A little smoke and a blown fuse, low B-
I built a TSE 300B for my adult son a couple of years ago. He and his girlfriend seem to have a habit of turning it on and off without waiting for it to cool off. (I replaced the fuse once before.) He says before it stopped working, he smelled something and then it blew a fuse. The amp has a CL-90 in it, but I’m thinking the quick on-off-on was the cause of the problem.
I replaced the fuse and put in the tubes and started it up; one of the bias meters went to zero while the other was pinned and then the fuse blew.
I took it apart and looked at the circuit board and didn’t see any problems.
Next, with only the rectifier tube and the 5842’s in place I got a B+ of over 400 and a B- of 170 and then after a minute some smoke. This time I checked and the solder around R25(36K, 3W) was burned off.
Am I correct in assuming that the problem is the low B- and that the either D2 or D3 (FRED’s) are bad?
Any suggestions on how to troubleshoot?
Thanks, Jacques
I built a TSE 300B for my adult son a couple of years ago. He and his girlfriend seem to have a habit of turning it on and off without waiting for it to cool off. (I replaced the fuse once before.) He says before it stopped working, he smelled something and then it blew a fuse. The amp has a CL-90 in it, but I’m thinking the quick on-off-on was the cause of the problem.
I replaced the fuse and put in the tubes and started it up; one of the bias meters went to zero while the other was pinned and then the fuse blew.
I took it apart and looked at the circuit board and didn’t see any problems.
Next, with only the rectifier tube and the 5842’s in place I got a B+ of over 400 and a B- of 170 and then after a minute some smoke. This time I checked and the solder around R25(36K, 3W) was burned off.
Am I correct in assuming that the problem is the low B- and that the either D2 or D3 (FRED’s) are bad?
Any suggestions on how to troubleshoot?
Thanks, Jacques
I would remove the output valve from the channel that made the meter go full range. You may find that a low -B caused stress on the valve and it has failed. Remove the valve, check the -B voltage. If it has recovered, replace any damaged resistors and fit a new output pair.
The usual suspect here is a shorted mosfet. It will cause excessive tube current, low B+ and possible smoke from R25. Measure source to drain with an ohmmeter. If shorted, replace the mosfet and R25.
I've replaced the mosfet and R25 and am still seeing the solder bubbling and smoking at R25. Any other "usual suspects"? When I measure the two mosfets on the board with my ohmmeter they read differently. Could there be something else in the circuit that could cause this?
Thanks.
Thanks.
I had same problem. Check if pcb traces are ok and mosfet itself is ok and genuine.
In my case R33 & R29 were also burned with pcb traces, power tube however survived.
In my case R33 & R29 were also burned with pcb traces, power tube however survived.