10-13-2020, 12:06 PM
I rebuilt the filter block in a model 87 with electrolytic caps which is very similar to the 96 using small value capacitors and a lot of inductance. Got the rest of the chassis sorted and enjoyed the set for abt 4 months. Till one day I was listening to it and the volume dropped to a whisper. I looked in the back to find to find the 80 tube glowing beet red. Quick turned it off.
Have seen this happen before when servicing ham gear and audio amps where the tubes are pushed to their limits voltage wise. In audio amps what happens operator tried to use it w/o the speaker connected. This causes one of both of the output tube to short. Next in line is the output transformer and then choke in the power supply. Finally the rectifier tube and power transformer are toast. From a service standpoint it's not so bad you can make a few dollars but not so great for the owner.
In the case of the 87 it wasn't as bad as it could have been if I wasn't right there to take note of what was happening. I should backup a bit. At this time we where remodeling the house (2013) so the set went off to the shed till a couple of weeks ago. Got it on the bench and apart to find that the first cap in the block was a dead short. Killed by ripple current. This is what caused the rectifier to get red and angry. I'm not sure if I hadn't caught it quickly if it would have taken the transformer. I kinda wish that the diagrams that do have the + and - notations for these later '20s set didn't have them. Caps got replaced with film jobs. It's back working but need to be aligned which requires a long thin hex driver maybe longer than Steve's tool.
Off the soap box
Have seen this happen before when servicing ham gear and audio amps where the tubes are pushed to their limits voltage wise. In audio amps what happens operator tried to use it w/o the speaker connected. This causes one of both of the output tube to short. Next in line is the output transformer and then choke in the power supply. Finally the rectifier tube and power transformer are toast. From a service standpoint it's not so bad you can make a few dollars but not so great for the owner.
In the case of the 87 it wasn't as bad as it could have been if I wasn't right there to take note of what was happening. I should backup a bit. At this time we where remodeling the house (2013) so the set went off to the shed till a couple of weeks ago. Got it on the bench and apart to find that the first cap in the block was a dead short. Killed by ripple current. This is what caused the rectifier to get red and angry. I'm not sure if I hadn't caught it quickly if it would have taken the transformer. I kinda wish that the diagrams that do have the + and - notations for these later '20s set didn't have them. Caps got replaced with film jobs. It's back working but need to be aligned which requires a long thin hex driver maybe longer than Steve's tool.
Off the soap box
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got it!
Terry
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