Again, guys, many thanks for your assistance. No hurry at all George. This evening's trouble-shooting was interrupted by a call from work, so you have my complete understanding when it comes to work demands.
As I said, I've been through every connection and solder joint. No, no over-sized cap leads "requiring" drilling out the thru-holes.
The input connections, from the jack, through the pot (attenuator, actually), and to the board all check out OK. But taking the pot out of the circuit is on my list of hail Mary's.
I'll try swapping out OPT's. Continuity is good on both primary and secondary, with about 126 ohm on the primary. Sound about right? Anyway, I suppose something could have failed subtly and only shows up in operation.
That leaves the speaker jacks. The plate they're mounted on is metal, so I suppose a shorted hot is possible.
Failing those things, I'll set up for George's channel-jumping procedure. Probably won't have time for that until the weekend.
As I said, I've been through every connection and solder joint. No, no over-sized cap leads "requiring" drilling out the thru-holes.
The input connections, from the jack, through the pot (attenuator, actually), and to the board all check out OK. But taking the pot out of the circuit is on my list of hail Mary's.
I'll try swapping out OPT's. Continuity is good on both primary and secondary, with about 126 ohm on the primary. Sound about right? Anyway, I suppose something could have failed subtly and only shows up in operation.
That leaves the speaker jacks. The plate they're mounted on is metal, so I suppose a shorted hot is possible.
Failing those things, I'll set up for George's channel-jumping procedure. Probably won't have time for that until the weekend.
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I suppose anything is possible, but I have never seen it happen. 126 ohms is a realistic number, and if both OPT's measure within 10% of each other, they are probably OK.
There was a builder in England who had a dead channel. It turned out to be a short in one of the speaker jacks. Took a while to find it because it was pretty far down on the "likely suspects" list. How to test for it? Unsolder the OPT wires from the jacks and connect them directly to a speaker with clip leads. It is also possible to create a short if the cathode feedback connections are used, but one channel has the leads swapped at the PC board.
Is the dead channel completely dead, or is there some faint sound coming out? Often you need to disconnect the input from the good channel to be able to hear it.
Another simple test. Connect a junk speaker to the dead channel. Remove the output tube from the good channel (we want to kill that channel and you should never run a tube amp without a speaker). Connect no input to the amp and turn the volume control all the way down. Turn the amp on and allow it to warm up for about a minute, but do not let the tubes get too hot to touch, or swipe an oven mitt from the kitchen. With the amp on and running pull the 12AT7 from the socket. You should hear at least a loud pop, and probably some scratching sounds as the tube is wiggled. If so the output stage and speaker wiring is OK. If there is no sound at all pull the output tube. Again there should be a pop. If none the problem has to be in the OPT / speaker jack connection. Unplug the amp and allow 5 minutes for every thing to discharge before replacing the tubes.
Fixed.
Swapping OPT's led to the discovery of the failed component, though it wasn't an OPT. I'd decided to work my way in from the back of the amp. So I swapped secondary connections from R to L and L to R. The right speaker came up, plain as day. Left channel is clearly good all the way to... the binding posts.
The hot binding post on the left channel was open. Not shorted, open. The shaft that passes through the insulated barrel was snapped right in two (they were cheap), but the ends were apparently in close enough proximity that the force of my test probe (in my first pass of troubleshooting) would close it at least long enough for my brain to say, "There. The meter beeped. You've got continuity so let's move on to what's really wrong". The lightweight wires hanging off the post wouldn't torque it enough to do the same thing.
Thinking back, the only thing I can come up with is that I snapped the post while moving the amp to change tubes the morning it stopped working (unlikely - seems like I would have noticed something that forceful) or I just jostled it enough to pull it apart.
Anyway, thank you, everyone, for your generous input.
Swapping OPT's led to the discovery of the failed component, though it wasn't an OPT. I'd decided to work my way in from the back of the amp. So I swapped secondary connections from R to L and L to R. The right speaker came up, plain as day. Left channel is clearly good all the way to... the binding posts.
The hot binding post on the left channel was open. Not shorted, open. The shaft that passes through the insulated barrel was snapped right in two (they were cheap), but the ends were apparently in close enough proximity that the force of my test probe (in my first pass of troubleshooting) would close it at least long enough for my brain to say, "There. The meter beeped. You've got continuity so let's move on to what's really wrong". The lightweight wires hanging off the post wouldn't torque it enough to do the same thing.
Thinking back, the only thing I can come up with is that I snapped the post while moving the amp to change tubes the morning it stopped working (unlikely - seems like I would have noticed something that forceful) or I just jostled it enough to pull it apart.
Anyway, thank you, everyone, for your generous input.
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