I've poked around looking at other threads about this amp with this same problem, so I am gonna run this by the pros in hopes to gain from your past experiences. Amp came to me with a complaint that it was "blowing the F4 1A fuse."
Pulled ALL tubes and tested, finding three dead 12AX7s and two bad 5881 power tubes. Then checked all fuses, opened up the chassis and the 1.6A filament fuses (F2, F3) were intact as was the 5A mains fuse. (schematic shows 1.5A for F2, F3 but the PCB is labeled 1.6A, so apparently this amp is newer than the schem). Replaced the F4 1A fuse.
Installed just the preamp tubes, including three new 12AX7s. With NO POWER TUBES installed, powered up on LBL and noticed a significant HV arc near the V8 power tube. A closer look revealed some smoked up places near where the OT blue secondary wire connects to the PCB. The brown wire is labeled P9 but the blue wire has enough damage around the lug that I cannot make out the label - the wire connections are not labeled on the schematic.
POWER OFF
measured ~38 ohms between brown/blue OT secondary wires.
Blue OT sec to gnd measured >1.5Mohms
Brown secondary to gnd measured >1.5Mohms
POWER ON, connected to LBL
with B+2 wire disconnected from PCB, B+2 wire measures ~240vdc
with B+2 wire connected to PCB - Blue OT secondary measures ~255vdc (not connected), Brown secondary wire measures ~260vdc (not connected)
With B+2, Brown/Blue OT secondaries all connected I measure ~240vdc on both sides of R104, R105, R106, and R107
I can measure the same ~240vdc at pin 3 of each power tube socket
Bias at pin 5 of each power tube socket is -23.5vdc
The B+ seems low to me, even with the amp on the LBL. The schematic doesn't provide any voltages, so I have no clue whether it's in the normal range or if something is pulling down the voltage. The bias seems awfully hot as well, but again, I just don't know.
Besides cleaning up all the smoke and carbon, what advice can you offer with what I have here?
Schematic attached, although I believe the amp is a newer revision.
Pulled ALL tubes and tested, finding three dead 12AX7s and two bad 5881 power tubes. Then checked all fuses, opened up the chassis and the 1.6A filament fuses (F2, F3) were intact as was the 5A mains fuse. (schematic shows 1.5A for F2, F3 but the PCB is labeled 1.6A, so apparently this amp is newer than the schem). Replaced the F4 1A fuse.
Installed just the preamp tubes, including three new 12AX7s. With NO POWER TUBES installed, powered up on LBL and noticed a significant HV arc near the V8 power tube. A closer look revealed some smoked up places near where the OT blue secondary wire connects to the PCB. The brown wire is labeled P9 but the blue wire has enough damage around the lug that I cannot make out the label - the wire connections are not labeled on the schematic.
POWER OFF
measured ~38 ohms between brown/blue OT secondary wires.
Blue OT sec to gnd measured >1.5Mohms
Brown secondary to gnd measured >1.5Mohms
POWER ON, connected to LBL
with B+2 wire disconnected from PCB, B+2 wire measures ~240vdc
with B+2 wire connected to PCB - Blue OT secondary measures ~255vdc (not connected), Brown secondary wire measures ~260vdc (not connected)
With B+2, Brown/Blue OT secondaries all connected I measure ~240vdc on both sides of R104, R105, R106, and R107
I can measure the same ~240vdc at pin 3 of each power tube socket
Bias at pin 5 of each power tube socket is -23.5vdc
The B+ seems low to me, even with the amp on the LBL. The schematic doesn't provide any voltages, so I have no clue whether it's in the normal range or if something is pulling down the voltage. The bias seems awfully hot as well, but again, I just don't know.
Besides cleaning up all the smoke and carbon, what advice can you offer with what I have here?
Schematic attached, although I believe the amp is a newer revision.
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