Magnavox CG Phantom Remote Alignment / Repair

[FrankieKat]

Greetings VK! I have a '61 Mag Concert Grand that I have been making steady restoration progress and currently trying to get the Phantom Remote control working correctly now. The transmitter functions generally, however no matter what I do, I cannot correctly set all of the frequencies for each button.

If I play with them I can intermittently get volume up/down and on/off to work, but never reliably. I can sort-of tune in the volume up button to trigger a reject/search -- so I know the receiver is working on all functions -- however can never get the actual reject/search button (the first tuning pot in the series) to do anything. Unfortunately the alignment instructions say "the unit requiring alignment should be adjusted against a transmitter which has been checked for accuracy and proper operation". Slim chance of that I'm afraid.

Not dissimilar to other old RF remotes of the era, this one has a transmit-RF carrier of 37KHz and modulates each keypress to an AF frequency for each relay in the receiver: Search/reject: 95Hz, Vol up: 85Hz, Down: 76Hz, On: 67Hz, Off: 53Hz. The key contacts for the five functions (Search / Reject is the same command, and the head unit decides which to do based on the source selector) are connected to five 800ohm pots in series, so setting them in order is obviously critical. Perhaps somehow the Search/Reject tap is no longer providing enough resistance to set the AF oscillator to 95Hz, which is why I can get it to search/reject on the Vol Up by tuning a combination of the first and second pot.

Schematic for quick ref:



Here's what I've checked:

1. Key contacts are making good contact on all functions
2. Verified good series resistance and function of all pots and values
3. Spot checked resistors and original caps for leakage and correct values (all seem fine on my Solar bridge -- don't want to re-cap this unless I have to in order to preserve originality as much as possible!)
4. 22.5 V battery good
5. Used wire loop antenna to scope to verify correct RF carrier frequency (37KHz)

I have tried probing the output of the AF oscillator to see the frequencies that are actually being produced, but have not really received conclusive results. If I probe at the junction of C4 and Q3 I get a pulse that I measure as 277Hz and the frequency pot adjustments seem to affect the amplitude, not the frequency.



If I take C4 out of circuit and probe from there I get a very rough waveform that I measure as 338Hz (which is 4x 95 Hz, the frequency for Search/Reject… coincidence?). Both of these are with R13 also out of circuit… perhaps I need to test again with that back in.



I even tried using SDR# connected to a PC sound card to narrow in on the 37KHz band and demodulate audio, which seems to pick up a lot of the low frequencies but the FFT isn't sensitive enough to give me a clear measurement of 95Hz vs 85Hz, etc… oh well, worth a shot.

Would love to hear any tips or experience with working with these. Given the fairly unpredictable output, and the fact that the receiver does receive and trigger all of the functions it seems to me that the issue is in the transmitter/remote. Wondering if perhaps it's that the AF oscillator is running way too fast which is why I'm seeing the 277-338Hz frequencies, and then perhaps it's the second or third order harmonics that are strong enough to trigger the receiver, and would explain why I can get maybe 2 or 3 buttons out of 5 to work at a time but never all 5. If so, would this mostly likely point to a bad passive component or perhaps a bad transistor?

Thanks for any help or wisdom!

-FK