[NostalgiaRadioTime]

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.

   

[klondike98]

Just a thought...any chance you have an extra 12SK7 to try?

[Ron Ramirez]

I agree with Bob. My first thought was R4 and/or C10, but if they are new and good, it isn't those parts. That leaves the 12SK7 RF amp tube.

Although, if the primary of T2 is doing odd things (e.g. intermittent), that could cause it as well. But try the 12SK7 first. If it is ok, then it would be prudent to monitor the plate voltage of the 12SK7 RF amp for fluctuations.

[RodB]

Check the screen voltage of the first three tubes. It should follow the fluctuation of the 12SK7 plate. If it does then check the voltage at the right side of R4 to see if that is low or fluctuates. Strange symptoms.

[NostalgiaRadioTime]

Thanks for the ideas, much appreciated. I managed to find a schematic with a full voltage chart and spent the afternoon checking voltages, and tracing through the schematic, double checking that everything was connected correctly. Thankfully it was. Here's what I measured, and you can compare those to the chart on the attached schematic:


Tube             Pin 1          Pin 2                  Pin 3             Pin 4            Pin 5           Pin 6             Pin 7             Pin 8


12SK7 RF       0           27.4 VAC               0                  -1.6 VDC         0             38.3 VAC         40.8 VAC        36.4 VDC

12SA7           0            41.1 VAC              114 VDC         38.2 VDC    -2.6 VDC      0                     55 VAC         -.55 to -.70 VDC

12SK7           0            27.5 VAC              0                    -.35 VDC        0             38.5 VDC          14.2 VAC          116 VDC

12SQ7          0              -.7 VDC               0                    -2.8 VDC        0             49 VDC              14.2 VAC          0

50L6            0               89 VAC            120 VDC             115 VDC         0            0                       54 VDC           6.4 VDC

35Z5            0               121 VAC           116 VAC              0              115 VAC      0                       89 VAC            114 VDC


I monitored Pin 8 of the 12SK7 RF tube and was 36 VDC at normal volume. As volume fluctuated down to minimal, it dropped to 26 VDC

I monitored the right side of R4 and it was 115 VDC at normal volume, dropping to 110 VDC when quiet.  Measured at the positive side of C11

I actually have two of these radios, and I just replaced the electrolytics on the second chassis, plus two wax capacitors and tried it and it has the same volume fluctuation as this one! 

One abnormality on both is R9 is shown as a 1 meg resistor, yet on both chassis, it's a 220K ohm. I also noted the center gang connection on the tuning capacitor lights up my test light to chassis ground, as does the other end of the wire it's connected to on the OSC coil. 

So I'm still at a loss. I had it playing over 30 minutes again flawlessly, then without touching it, the volume crackles and drops. I am wondering if its a tube socket, yet I get no crackling or any change in volume by wiggling on the tube or poking against the socket pins underneath. 

   

[NostalgiaRadioTime]

Here are the pin 8 voltages that got cut off:

12SK7 RF 36.4 VDC

12SA7 -.55 to -.70 VDC

12SK7 116 VDC

12SQ7 0

50L6 6.4 VDC

35Z5 114 VDC

[RodB]

So maybe there's a problem in a socket connection.

[Ron Ramirez]

I'm frankly surprised the plate (and screen) voltage of the RF amp tube was designed to be that low.

[NostalgiaRadioTime]

It's been a long time since I've had a radio this stubborn and uncooperative! I spent the afternoon on it, totally redoing the joint on the terminal strip where R4 and C10 join, even replacing both the cap and resistor to eliminate the possibility of one or the other being defective. That caused no change. So I monitored voltage fluctuations at these points when the radio was playing normally, verses when the volume would change (now either really loud, or really quiet and somewhat garbled initially before clearing itself up again):

On Pin 6 of the 12SK7 RF tube at normal volume I get 38 VDC, when the fluctuation and sputtering begins, it drops to 27 VDC

Measuring on each side of the 1200 ohm resistor between the e-cap positives (R3), I measure 115 VDC normal / 113 VDC when the volume drops at the junction with R4, and 127 VDC on opposite side of R3. I re-soldered the joints on either side of R3 just to make doubly sure there's no bad or cold solder joint there.

Measuring right on the point where C10 & R4 join, I get 38 VDC at normal volume, and as it sputters and fluctuates so does the voltage, down to 28 VDC at low volume.

