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Here’s a new thread on an old subject. Valvehead here posted a thread titled “Marshall Short” that described all of the symptoms that I am experiencing. His was a model 8100 mine is the 80 watt combo the 8080. Same schematic with the exception of a few different resistor values. Let me say that I am very much an amateur in troubleshooting when the problem is not obvious (shorted, burnt in half, broken traces, bad solder joints, etc.) Last summer I tried the shotgun parts replacement method a couple of times. The first two times I had TR8 and TR10 swapped……..duh. When I realized it I replaced several parts in the power section in case damage had spread. Sorry if this seems long winded but I have done a lot and don’t want to leave anything out. Here is what was last replaced:
D1, 2, 4, 5
ZD1, 2
TR4, 5, 6, 7, 8, 9, 10, 11, 12
IC7, 8 (just because)
R73, 74, 96, 97, 112, 113, 114, 115, 116, 117, 118…….(112, 113, 114 were really cooked but probably not the problem)
BR1
C17, 20, 40, 41, 53, 55, 59, 61, 62
After doing this it worked plugged into a light bulb current limiter. Played it for at least 20 minutes. Plugged it directly into the wall and it lasted 20 seconds. Blew the fuse and both TR8 &10 are shorted again. I left it alone for about 6 months and decided to have another go at it. I hate it when things get the best of me.
Recently I took the long road. I lift every resistor out of the circuit and checked their value. All but one were within tolerance.R118 which is a .33 was reading .7…….just wondering if that is enough imbalance to cause the problem. I decided to try some voltage readings based on points on the schematic. TR8 and TR10 are out of the PCB. I soldered some leads in their place to make it easy to read with my meter. Also left the preamp tube out……thought it was unimportant for this round of testing. Correct me if I’m wrong. Most of the points were for the + and – 15V rails. Here’s the results from that:
R65 = +15.2V
R64= -15.2V
R78= -14.9V
R83= +15.2V
R96= +2.3V
R118= +2.3V
IC 2 thru 10 ……pin4 -15.2V, ………pin8 (and 7 on a couple) +15.5V
Here’s where it gets different from what I have read of others with the same issue:
TR8 B= +47.6V
C= +47.7V
E= +2.3V
TR10 B= -47V
C= -47.7V
E= +2.3V
TR9 B= -46.9V
C= +46.9V
E= -46.9V
Anything seem incorrect to you guys? Remember don’t be too technical because I’m an amateur. Thanks!!!!
D1, 2, 4, 5
ZD1, 2
TR4, 5, 6, 7, 8, 9, 10, 11, 12
IC7, 8 (just because)
R73, 74, 96, 97, 112, 113, 114, 115, 116, 117, 118…….(112, 113, 114 were really cooked but probably not the problem)
BR1
C17, 20, 40, 41, 53, 55, 59, 61, 62
After doing this it worked plugged into a light bulb current limiter. Played it for at least 20 minutes. Plugged it directly into the wall and it lasted 20 seconds. Blew the fuse and both TR8 &10 are shorted again. I left it alone for about 6 months and decided to have another go at it. I hate it when things get the best of me.
Recently I took the long road. I lift every resistor out of the circuit and checked their value. All but one were within tolerance.R118 which is a .33 was reading .7…….just wondering if that is enough imbalance to cause the problem. I decided to try some voltage readings based on points on the schematic. TR8 and TR10 are out of the PCB. I soldered some leads in their place to make it easy to read with my meter. Also left the preamp tube out……thought it was unimportant for this round of testing. Correct me if I’m wrong. Most of the points were for the + and – 15V rails. Here’s the results from that:
R65 = +15.2V
R64= -15.2V
R78= -14.9V
R83= +15.2V
R96= +2.3V
R118= +2.3V
IC 2 thru 10 ……pin4 -15.2V, ………pin8 (and 7 on a couple) +15.5V
Here’s where it gets different from what I have read of others with the same issue:
TR8 B= +47.6V
C= +47.7V
E= +2.3V
TR10 B= -47V
C= -47.7V
E= +2.3V
TR9 B= -46.9V
C= +46.9V
E= -46.9V
Anything seem incorrect to you guys? Remember don’t be too technical because I’m an amateur. Thanks!!!!
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