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I need some help deciphering what's going on with this amp. I've attached a picture of the schematic on the actual base of the amp I have in front of me, and also a more readable one I found that matches mine.
Customer says it worked fine for years, had a replacement 15" put in at some point.
One day it just didn't turn on/blows fuses.
I mistakenly told him just to bring the amp, not the cab. So my repairs started with just using the external speaker jack, to test.
My first question, am I correct in that if the normal internal speaker isn't connected via it's native odd 4 pin XLR-ish plug, that the external jack will not work?
I had the amp stable with simply a new fuse and power tubes, but could only get sound using the "ext amp" jack. So at least I know the preamp works. No sound using ONLY the ext speaker jack.
I didn't want to start rewiring the speaker jacks just to be able to test using my shop speaker via a normal 1/4" cable, so I had him bring in the cab.
I plugged in the cab using the 4 pin connector (nothing in the ext jack) and the amp is blowing fuses.
I'm having a hard time deciphering the schematic as far as the OT secondaries at the 2 diagrams for the 4 pin plugs, and also the "F" notation.
I *think* that since F is notated at the cathode of the PI, that the PI Rk isn't grounded unless the 4 pin lug is connected to the cab? I'm not sure if that explains the fuse blowing.
To further cloud things, I've seen schematics where the standby connects to this matrix of madness, but I can't confirm or deny that in my actual amp . I should probably do that.
The only other thing I'm able to decipher is that the OT secondaries are 16r and 8r, and the ext speaker jack switches to 16r and wires the speakers in parallel.
All solder joints seem to be original on the speaker jacks, so I think I can rule out any funny business with previous repairs. Plus, the amp has been working fine for years until the other day. Honestly, I don't think the output tubes that it came in with were bad. I re-inserted them and they are pulling current.
Customer says it worked fine for years, had a replacement 15" put in at some point.
One day it just didn't turn on/blows fuses.
I mistakenly told him just to bring the amp, not the cab. So my repairs started with just using the external speaker jack, to test.
My first question, am I correct in that if the normal internal speaker isn't connected via it's native odd 4 pin XLR-ish plug, that the external jack will not work?
I had the amp stable with simply a new fuse and power tubes, but could only get sound using the "ext amp" jack. So at least I know the preamp works. No sound using ONLY the ext speaker jack.
I didn't want to start rewiring the speaker jacks just to be able to test using my shop speaker via a normal 1/4" cable, so I had him bring in the cab.
I plugged in the cab using the 4 pin connector (nothing in the ext jack) and the amp is blowing fuses.
I'm having a hard time deciphering the schematic as far as the OT secondaries at the 2 diagrams for the 4 pin plugs, and also the "F" notation.
I *think* that since F is notated at the cathode of the PI, that the PI Rk isn't grounded unless the 4 pin lug is connected to the cab? I'm not sure if that explains the fuse blowing.
To further cloud things, I've seen schematics where the standby connects to this matrix of madness, but I can't confirm or deny that in my actual amp . I should probably do that.
The only other thing I'm able to decipher is that the OT secondaries are 16r and 8r, and the ext speaker jack switches to 16r and wires the speakers in parallel.
All solder joints seem to be original on the speaker jacks, so I think I can rule out any funny business with previous repairs. Plus, the amp has been working fine for years until the other day. Honestly, I don't think the output tubes that it came in with were bad. I re-inserted them and they are pulling current.
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