This was a total rebuild of a 37-610 Code 121 chassis. Except for the tuner, which had the resistors and paper caps replaced. I was able to calibrate the 470 kc IF and both shortwave bands, but get nothing on AM. I've done a thorough point to point check against the schematic and everything looks OK. The only discrepancy I see is at the AM oscillator coil. (see image) It measures 8 ohms where the schematic shows its value at .8 ohms. Seeing that it has continuity, I wouldn't expect its value to drastically change. Perhaps this is a typo? If not, could it be preventing the signal flow? I only get some static when the tuner cap is turned wide open. I've spent several days trying to find the source of this problem. Where else can I check? I need some expert guidance please. And thanks in advance!
The first question I would ask in the troubleshooting process is : "did it receive any stations before the restoration process started"? If so, then the problem was created during the servicing so recheck your work. If it did not play before, then given that the IF strip and audio do seem to work, your suggestion that the oscillator is at fault seems reasonable. I'm no Philco expert, but have read many times that Philco oscillator coils are susceptible to wire breaks caused by corrosion. One common test to see if an oscillator is working, is to hold a transistor radio near the troubled set and tune the tube set across the band, listening for the signal (or a multiple of the osc frequency) in the transistor set. If you don't hear anything, it's likely a non-functioning oscillator?
The coil in question appears to be intact, but measures at a higher value than spec. These coils are nearly impossible to access without disassembling the tuner section, which I have done to troubleshoot this problem. I didn't use the transistor radio test, but I had passed an RF generator line around the chassis at full output and got zilch, except when it was attached or in close proximity to the 6A8G oscillator tube, but that bypasses the coils. I need to verify everything is in correct order before reassembling this section. This radio was in very bad condition and hadn't run since the 1950's.
You can also have a look at pin5 of the 6A8. Should see a volt or two negative on it when the osc is working. Compare it to the working SW band. 0 or positive voltage indicates no feedback.
If you want to listen for it the osc frequency= dial frequency+ the IF (470kc).
If you do find that the coil is inoperative there are universal replacements available, not sure if they have a tap but may not matter as they are slug tuned so you could use the slug to set the low end of the band instead of #13.
Thanks for the good advice Terry. The tuner section is out of the chassis, so I can't take any measurements right now. If the coil proves to be bad, I have a scrap 37-610 chassis with the coils intact, but it's a code 124 and the tuner section is a different design altogether. Its AM oscillator coil has no tap and has a different part number in the documentation. Would it be a safe bet to try it and would the trimmer compensate for any differences? And regarding my original question; what could cause the coil to measure 8 ohms, when it's spec'd at .8 ohms? Aren't they usually open if bad?
I find it strange that you are measuring a direct multiple of what it says on the schematic, 8 ohm rather then .8 ohm, it could be a error on the schematic. Just out of curiosity what does the AM B.C oscillator coil read on the parts chassis?
Regards
Arran
Strange but true Arran. And the coil from the parts chassis measure about the same. HOWEVER, I should have studied the schematic a little closer. The coil value text is aligned with the tap (pin 2) and pin 3. Measuring between those two points gives me .8 Ohms. Voila! The 8 Ohm value isn't called out. So, it appears that this is a good coil and I should be looking elsewhere for a solution.
Problem solved! Found a miswire in the AM section. Completed the restoration this evening and all is well.
Thanks for the help!
Been there and done that!!!
Good work!
(12-02-2016, 10:27 PM)klondike98 Wrote: [ -> ]Been there and done that!!!
Good work!
Thanks! This was a long project I did for a friend. Took more than a year. The radio began as a basket case and everything, I mean Everything needed attention. Complete restore from a bare chassis. Works great now!
Frustrating at times, but the result was worth it! Now on to the next project!
