Hello, I'm having a hard time dialing in the RF alignment on this radio. IF alignment has been done with all four compensators peaked. I set the high frequency at 1500 and adjust #7 to peak on the second maximum peak then attempt to do the low frequency #9 at 580 I can't find the tone anywhere. The best I can do is set the gen to 740 then I find the tone way down at 500. I can not adjust #9 to bring it up to 740. The closest I can get is 700. When I go back to 1500 I find it is way off. There is a huge amount of interaction between #7 and #9. I am using an Eico 330 for the gen and a frequency counter to verify I am on the correct frequency. Bottom line is I can't get anywhere close to the low frequency setting of 580.
I have cleaned the 7/9 compensator fins and micas. They were pretty clean as it was.
See note (bottom right) in the alignment instructions.
I would take a stab at Cap # 6 and resistor #5. More modern sets have tuning caps where the oscillator section has a proportionately lower capacity than the RF section. I am guessing that if Cap # 6 leaks or has a higher than stated (or lower than stated?) capacity, you will not be able to get the oscillator to track correctly, no matter how much you adjust the compensators (trimmers).
Hope this helps.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
MrFixr55, Items 5 and 6 are osc. coils. Am I using the wrong schematic or are you? I did replace resistor #11 (70K) with a 68K. I noticed my radio has a 20 ohm resistor between the cathode and ground of the 6A8 tube where the schematic does not show this but instead cathode is wired directly to ground. Is this likely a possible cause?
I have been doing some checking and found caps 23 and 24, both 100pf, are supposed to be inside the 2nd IF can but they are not there or anywhere else in the radio. On further investigation it looks like the caps are part of the 20A or 20B trimmers as there are several mica layers in there. I did change the resistor #21 (51K) with 50K. This probably is not the problem.
Dan
(This post was last modified: 02-16-2026, 03:11 PM by dconant.)
Oopsie, my bad. Was looking at 38-12, clicked on the wrong one, But I think that the issue is the same. IF frequency on this set is 470KHz Check Cap # 10, a 3500 pF (0.0035 uF) mica cap and Cap#14, a 250 pF (0.00025 uF) cap. My theory is that if either of these caps are bad, you will have to overcompensate to get the beat frequency to be 470 KHz and therefore can't get the dial to "track". I may be wrong, but this is my guess. The IF is a fixed frequency. If you can peak the IF to 470 KHz, then the IF transformers are good.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
The IF is peaked up nicely. I checked the #10 3500pf cap and it measures pretty close. I don't have one but I do have a 2200pf and 1400pf I could wire in parallel. Would that be close enough? As for cap #14 I believe that is one of the leaves inside the #7 trimmer. This is similar to what is in #20 (the 100pf caps). So if #14 is part of the #7 trimmer wouldn't that cap change value as the trimmer is adjusted along with what ever else is going on in that tree. The parts list says #14 is part of #7. If I start with #7 fully clockwise and back off I can hear the tone multiple times as I back off the screw when the dial is at 1500 some strong and some weak. If I adjust as the note in the instructions say I can't find 580.
General question: As is back off a trimmer is the capacitance increasing or decreasing?
Turning the trimmer (compensator) screws CCW (Counterclockwise) lowers the capacitance because the distance between the plates increase. It is a little strange that they have Cap # 14 as part of Cap #7. It is possible that a shoulder washer between C14 and C7 would prevent squeezing the stack from changing the value of C14. Consider reassembly but not putting C14 in the circuit but replacing it with a fixed sealed cap.
I would consider replacing the 3500pF cap with the 2 caps that you have. The error would be about 3%, well within the 10-20% error allowed for radios of the time.
I suppose that the diagnosis would go 2 ways: First off, verify that your signal generator is accurate by checking against a radio with digital tuing. Your car radio would work fine if you don't have a portable or home radio that is digitally tuned. Next, determine if the RF section of tuning is accurate on this radio. this would be done by disconnecting the grid cap and connecting it to the antenna of another radio, tuning the other radio and signal generator to the same frequency and rotating the dial of the 38-10 that you are trying to repair to verify that the signal in the radio used to detect the signal generator peaks at the correct dial position on the radio under repair.
The final part is more difficult and that would be to verify that the oscillator section of the radio under test tracks at 470KHz above indicated frequency on the dial of the radio under test. I have to do some thinking on that if you don't have a frequency counter.
I don't think that the 20 Ohm resistor between cathode and ground makes much difference. Considering that the plate resistance of this tube is likely over 100KOhms, the 20 Ohm resistor is a drop in the bucket. I don 't know what the advantage of having that resistor in the cathode circuit is, given the total tube cathode to plate resistance. The EEs in our gang here may have a better explanation.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Well well well looky what I found. Read the last paragraph in the picture. Got the radio much closer now but not close as I'd like. My favorite stations, 740 comes in at 710, 800 at 780 and 1180 at 1210. If I dial in 1180 in to 1180, 740 and 800 get weaker. So I guess I'll leave it at that. I'll change the 3500 cap and see if that helps. The lower frequencies seem to get weaker the lower in frequency I go but that might be weather related as it is kind of foggy here. The top paragraph talks about the 20 ohm resistor.
BTW I'm not sure where I found this as I downloaded a changes PDF from somewhere but don't remember where. I guess I just got lucky I found I had this document.
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