper matching from what was in place, and now have a working amp again.
I forgot to record which Revision this JCM2-60-02 board this was. It was lighter in color regarding the solder mask, while my parts board was a later revision, also being much darker in it’s solder mask color.
jcm26002_ISS5.pdf
jcm26302_ISS5_Reverb PCB.pdf
I clipped out C23, thinking that was the fault, and also removed V3, just in case. Tried again, and I still had the same voltage present, and checking the cap I had just removed, found no short across it. So, at this point, having already replaced R44 from the top side of the board, I was forced to pull the main PCB out of the chassis.
With this board removed, and having a damaged spare board intact in this circuit area, I got my Kikisui 7372A 0-300VDC/0-150mA power supply out, so I could apply DCV to the junction of R63/R62 and ground, with no tubes installed. The spare board had no voltage present at R44, while this amps’ PCB still had DC Voltage leaking from someplace.
Looking at what was subject to this high voltage, I replaced TR1 and TR3 JFET’s (J174’s). Not finding any obvious path to provide this voltage, and knowing it wasn’t from the plate of V2, that led me to the top side PCB voltage trace from R61 to R24, which ran underneath R30, R31, R32, C23, R37, R38, R39, R40, R41 and R42 in this immediate region on the board. I unsoldered the resistors from the bottom side of the board, since all the resistor leads are folded over, preventing simple heating of the solder pad and raising the resistor body up off the board. Now with that obstacle out of the way, I one by one heated the solder pad and lifted each end of these resistors up off the surface until I had all lifted off. Re-soldered each into their new position, and re-applied the external DC voltage. Now, no voltage was present at R44, which had been present when all were flush on the board.
I was able to get some Corona Dope applied to the top side of the board in that location to insulate the PCB trace as well as get coating onto the resistor bodies. I never got as far as finding exactly where the fault was, other than it was from that trace making inadvertent contact to cause all this.
The Reverb circuit was also damaged, as the input cap to the LM348 IC was only a 47nF/63V part. I replaced that, along with the IC.
This morning, I put the PCB back into place, had to also go thru some power tubes to get proper matching from what was in place, and now have a working amp again.
I forgot to record which Revision this JCM2-60-02 board this was. It was lighter in color regarding the solder mask, while my parts board was a later revision, also being much darker in it’s solder mask color.
jcm26002_ISS5.pdf
jcm26302_ISS5_Reverb PCB.pdf