04-03-2015, 12:39 PM
My turn to be perplexed...
Been working on this Stewart Warner 950. It's the early version, 7 227 tubes, an 80 and a pair of 45's.
Tonight, I slid the coil shields into place and checked out the reception (yesterday I had checked it out without the shields in place, and got just one station and of course lots of oscillation). It played pretty well, for a while. After a bit, it started sort of sputtering and then went away completely.
I did some troubleshooting and found that the large voltage divider resistor was open on both halves (it had some tell-tale corrosion in spots). I subbed in some temporary replacements... B+ on each tap now, but still no sound.
Began tracing things out. Good B+ on the 45's, but none on the driver tube. Darn, I thought to myself.. the interstage transformer must have opened up. Check B+ on the other side of the input.. hmmm.. nothing there, either.
Checked the B+ on the plates of all the other 27's.. none there either. Curiouser and curiouser. Especially since according to the schematic, there's nothing but a solid wire between the divider resistor and the bus that goes to all of those plates.
Tracking where that open is will be a MAJOR PITA, since the wiring harness has this beautiful truss holding it all together, and to trace the wire from the divider to where ever that bus is will require that I remove that truss.
Sooooo... I decided to jumper in a 10K resistor between the proper tap on the divider to the B+ side of the interstage transformer (which should, and did, put B+ onto all those other plates as well). Turned on the radio and VIOLA! (as I brought the voltage up on the variac it made a noise not at all unlike an out of tune string instrument.), there was sound again.
Next, I bypassed that extra 10K resistor and everything is working very well indeed.. better even than it was before.
So much for the troubleshooting.. now I'll have to actually trace down the open circuit.
Been working on this Stewart Warner 950. It's the early version, 7 227 tubes, an 80 and a pair of 45's.
Tonight, I slid the coil shields into place and checked out the reception (yesterday I had checked it out without the shields in place, and got just one station and of course lots of oscillation). It played pretty well, for a while. After a bit, it started sort of sputtering and then went away completely.
I did some troubleshooting and found that the large voltage divider resistor was open on both halves (it had some tell-tale corrosion in spots). I subbed in some temporary replacements... B+ on each tap now, but still no sound.
Began tracing things out. Good B+ on the 45's, but none on the driver tube. Darn, I thought to myself.. the interstage transformer must have opened up. Check B+ on the other side of the input.. hmmm.. nothing there, either.
Checked the B+ on the plates of all the other 27's.. none there either. Curiouser and curiouser. Especially since according to the schematic, there's nothing but a solid wire between the divider resistor and the bus that goes to all of those plates.
Tracking where that open is will be a MAJOR PITA, since the wiring harness has this beautiful truss holding it all together, and to trace the wire from the divider to where ever that bus is will require that I remove that truss.
Sooooo... I decided to jumper in a 10K resistor between the proper tap on the divider to the B+ side of the interstage transformer (which should, and did, put B+ onto all those other plates as well). Turned on the radio and VIOLA! (as I brought the voltage up on the variac it made a noise not at all unlike an out of tune string instrument.), there was sound again.
Next, I bypassed that extra 10K resistor and everything is working very well indeed.. better even than it was before.
So much for the troubleshooting.. now I'll have to actually trace down the open circuit.
