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Philco 71 code 123 oscillator - Printable Version

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Philco 71 code 123 oscillator - tmr63 - 02-13-2024

Hello all
It's been awhile since my last post.  Finally have time to get back to my7 radio bench

I am currently working on a philco model 71 code 121.
  As expected it has a bad oscillator coil

I replaced the electrolitics and brought it up monitoring the current which was within what was specified.
I put a modulated IF signal through it and all as ok.  Loud tone with volume control so everything is ok prior to a complete recapping.

I have a question as to rewinding the coil.  It’s a small winding.  Not sure where to start.  The coil wire falls apart trying to unwind it.  It is very fine wire wrapped around a cotton thread.  I have replacement wire that is close to the same size but slightly thicker.  I plan to measure the width of the winding and wrap it close to that dimension.  I doubt the frequency of the local oscillator will be correct.  Can I bring the spread in using the existing adjustments for the oscillator adjustment or will I have to start changing parts in the oscillator circuit.  Has anyone else tried this?
Thanks
 
Tom

RE: Philco 71 code 123 oscillator - Radioroslyn - 02-14-2024

Take a closeup picture of the winding and enlarge it so you can count the turns. Wire gauge is going to be 38. It's important that you note the direction that it is wound as the new winding must be wound the same. It does not have anything to do w/the osc frequency but it's the feedback winding. Replace the rotten film strip under it.

GL

RE: Philco 71 code 123 oscillator - tmr63 - 02-19-2024

Terry
Thanks for your input.  I thought the inductance of the winding would figure into the overall frequency but as you stated it does not.
I replaced the plastic, rewound the coil and covered it with bees wax. Its working fine and the band spread is completely correct.

Ready to replace all the caps and check the resistors to settle some of the noise down.  Thanks for your help and advice.  Your suggestion was a great way to count the windings

Thanks again
Tom

tmr63

RE: Philco 71 code 123 oscillator - dmblackburn - 04-16-2024

I have been working on a Philco 71, Code 123 . Restuffed all the Bakelites but not testing the voltages on each tube. The radio will turn on and tubes light up safely on a dim bulb, isolation, and Variac. On the Type 37 tube, I am not getting a plate voltage but can see from the schematic that both plate and cathode are connected to ground. Would that account for the difference in the tube characteristics noted in the specifications for the tube which call for anywhere between 90-250 volts ?

RE: Philco 71 code 123 oscillator - tmr63 - 04-16-2024

Table 1 in the schematics lists the plate as 0V as it is tied to ground
so that is correct
When you re-stuffed the electrolytic's did you tie them to ground?  C56 has an insulation sleeve which keeps it above ground

RE: Philco 71 code 123 oscillator - dmblackburn - 04-19-2024

Thank you for your response and confirmation. I did connect the electrolytics to ground and applied shrink tubing. I'm going to take a look at P/N 21 (filter bank). I believe it is different on the Code 123 (one less capacitor). Working with the Code 123 is challenging and figuring it out as I go given the lack of a schematic. This set does not oscillate and also looking at the oscillator coil that doesn't meet the ohms specifications (but again I'm looking at the specs for a code 121 and I believe the code 123 that I'm working on has a different oscillator coil

RE: Philco 71 code 123 oscillator - dmblackburn - 05-08-2024

I tested voltages on all of the tubes on the Code 123 and all but one of them were very close to the published Tube Socket Data for the Code 121.
The detector oscillator tube (Type 36) testing results were bizarre as follows: Normal specs in parentheses ( )

F to F  6.24: VAC (6.3 VAC)
P to K:  -2.6 VDC (235 VDC)
SG to K : 16 VDC (90 VDC)
CG to KL : 30.8 VDC (2.3 VDC)
K to F : 31 VDC (20 VDC)

Could these readings be a result of a defective oscillator coil or should I be looking elsewhere ? I've replaced all resistors, electrolytics, large caps.
Also, a general question regarding ohm readings on the oscillator coil. From the chart in the documentation, I see a 5.2 ohms reading for inner and outer primaries, and 15.7 for the secondary. What lug numbers correspond to where I should be taking the readings ? Are the readings taken from various lugs or a particular lug and chassis ground ? It also appears that the physical wiring on the 123 to the oscillator coil is different from the Code 121 schematic.