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38-12 Oscillator Replacement - Printable Version

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38-12 Oscillator Replacement - Radioguy52 - 04-28-2022

Revisiting this topic , Coil was bad on one winding. I ordered the Universal RR:  The new coil restores functionalit but two symptoms.
1) Stations in the 1080 + work great no static.
2) Low end no reception only stattic.

I have read adjusting the slug in the universal might help? Is there any method that makes senses where to start?

RE: 38-12 Oscillator Replacement - Radioguy52 - 04-28-2022

A quick progress/ no progress. I tried adjusting the Universal P C70 OSC (IF 455KC ) because the station i.e 1080 are playing down in the 14KC range. No major change in the frequency.

My initial question is if the universal is a 455KC oscilator and the radio 38-12 call for 470KC will it ever work?

RE: 38-12 Oscillator Replacement - Radioroslyn - 04-28-2022

Seems like the lo should be running between 1020kc @ the low end to 2070 to @ 1600kc @ the high end. How does it track from 1080 to 1600kc? Is the lo running at the low end of the band?

RE: 38-12 Oscillator Replacement - Radioguy52 - 04-28-2022

The higher end from 1080 to 1400 sound ok but show on the dial about 20% higher. As I go to below 820 )which is at around 10000 on the dial I begin to loose reception to more static vs signal. Not sure if that explains it per your comment. and my concern that the replacement OS is IF = 455 and the 38-12 Philco is spec at IF 470? Mid range is shifted up and low range hardley there.

RE: 38-12 Oscillator Replacement - Radioroslyn - 04-28-2022

The 15kc difference between 455 and 470kc can be adjust by using the slug in the coil and the trimmer on the tuning cap. So you've got it wired terminal 1 to the tuning cap stator and grid leak, 2 has nc, 3 to chassis ground, 4 to the 6A7 screen grid, and 5 back to B+ correct?

RE: 38-12 Oscillator Replacement - Radioguy52 - 04-28-2022

I double / triple checked and yes I am 5 to B+, 4 to 6A7 screen grid, 2 not ued (although I tried to switch 3 with 2, but no change, I removed the slug all the way out and no change.. I must be missing something BIG? Also I ran a 5ooKC signal to the external antenna and it sings at the 700 scale.

RE: 38-12 Oscillator Replacement - Chas - 04-28-2022

If the old oscillator coil has not been physically damaged as to obscure how it is constructed then rewinding the oscillator coil could be done to get the radio operating properly.

Rewinding is a process of most exactly replicating the wire size, the number and direction of the turns and using a new layer of plastic sheet to replace the celluloid sheet that may have caused wire corrosion...

Chas

RE: 38-12 Oscillator Replacement - Radioroslyn - 04-29-2022

The local oscillator should be operating at the incoming frequency (the dial frequency) + the IF frequency. At 550kc the osc needs to be at 1020kc. According to the information on the replacement coil w/o the slug in place should put the osc very close to the proper frequency. 
Feeding a 500kc signal to it just confuses things as it's too high to be at the IF and too low to be it the bcb. It serves no purpose other than to muddy the waters. Have you aligned the IF stage @ 470kc with an accurate generator?

If so I would I'd check rotor for the end plates being bent in shorting against the stator. Or a weak 6A7.
OBTW You didn't mention terminal 1 on the coil. That will cause the circuit not to work if not connected

RE: 38-12 Oscillator Replacement - Radioguy52 - 04-29-2022

Sadly the old is too far gone. I noted I put a side radio next to the Philco and set it for aprox 1000 kc.. The philco was creating interference squeling at arounf 600kc,, (I assume the OSC might be producing, but moving the slug creates zero affect.

RE: 38-12 Oscillator Replacement - Radioguy52 - 04-29-2022

Enclosed is the schematic?

RE: 38-12 Oscillator Replacement - Radioroslyn - 04-29-2022

Yes that is the proper schematic for your set. If the osc is working and the dial is set @ 1000kc you would hear the osc signal @ 1470kc on another receiver. It will unmodulated so there isn't any tone or voice just a loud hiss.

You may try reversing the B+ and screen grid connections. The phasing of the windings is important. If it's out of phase it won't oscillate due to a lack of feedback. According to coil info it's wired correctly but...

RE: 38-12 Oscillator Replacement - Radioguy52 - 04-29-2022

You have me very curious.  I connected pin 4 of the OS coil to the grid of 6A7  and B+ to pin 5.. Are you indicating reversal may help?i.e B+ to pin 4 and pin 5 to the 6A7 Grid?

Specification Sheet:

[Image: https://www.tubesandmore.com/sites/default/files/associated_files/p-c70-osc_specsheet2.png]

RE: 38-12 Oscillator Replacement - Radioroslyn - 04-29-2022

Check #11 for proper value and replace 5 and 6 (grid leak) resistor and cap. Should have hv on grids 2,3, and 5 on the 6A7. And hv on the plate.

RE: 38-12 Oscillator Replacement - Radioroslyn - 04-29-2022

Sorry missed your post.
Yes switch the screen grid and hv connections on the coil. This inverts the phasing. If it doesn't do anything then wire it back the other way. We are using the 1st diagram and terminal 3 goes to chassis ground. There is no series capacitor (padder) in the original circuit.

RE: 38-12 Oscillator Replacement - BrendaAnnD - 04-30-2022

The oscillator coil should NEVER be connected to the grid cap on top of the 6A7. That connection is for signal incoming from the antenna coil and AGC.

*oops... I somehow read Grid CAP, where it just says grid...