Model 71 oscillator - Printable Version

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+--- Thread: Model 71 oscillator (/showthread.php?tid=21611)

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Model 71 oscillator - WB5NEN - 09-06-2020

This problem is intermittent. When it occurs, the receiver is dead below about 1200 kHz. Looking at the cathode of the 37 oscillator with an oscilloscope, I can verify the 260 kHz oscillator output declines to zero at about 1200 on the dial. (A second receiver confirms this.) Thoughts on this? Yes, the receiver had been working well. I’m thinking it’s the oscillator coils, or (gasp) the autodyne circuit? But this is a new problem for me. Thanks.

Richard

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RE: Model 71 oscillator - Steve Davis - 09-06-2020

Richard, try lowering the cathode resistor from 15K to 10K or even 7500 ohm. If the oscillator still drops out, the oscillator coil form may have moisture. Remove the coil and bake it @ 200 degrees for about 20 minutes. 

Steve

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RE: Model 71 oscillator - Radioroslyn - 09-07-2020

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RE: Model 71 oscillator - WB5NEN - 09-07-2020

Steve and Terry,

It’s working. I noticed that, by touching the grid cap on the oscillator, it sprang to life. But next time, it was dead until I touched the grid cap again. Rinse, repeat. This struck me as odd, so I paralleled a resistor across the 15k cathode dogbone resistor (which actually measured 12.5K.) The  current value of these two resistors is 8.4K. Right now, that oscillator starts up automatically every time I turn the set on. But I’m leaving it on the bench for awhile. Fingers crossed.

I did remove and examine the oscillator coil. I just wanted to see it. It’s fine, as I suspected, because the large oscillator coil showed continuity and 4.3 ohms. The “input” coil From the RF stage in this radio is just a turn of wire...and one end is unterminated. It just floats.

Now, can someone educate me on this cathode resistor...I’m not clear on what I’ve done, or why. Sorry for the ignorance.

And thanks, gents.

Richard

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RE: Model 71 oscillator - WB5NEN - 09-07-2020

Oh. I forgot to mention I did not bake the coil form to reduce moisture, since I live in the New Mexico high desert, we are in a terrible drought, and...humidity, what’s that? But I understand how moisture in the form could affect it.

Richard

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RE: Model 71 oscillator - Radioroslyn - 09-07-2020

By lowering the value of the cathode resistor is causes the tube to conduct more (raises the gain a bit). This in turn can cause the oscillator to start easier.

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RE: Model 71 oscillator - WB5NEN - 09-07-2020

I can understand that. Of course. Thank you.

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RE: Model 71 oscillator - WB5NEN - 09-08-2020

This Model 71 is pretty fussy. But also pretty to look at. It appears the oscillator is starting reliably now (thanks again for the help), but I see that injection falls off fairly fast as I tune from the high end to the low end of the band. I can’t say I’ve checked that on other receivers, but I will, going forward, so I can learn. Is this normal? I suspect not. 

Richard

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RE: Model 71 oscillator - Radioroslyn - 09-08-2020

Haven't check the amplitude of the oscillator on a 71.
I did on an early 16B, Two of the SW bands would die about a mc the low end of the band.  Drove me nutz for a while only to find it was a missing resistor in the circuit. I didn't use a very sophisticated system just a modern ham transceiver with a S meter to track the oscillator.
In your case it could very well be a lack of feedback.

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RE: Model 71 oscillator - Steve Davis - 09-08-2020

Richard, you could do Ron's "Super 71" modification where he replaced the 36 tube with a 6A7. Look here    https://philcoradio.com/phorum/showthread.php?tid=14917&highlight=super+71    

Steve

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RE: Model 71 oscillator - WB5NEN - 09-09-2020

Well, the local oscillator has abruptly quit...again. This is a very perplexing problem. I had kept it on the work bench, playing it on and off for several days, and yesterday I slipped it back in the cabinet. My confidence was high that dropping the cathode resistor on the oscillator from 12.5 K to about 8.4 K had finally kept the oscillator going. But it didn’t. It quit again. Should I consider changing the bypass silver mica? It specifies 700 pf, but I have not actually checked it. I believe I will do so.

I do not have a 36 spare tube, but I’m open to ordering one. 

I’m not too eager to do a chassis rebuild for a more reliable tube and circuit, but it might be necessary.

I’m trying to understand the feedback circuit in this strange old LO. I know a grid is always out of phase with the plate, but I think the cathode is in phase with the plate. Is that correct? And so, in the schematic, there is a little coil in the cathode circuit below the primary of the first IF. Is that feeding back in phase to the plate to create oscillation? I’m lost. 

Thanks. 

Richard

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RE: Model 71 oscillator - Radioroslyn - 09-09-2020

Part #13 is the oscillator coil and that little coil (feed back winding) is part of it. #20 is the first IF transformer.

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RE: Model 71 oscillator - RodB - 09-09-2020

Hi Richard. I think the way this one works is that when the tube starts conducting the oscillator primary gets a pulse from the plate circuit which starts the secondary oscillating. This in turn will induce a signal in the secondary of the cathode circuit. This local oscillator signal will then mix with the grid input signal to produce the IF. The oscillator continues to oscillate from capacitor 12 in the plate circuit. Maybe if you haven't changed cap 12 you might try replacing it.

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RE: Model 71 oscillator - Radioroslyn - 09-09-2020

12 is the trimmer to adjust the primary of the 1st IF transformer.

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RE: Model 71 oscillator - WB5NEN - 09-09-2020

Terry,

So that little oscillator coil must be on coil form 13? I’m thinking that little coil might need an additional winding to increase the feedback? Almost like a tickler...

R

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