02-18-2023, 03:29 PM
I am looking for help and advice on my 38-690 Code 125 set. I completely went through the radio in 2017. New caps, resistors and all the normal work needed. I did a proper alignment and the magnetic tuning works perfectly. Sensitivity is between .5 and 1uv on the broadcast band. A couple years ago the interstage audio transformer developed a case of the crackles, so I did a capacitor resistor mod to keep the B+ off the primary. Audio is excellent.
I've always had an oscillation/motorboating condition on bands 2-5, and the dial does not track properly there as well. the lower portion of the band is squeezed up higher on the dial. For example, when setting the 18 Mhz osc adjustment, it peaks at 18 on the dial with the image below by 2X the IF freq. I verified this on the spectrum analyzer, and the oscillator tracks above the dial. 15 Mhz comes in at about 15.6 on the dial, so the tracking rapidly deteriorates. There is a 1300 PF padder, 43, that is common to all 4 bands that have this condition. I tried a fixed cap, and there was no difference.
The motorboating is on parts of the dial that are most sensitive, and appears to be feedback into the RF amplifier. If I short the antenna and ground connections, most of the noise goes away. Adjusting the RF and Antenna compensators creates more motorboating as the circuits reach resonance or peak tuning.
The main chassis is Run 4 as stamped, but the RF unit doesn't seem to match any of the run numbers as based on the Philco service notes. I can't match any paint dots on the osc coil, nor do I find the extra terminal for the 250 PF cap in run 3. I did add the 100 ohm resistor based on the run 4 service notes. I even looked at the 38-116 Code 125 service notes, with nothing of note there.
I have the chassis on the bench, and will be pulling the RF unit out again, tracing the wiring etc. I did verify that the correct sections of the tuner cap were switching in and out on the correct bands, which was an easy place to mess up. This unit was also sort of hacked when I got it, and the Osc tube socket was replaced.
At this point, I am not sure if it makes more sense to look for another RF unit from a 38-116, 690, Code 125, or a 38-116 earlier run with the different RF unit. I know Ron has done extensive work on these sets, and I read his restoration post on this model, which seemed to be a pain as well.
I appreciate all your suggestions and comments.
I've always had an oscillation/motorboating condition on bands 2-5, and the dial does not track properly there as well. the lower portion of the band is squeezed up higher on the dial. For example, when setting the 18 Mhz osc adjustment, it peaks at 18 on the dial with the image below by 2X the IF freq. I verified this on the spectrum analyzer, and the oscillator tracks above the dial. 15 Mhz comes in at about 15.6 on the dial, so the tracking rapidly deteriorates. There is a 1300 PF padder, 43, that is common to all 4 bands that have this condition. I tried a fixed cap, and there was no difference.
The motorboating is on parts of the dial that are most sensitive, and appears to be feedback into the RF amplifier. If I short the antenna and ground connections, most of the noise goes away. Adjusting the RF and Antenna compensators creates more motorboating as the circuits reach resonance or peak tuning.
The main chassis is Run 4 as stamped, but the RF unit doesn't seem to match any of the run numbers as based on the Philco service notes. I can't match any paint dots on the osc coil, nor do I find the extra terminal for the 250 PF cap in run 3. I did add the 100 ohm resistor based on the run 4 service notes. I even looked at the 38-116 Code 125 service notes, with nothing of note there.
I have the chassis on the bench, and will be pulling the RF unit out again, tracing the wiring etc. I did verify that the correct sections of the tuner cap were switching in and out on the correct bands, which was an easy place to mess up. This unit was also sort of hacked when I got it, and the Osc tube socket was replaced.
At this point, I am not sure if it makes more sense to look for another RF unit from a 38-116, 690, Code 125, or a 38-116 earlier run with the different RF unit. I know Ron has done extensive work on these sets, and I read his restoration post on this model, which seemed to be a pain as well.
I appreciate all your suggestions and comments.
Tony
“People may not remember how fast you did a job, but they will remember how well you did it”
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