Philco 38-690 restoration - Printable Version
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+--- Thread: Philco 38-690 restoration (/showthread.php?tid=3166)
RE: Philco 38-690 restoration - morzh - 02-17-2014
The colors seem similar (black top)
RE: Philco 38-690 restoration - FStephenMasek - 06-22-2014
The cabinet is completely finished. I'll take a photo when I have good light.
The good news is that a cd player to the top of the volume control gives audio output. The bad is that there is no radio reception. When I touch the grid caps on the 1st IF, 2nd IF, and RF-AVC tubes it produces noise in the speaker(s). The same with touching the grid caps on the RF, mixer, and oscillator tubes, although less so.
The next step may be using a signal generator to try to inject a signal at various locations to isolate the problem area(s).
RE: Philco 38-690 restoration - morzh - 06-22-2014
Wow, that is one long effort!
I thought it was "playing with crackling", what happened to reception?
RE: Philco 38-690 restoration - FStephenMasek - 06-22-2014
Who knows, as the hack who worked on it previously left long solder blobs, loose solder joints, and paralleled electrolytic capacitors, had a jack added to feed into the volume control, and had an antenna wire connected to the RF section of the tuning gang, bypassing the antenna coil.
Many other things have happened since this project started, but I am making a serious effort to finish all of my approximately 100 radios this year.
The light and angle make a big difference in how a cabinet looks:
[Image: http://i269.photobucket.com/albums/jj77/FStephenMasek/Philco38-690cabinetfinishedoblique.jpg]
[Image: http://i269.photobucket.com/albums/jj77/FStephenMasek/Philco38-690cabinetfinishedfront.jpg]
The chassis being debugged:
[Image: http://i269.photobucket.com/albums/jj77/FStephenMasek/Philco38-690chassisbeingbebugged.jpg]
RE: Philco 38-690 restoration - FStephenMasek - 06-22-2014
Ah, I found that the variable IF control needs to be to the far left. If I put it there, then connect the antenna to the mixer grid, it receives. That proves that the IF is working. I tweaked the various transformers just by ear on a strong local station to get them closer to where they will be when I align it.
When I touch the antenna to the RF tube grid, I get noise, but no reception. So, I've got to look at the antenna coil, RF coil, and band switch. I may also shorten the leads on some of the replacement capacitors put in by the other guy, but want to avoid removing the RF deck unless it can not be avoided.
RE: Philco 38-690 restoration - morzh - 06-22-2014
Worh the effort.
RE: Philco 38-690 restoration - gregb - 06-22-2014
Your cabinet looks great! Well done.
Gregb
RE: Philco 38-690 restoration - PhilcoJohn - 06-23-2014
It is coming along great! I am so very jelous! Keep us up to date with the pictures. How does this radio compair to my 38-116 console radio? Thanks.
RE: Philco 38-690 restoration - morzh - 06-23-2014
Don't forget about that tweeter SNAFU Philco did
RE: Philco 38-690 restoration - FStephenMasek - 07-06-2014
It turned out that there was an open 1,000 ohm resistor in the circuit feeding B+ to the RF and mixer tubes, number 64 on the schematic:
[Image: http://i269.photobucket.com/albums/jj77/FStephenMasek/037-2.jpg]
I've got one of the three chassis sections done, and it plays much better now, receiving many stations. I suspect the tweeters are not working, but I'll get to that issue after finishing the radio chassis work. The problem may even be in the radio chassis.
The guy who previously worked on it did not bother to replace 0.02 mfd capacitors with the proper value, instead using 0.01mfd units, even though he could have put two of those in parallel ...
I'm going though and replacing the resistors and correcting capacitor errors.
RE: Philco 38-690 restoration - morzh - 07-06-2014
Are they cancelling or they indeed are not being driven?
RE: Philco 38-690 restoration - FStephenMasek - 07-06-2014
On the tweeters, I corrected the wiring error and changed the capacitor value. I'll do more testing.
RE: Philco 38-690 restoration - morzh - 07-06-2014
Because as I recall the tweeters are simply driven by the same stage as the main speaker, just use a differentvxfmr and a cap in series with its primary, so there are only so many possibilities for them not to work.
RE: Philco 38-690 restoration - FStephenMasek - 07-10-2014
I just finished replacing resistors and correcting the capacitor errors of the guy who worked on it before I bought it. It sounds nice on my work table on AM. I fed it signals with my signal generator and all of the other bands are receiving, very good news. Now to clean the chassis, repair a cracked tweeter cone, and align it. I'm glad it does not have as many trimmers as the 38-116 I restored.
Using a VTVM, I see tweeter output only once the volume is quite high, so I should also investigate that. It may be designed that way, but it never hurts to check the circuitry and study the schematic.
The audio shorting switch, like the one on my 38-116, seems to always be in the shorted position. I may take the dial shaft mechanism apart, but think the shirting switch arrangement is a bad design, so may just cap that wire.
I'm still missing the hooded dial light assemblies, so will search for them (rather than repair them, the other guy just threw them away...).
RE: Philco 38-690 restoration - morzh - 07-10-2014
yes, the audio shorting construction is less than.....works in my radio but looks very unreliable.
As for the tweeters, there gotta be something wrong with the way you are measuring.
The tweeters are simply fed from an extra winding of the main interstage transformer with 1uF cap in series with the output (similar to 37-690 but there they use a separate transformer in parallel to the main one with the cap in the input winding). So there are no threshold elements etc, the output is always there, and the sound/signal cannot come and go abruptly. You have to use AC setting and make sure that your device can work as a true RMS at the frequency you are working at.
I use my Fluke DMM for it.