So then I did another test. When the volume dropped, I plugged in my iPod (the 3.5mm cord that is attached to the high side of the volume control). This cuts off the RF section. I had to turn the volume up on the radio, which is normal when using the cord, and the volume level never changed. At one point, I could hear the strong local station bleeding through, which indicated the volume again changed back to normal on the RF side. What this seems to indicate is the problem is indeed isolated to one or more of the tubes before the volume control (the two 12SK7's and the 12SA7) and not the Audio section 12SQ7 and 50L6.

The next test I did, as i monitored the voltages above, was when the volume dropped, I immediately poked on each of the pins on those 3 tubes, and none of them caused any change. I thought that would rule out a bad tube pin connection in the socket. But now I'm not so sure.

I then tried once again a completely different set of NOS tested good tubes. Every tube except the rectifier. And the problem was still there. So I reinstalled the original tubes I had in all along and the radio was all but silent. Very low volume and a bit garbled. Eventually it came back to life at normal volume. But this is with the same tubes I had been using all along, even put the 12SK7's back in the same tube sockets. So I still can't entirely rule out a bad tube socket, but which one!

I'm totally at a loss as to what to replace next. There's not much left that I haven't! Could it be:

1.) a faulty volume control that only acts up at odd times and only at lower volume levels? (I checked the connections there too and nothing I poked on triggered the problem). The radio has TONS of volume when playing normally

2.) Faulty RF coil (the plate of the 12SK7 RF tube connects directly to it)?

3.) Faulty OSC coil?

4.) An intermittent short in one of the IF cans?

5.) Replace tube sockets (these are the type of sockets with a molded bottom, so the pin sleeves are not exposed on the bottom where they could be crimped tighter for a better connection)?

All of these replacements require a lot of work, especially the tube sockets. I really don't want to go down that rabbit hole unless I could isolate it to one single socket because I know what will happen. Replace #1, no change, then do another, and another until finally I have all 6 replaced and it still acts up. I think I have one or two of the Universal RF and OSC coils here yet that AES used to sell but if there's an intermittent short in the IF can, that might be a deal breaker unless I could locate a similar can to replace it with.

Any thoughts on where to proceed next would be greatly appreciated!

[morzh]

I would not go as far as replacing sockets.

However, I would

- when the radio warms up, and the problem surfaces up

1. Wiggle the tubes, applying a bit more force than the poke of a wooden dowel. Especially the 12SQ7
2. Maybe wiggle (forcefully) the volume pot.
3. Maybe during the low vol touch grids with a 1nF cap (or if on dry non-basement surface - with 10nF, or if with isolation transformer - with a screwdriver touched by finger), starting from the output and back to RF, and see where the vol goes from Hi to Low.

That would be my start.

And, yes, if possible, the 12SQ7 should be replaced.

[NostalgiaRadioTime]

Thanks, Mike. Oh believe me, I did wiggle the tubes from the top and the bottom with considerable force, enough that it if it was a poor pin connection, it would've made a crackle or changed in sound. I'm confident the 12SQ7 is good. I tried 3 different tubes that are all NOS and tested perfectly, diodes too. No rattles, etc. The vendor I got them from is very reliable and he tests them before selling them (and marks the settings on the box), and I double checked them myself and found them to be as he indicated.

The only reason I thought of needing to change a socket is because these black molded sockets cannot be serviced. if the tube is too loose or not making a good connection around the pins, there's no way to get inside to crimp the hole tighter.

But I will try your suggestion with the 1nF cap tomorrow. Thanks!

[morzh]

1nF is for safety reason, to limit the leakage current in case of no isolation xfmr and a wet basement.
Otherwise, as I said, you could touch it, or use a larger cap.

[RadioSvit]

Hi to all . Hello Greg, it seems to me that you need to pay attention to the AVC system, - R 9, R10, C 2, C7, C8. And first of all, I would replace the radio tube 12SQ7 .One of the two diodes of this tube can also greatly affect the operation of the AVC

[462ron]

Hi Greg, just a thought, do you have any freeze spray? If so, give some components in suspect circuits a shot when the problem is happening.

Ron

[morzh]

Peter,

Greg said he replaced the 12SQ7 three times by NOS tubes.