[LarryFS]

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.

[morzh]

Start from replacing the rectifier tube.

Measure the B+ voltage, see how it behaves.

[Bill Bacco]

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

[LarryFS]

Thanks, guys! I swapped the rectifier tube.  Same quirk.  I'll start looking at voltages.

[morzh]

have you tested all tubes?

[LarryFS]

Yes, all tested on a Jackson 648S.  All ok except the second section of the 6A7 tested weak.

[morzh]

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.

[radio1]

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.

[LarryFS]

I swapped out the 6A7 oscillator tube with another and still had the same behavior.  Haven't done voltage checks yet.

[Scott D]

Have you had the time to measure the voltages yet? Just curious...

[LarryFS]

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.

[462ron]

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

[LarryFS]

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.

[Scott D]

That solution would be consist in what I found in one of my Philcos. The squeal is gone. Thanks for letting us know.

[Phlogiston]

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.