Model 70 air cap ground strap. - Printable Version
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RE: Model 70 air cap ground strap. - Radioroslyn - 07-03-2020
>The original OSC. wire measures most like 32AWG by my mic, .02 off. The way it is coupled back to the det coil may have something to with it, parallel.
Ok the next way to re it would be to rewind it so that windings match the holes in the form. If you are sure abt the gauge of the wire. A gdo can be handy for roughly reading the resonant frequency. Or a regenerative receiver, if you put the coil near the coil in the regen rx at it's resonant frequency the regen det will have a null (dead spot)
Retuning the IF to a different frequency so the osc will track is a bad idea. The transformers are designed to have a certain selectivity curve and Q @ 260kc. Above and below this frequency not so much. There is a few more reason too.
RE: Model 70 air cap ground strap. - radio1 - 07-03-2020
I meant to say series to the rf, I edited that. Yes, I am trying to reach the 260, and did wind the coil to the original holes comparing against my photos of the original coils. Thought that should get me there, then back off turns if needed, but just cannot seem to get to the the inductance bounced against 410 PF. I wonder of purity of the original wire copper against what we buy today. I measured inductance as is, and took off turns to measure again, that's much to derive and absorb, so I am dealing with the original, coatings, purity, and the the cotton coated outer wrap and the distance between the windings. At the moment, added turns to the RF coupling to the osc. and mounted them a bit higher on the coil, just experimenting. It is difficult to match the original, but will keep my efforts to see when it comes into play. Listening to it right now is great 1140 Richmond down here in NC, wavers, but no AVC, but sounds like they are here with me, ha! I will try your methods too. Joe Pags 1210 a bit more difficult, but coming in.
RE: Model 70 air cap ground strap. - Radioroslyn - 07-04-2020
<Yes, I am trying to reach the 260 >
I think there maybe a misunderstanding of what the goal is. It's not to make the oscillator operate @ 260KC. It's to make the oscillator operate from 810kc (tuning cap closed) to 1760kc (open). This explains it well: https://www.youtube.com/watch?v=hz_mMLhUinw They are going to talk about a 455kc IF, in your case it's 260kc so the math is a little different. The oscillator operates 260kc above the incoming signal.
The pentagrid converter is a multi element tube that equals your 24A mixer and 27 osc tubes all in one.
RE: Model 70 air cap ground strap. - radio1 - 07-04-2020
No, I understand that, I am trying to reach the 260 resulting IF (the difference f1-f2). Thing is, it seems this radio was designed to track 260 below the band, not above, and the difference is still 260 with the audio modulation on it. That's why I was asking others what inductance they get to oscillate against 410 PF, I am at a resulting 232khz, not 260khz at the moment, and by inductance/capacitance calculations based on my current inductance, the osc. riding below the broadcast band.
RE: Model 70 air cap ground strap. - rfeenstra - 07-04-2020
"it seems this radio was designed to track 260 below the band"
I'm trying to remember but I think I aligned my 70's with the oscillator running above the tuned frequency.
RE: Model 70 air cap ground strap. - Steve Davis - 07-04-2020
Yes, the oscillator runs above the tuned frequency
Steve
RE: Model 70 air cap ground strap. - Radioroslyn - 07-04-2020
<Thing is, it seems this radio was designed to track 260 below the band, not above, and the difference is still 260>
I think what you will find is if you are able to get the oscillator to run at the low of the band at 310kc with dial set at 550kc when you try to get to the high end of the band 1500kc the oscillator won't tune high enough. You'll have too much L in the circuit. If you remove turns to get up to the high end then the low end suffers by loosing the bottom 50 to 100kc. In short the tracking will be poor.
GL
RE: Model 70 air cap ground strap. - radio1 - 08-13-2020
Found this pic on the bay of an original and the hookup. Similar to the one above, strange how they hooked up raw to one side, but used a insulated washer and star lock washer on the other side, not that it makes that much difference, it is what it ends up with, then again there may have been a reason.