04-27-2023, 11:30 AM
Just finished a careful (no shot-gunning, following the schematic as I replaced) total recap of one of these Frog Eyes. Every capacitor, every resistor was replaced. I even checked the IF cans and there is no mica sheet to be found in them. Tubes all tested good (I tried 3 different known good tubes in each socket) Yet despite all of this, and trying the set periodically as I replaced components with no change, It will play beautifully for about 30 seconds, then the volume will drop considerably, and begin to fluctuate every few seconds, from silence, to where I had the volume set (it doesn't blast any louder). Poking around underneath on all the solder joints with a piece of wood dowel, I couldn't find any cold joints or any that would trigger the problem, or that would change the volume level as I poked.
I've only found this schematic, but there are no voltages listed. Does anyone have a different schematic that has a voltage chart so that I can verify the readings I'm getting are correct? And does anyone have any ideas on what could be causing this random volume fluctuation? I did install a 3.5mm cord, just a simple two wire connection to the "high" side of the volume control and ground, and when I play my iPod through the radio, the volume is perfectly fine, no fluctuation, so the problem is perhaps isolated to the RF section rather than the audio output section.
Tapping the tubes, even wiggling each tube as the set was playing did nothing. Poked around on every connection with a wood dowel. Tapped on the volume control shaft. Wiggled the tuning capacitor. Nothing would trigger the problem or correct it when it happened. It's very erratic too. I had it playing flawlessly for over 30 minutes, then all of a sudden the volume drop would occur and the somewhat distorted audio, only to correct itself a minute or two later. Then I'd turn the set off, let it cool down 10 minutes, turn it on and I'd have the volume drop almost immediately.
So I took some plate voltages, even though I don't know exactly what they should be, this is what I'm getting:
On pin 8 of the first 12SK7, I get between 24 and 32 VDC. I monitored it for a while and as the volume dropped, so did the voltage to the low end of the range
On pin 3 of the 12SA7, I get 116 VDC
On pin 8 of the second 12SK7, I get 117 VDC
On pin 6 of the 12SQ7, I get 50 VDC
On pin 3 of the 50L6 I get 125 VDC
On the 35Z5 I get 116 on the plate, and 127 on the cathode (pin 8)
It would seem to indicate a problem with the plate voltage on the first 12SK7, but there is only one wire connected to that tube pin, and it connects directly to the RF coil (T2), but I'm at a loss as to how that would cause the volume fluctuation.
This one is a real head scratcher to me, and I'd welcome any other ideas of what to try to pinpoint the cause.
I've only found this schematic, but there are no voltages listed. Does anyone have a different schematic that has a voltage chart so that I can verify the readings I'm getting are correct? And does anyone have any ideas on what could be causing this random volume fluctuation? I did install a 3.5mm cord, just a simple two wire connection to the "high" side of the volume control and ground, and when I play my iPod through the radio, the volume is perfectly fine, no fluctuation, so the problem is perhaps isolated to the RF section rather than the audio output section.
Tapping the tubes, even wiggling each tube as the set was playing did nothing. Poked around on every connection with a wood dowel. Tapped on the volume control shaft. Wiggled the tuning capacitor. Nothing would trigger the problem or correct it when it happened. It's very erratic too. I had it playing flawlessly for over 30 minutes, then all of a sudden the volume drop would occur and the somewhat distorted audio, only to correct itself a minute or two later. Then I'd turn the set off, let it cool down 10 minutes, turn it on and I'd have the volume drop almost immediately.
So I took some plate voltages, even though I don't know exactly what they should be, this is what I'm getting:
On pin 8 of the first 12SK7, I get between 24 and 32 VDC. I monitored it for a while and as the volume dropped, so did the voltage to the low end of the range
On pin 3 of the 12SA7, I get 116 VDC
On pin 8 of the second 12SK7, I get 117 VDC
On pin 6 of the 12SQ7, I get 50 VDC
On pin 3 of the 50L6 I get 125 VDC
On the 35Z5 I get 116 on the plate, and 127 on the cathode (pin 8)
It would seem to indicate a problem with the plate voltage on the first 12SK7, but there is only one wire connected to that tube pin, and it connects directly to the RF coil (T2), but I'm at a loss as to how that would cause the volume fluctuation.
This one is a real head scratcher to me, and I'd welcome any other ideas of what to try to pinpoint the cause.
Greg V.
West Bend, WI
Member WARCI.org
