I try to learn little bits and pieces as I go along, but this one has me stumped.
In a nutshell, I the oscillator is not working and I am having difficulty understanding the schematic to troubleshoot and repair the problem. The 38-116 has this complex oscillator setup and although I have checked voltages and replaced all the conventional caps and substituted tubes, nothing works on any band, so I'm assuming the problem is common to all bands.
Has anyone ever discussed this oscillator and it's design? Do any of the Phorum gurus have any advice??
Thanks in advance
Hello blurrrr,
The RF deck of the 38-116 Code 125 is one of the most complicated assemblies that Philco ever built. Only the RF deck of the 38-116 Code 121, identical to Models 37-116, 37-675 & 37-690, is more complicated.
Since you have changed tubes and checked voltages, check resistors (19), (21) & (22). Did you replace condenser (20)? Check continuity of the oscillator coils. It is very unusual that all five coils would be bad. How about the oscillator control circuit (6N7G and associated components)?
Have you tried feeding an IF signal to the set and seeing if it goes through? Inject 470 kc IF into the grid cap of the 6A8G mixer tube (the one directly in front of the 6U7G RF tube). Please note that it won't do any good to feed a signal into the grid of the other 6A8G tube - the one behind the 6N7G; the grid cap of the 6A8G oscillator tube is grounded. Anyway, when you inject a 470 kc signal into the mixer grid, you should hear a 400 cycle tone through the speaker. Your signal generator should be set to 470 kc with internal modulation on.
Thanks for the response, Ron.
What got me so exasperated is that I could put an IF signal though it--very clear and strong. I found the "Special section" in the front of Rider's Volume 8 that did a lot of simplification and explaining. Finally, a bit of advice from my old standby copy of "Elements of Radio Servicing" broke down the systems even further into simple tests of the oscillator components. It boiled down to testing each part which only showed a couple of resistors 10% over value(replaced, probably needlessly) and a check to see if the tuning condenser was somehow shorted to ground. I did not see anything but some dust and a couple of plates that were ever-so-slightly misaligned.
After some tweaking of plates and brushing and blowing the dust out, it fired right up!! There was a lot of dirt on this chassis when I got it, maybe that was it.
A couple questions/comments:
First, the schematic says the IF is 470; yet the chassis stamped 465. Which is right?
Second, I found resistor number 43 listed on the print as 40,000 ohms; yet the resistor installed in the chassis was 4,000 ohms. This resistor looked like a factory installation. What gives? Has anybody else noticed this?
Third, I suppose that there are always going to be changes during the production run; this radio had several differences to the published schematic. there were two bakelite condenser blocks near the output transformer instead of the individual components I expected to find.
Lastly, does anyone make a set of the station call letter windows for the dial? Or do others just cut up an old dial for the pieces?
Thanks to everyone on the Phorum, it's a GREAT resource. I have a 116, two 37-116's and a 38-690 waiting for attention; I suppose I'll be bugging you again soon
Blurrrr, to answer your questions:
1. Does this stamp specify "465 KC" or just "465"? If just 465 it may be an inspection stamp. If you could provide a photo, it may help.
2. Resistor (43) is indeed 4000 ohms, 1 watt. This is listed in a Philco Changes In Models sheet (see below).
3. As to the other production run changes you mentioned, these are also mentioned in another Philco Changes In Models sheet.
I suggest you download the
Philco Changes In Models package that I have made available. Please go
here for more details. The changes you asked about are all listed in this package, toward the end - the 40000 to 4000 ohm resistor is the very last change listed on the last page. Note that it does not mention that it was changed to 4000 ohms, it just gives the part number! Using
my guide on decoding Philco resistor part numbers, it revealed that 33-240439 is 4000 ohms, 1 watt.
4. To my knowledge, no one reproduces the station call letter tabs for 1937-38 Philco sets.
Good luck with your sets. Glad to hear your 38-116 works again. And please feel free to post here anytime.
Yes, the stamp says 465kc. (unable to upload pic)
I'm another potential customer for 1938 call letter tabs, if anyone ever reproduces them. If anyone has made tabs for these on their computer, I'd like to hear about it.