Philco 620B Power Up Quirk
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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.
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Start from replacing the rectifier tube.
Measure the B+ voltage, see how it behaves.
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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
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Thanks, guys! I swapped the rectifier tube. Same quirk. I'll start looking at voltages.
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have you tested all tubes?
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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Yes, all tested on a Jackson 648S. All ok except the second section of the 6A7 tested weak.
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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.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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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.
(This post was last modified: 11-26-2020, 08:25 PM by radio1.)
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I swapped out the 6A7 oscillator tube with another and still had the same behavior. Haven't done voltage checks yet.
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Have you had the time to measure the voltages yet? Just curious...
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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.
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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
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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.
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That solution would be consist in what I found in one of my Philcos. The squeal is gone. Thanks for letting us know.
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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.
"I just might turn into smoke, but I feel fine"
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(This post was last modified: 12-11-2020, 07:05 PM by Phlogiston.)
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