I recently refurbished a 1935 Philco 620B with new electrolytic caps, new tubular caps, and new resistors for those that were out of tolerance. The radio plays well but has a quirk in its power up. When I first turn it on, there is the expected delay as tubes warm up. After about 13 seconds I'll hear low volume audio. About 8 seconds later there is a click and the low volume audio instantly becomes normal in volume. The radio continues to play normally thereafter. Where do I start in trying to eliminate this quirk? Is it likely a tube issue? I've tried swapping the output 42 tube with another and still had the quirk.
Start from replacing the rectifier tube.
Measure the B+ voltage, see how it behaves.
You need to work your way back through the various sections with an oscilloscope or a meter to see if you can determine where the change occurs. I would start with the Power supply voltages to see if you see a change there when the volume switches.
Bill
Thanks, guys! I swapped the rectifier tube. Same quirk. I'll start looking at voltages.
have you tested all tubes?
Yes, all tested on a Jackson 648S. All ok except the second section of the 6A7 tested weak.
6A7 is known for that.....
Start measuring the voltages. See if some jump somewhere (either B+ or a plate of a tube)
correlates with it.
My 70 does this, somewhat different, using the old globe Philco 27 osc tube the radio ramps up smooth, using a newer ST RCA 27 the radio goes through these clicks with a high raspy sound just before it settles and clicks in around 12 to 15 seconds. The older tube test just above 50% on a Knight emission tester, the RCA tests around 70%. Don't know if this is symptomatic of some other problem as of yet, but strange you mention it when I have a similar sound. Both tubes test okay on this type tester with no shorts or gas.
I swapped out the 6A7 oscillator tube with another and still had the same behavior. Haven't done voltage checks yet.
Have you had the time to measure the voltages yet? Just curious...
I started but ran into a different radio behavior after I turned the chassis inverted. During power up I would initially get low volume, then the click, then a high pitch loud squeal. This behavior was not consistent. Sometimes the radio would work as it previously did with the chassis upright. I sprayed Deoxit into the volume control as best I could and I moved some wires slightly that looked like they might be in hard contact with a metal edge. Also, when I touched the cap of the 75 tube I got a lot of loud hum and some squeal. I replaced the 75 tube. The radio now powers up to a low volume state with no click. As I turn the volume control up, a high pitch tone/squeal starts about midrange on the volume control and gets louder as I increase volume. I will start again to measure voltages and see what I get.
Larry make sure the shield is on that 75 and that the wire to the grid cap is run inside the shield too or the radio will squeal!
Ron
That was it, Ron! I had the 75 tube cap wire outside the shield. Thank you!
The click I was initially getting is not present after installing a different 75 tube.
That solution would be consist in what I found in one of my Philcos. The squeal is gone. Thanks for letting us know.
One thing to add, is that the 27 (227) tube, Brobertson mentioned, was one of the first to have a cathode sleeve around the filament. These tubes often have high resistance shorts on the h/k. This will make noise. It can also cause hum. Often times the H/K problem will come and go as the tube heats up.