07-31-2018, 05:45 PM
http://www.nostalgiaair.org/Resources/207/M0013207.htm
Bart 260 came out from hiding and commenced to help find the issue. No screen grid voltage on the 6A8 and 6K7. Measured #18 and seemed ok but still no voltage at the sg's. Touched it and it can to life. Decided to replace it to find that it was cracked along it's length. Checked it's friend #16 and it was high it got replaced too.
So it's back working such as it was still has a hum issue, ac modulation, and a rattle in the spkr. When I got the set it was missing the 25Z6 and I didn't have one so I built an adapter so I could use a 25Z5 (tubes are electrically the same but use different bases). This made the tube kinda tall but worked.
Started in on the hum problem. Didn't change w/the volume setting so it probably would be in the p/s, leaky tube, or ground (b-) loop. Found that by adding more capacitance to the cathode of the 25Z6 the hum decreased but it was still very noticeable. But it had new filter caps and they where a bit larger than the original 2x 47mfd). Maybe I got a dud filter cap? Rebuilt another filter can. Made sure all of the connection were good installed it and no joy. Still hummed. Maybe my 25Z5 was leaky? Designed a solid state 25Z6 to replace the 25Z5 and adapter. Worked great no burning hot rectifier tube! But still hummed. So I wanted to see if I could find which stage the hum was coming from. To do this I built an other adapter that connected only the heater pins of the tube when inserted. Now I can plug in each tube and all of the elements are disconnected except for the heater so all the other tubes will have heater voltage. I found that the tube that killed the hum was the 25A6. Tried a couple on spares still hummed. [Image: http://philcoradio.com/phorum/images/smi..._crazy.gif] Started measuring resistances in the power supply and found something very curious. The resistance from #13 + side to pin 4 of the 25A6 showed 400 ohms ! (should show a short) Who ever replaced the filters the 1st time wired it wrong so the filter choke and #13 wasn't being utilize in the circuit. The hv output was connected from the 25Z6 cathode to the hv point in the chassis. Rewired the filter and hum is gone! [Image: http://philcoradio.com/phorum/images/smi...wisted.gif]
As with a lot of later '30s and early 40's Philcos the pilot lamp socket need a rebuild, the bulb burned out and it seem to have welded it self to the socket. Rubber was brittle so I couldn't push the bulb in the release it from it's socket. It met w/the irresistible force of a pair of vise grips. Replaced the rubber w/a nylon washer and all is good w/it.
It had picked up a bad habit of oscillating at the low end of the band. Seemed like a good time to the do an alignment. IF was way out a good turn and a half per trimmer. That cleared up the oscillating problem.
Spkr is better but still has a slight resonants. I used some thinned out white glue to reglue the edge and spider. It has a few small tears too that got a dose of glue.
It's been playing for the last few hours and it hasn't burst into flames so guess it's good.
Bart 260 came out from hiding and commenced to help find the issue. No screen grid voltage on the 6A8 and 6K7. Measured #18 and seemed ok but still no voltage at the sg's. Touched it and it can to life. Decided to replace it to find that it was cracked along it's length. Checked it's friend #16 and it was high it got replaced too.
So it's back working such as it was still has a hum issue, ac modulation, and a rattle in the spkr. When I got the set it was missing the 25Z6 and I didn't have one so I built an adapter so I could use a 25Z5 (tubes are electrically the same but use different bases). This made the tube kinda tall but worked.
Started in on the hum problem. Didn't change w/the volume setting so it probably would be in the p/s, leaky tube, or ground (b-) loop. Found that by adding more capacitance to the cathode of the 25Z6 the hum decreased but it was still very noticeable. But it had new filter caps and they where a bit larger than the original 2x 47mfd). Maybe I got a dud filter cap? Rebuilt another filter can. Made sure all of the connection were good installed it and no joy. Still hummed. Maybe my 25Z5 was leaky? Designed a solid state 25Z6 to replace the 25Z5 and adapter. Worked great no burning hot rectifier tube! But still hummed. So I wanted to see if I could find which stage the hum was coming from. To do this I built an other adapter that connected only the heater pins of the tube when inserted. Now I can plug in each tube and all of the elements are disconnected except for the heater so all the other tubes will have heater voltage. I found that the tube that killed the hum was the 25A6. Tried a couple on spares still hummed. [Image: http://philcoradio.com/phorum/images/smi..._crazy.gif] Started measuring resistances in the power supply and found something very curious. The resistance from #13 + side to pin 4 of the 25A6 showed 400 ohms ! (should show a short) Who ever replaced the filters the 1st time wired it wrong so the filter choke and #13 wasn't being utilize in the circuit. The hv output was connected from the 25Z6 cathode to the hv point in the chassis. Rewired the filter and hum is gone! [Image: http://philcoradio.com/phorum/images/smi...wisted.gif]
As with a lot of later '30s and early 40's Philcos the pilot lamp socket need a rebuild, the bulb burned out and it seem to have welded it self to the socket. Rubber was brittle so I couldn't push the bulb in the release it from it's socket. It met w/the irresistible force of a pair of vise grips. Replaced the rubber w/a nylon washer and all is good w/it.
It had picked up a bad habit of oscillating at the low end of the band. Seemed like a good time to the do an alignment. IF was way out a good turn and a half per trimmer. That cleared up the oscillating problem.
Spkr is better but still has a slight resonants. I used some thinned out white glue to reglue the edge and spider. It has a few small tears too that got a dose of glue.
It's been playing for the last few hours and it hasn't burst into flames so guess it's good.
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!
Terry
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