RF but no IF on 610 - Printable Version
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RF but no IF on 610 - Patrick - 04-02-2021
Without going into too much detail, I can inject an RF signal at various frequencies down to 500Hz (with var. cap fully meshed), and tune to that frequency, at which point I can adjust both the 1st and 2nd IF cans. But I get nothing when I try to inject the 460 Hz IF.
Can something be preventing me from hearing anything below 500Hz? Only tried it by ear so far.
I'll post a schematic if needed, but I thought this just might be a general question.
Thanks.
RE: RF but no IF on 610 - Radioroslyn - 04-02-2021
Do you mean hz or khz?
Would connect the generator to the grid cap of the 6A7 and set it for 460kc w/modulation. Adjust IF transformers for max signal. If there is no signal check voltages at the plate and screen of the 6A7 and 78.
RE: RF but no IF on 610 - Patrick - 04-02-2021
Khz. Cap of 6A7 is where I fed the signal in. No IF. Voltages are good. Can receive stations.
RE: RF but no IF on 610 - Radioroslyn - 04-02-2021
Ok voltages @ the 6A7 and 78?
RE: RF but no IF on 610 - Patrick - 04-02-2021
Edited last post
RE: RF but no IF on 610 - Radioroslyn - 04-02-2021
So the problem is you can't align the IF but the receiver works?
RE: RF but no IF on 610 - Patrick - 04-02-2021
Yes. Well...I can align by using an existing station, but not using the IF frequency. And I know my sig gen works.
RE: RF but no IF on 610 - Radioroslyn - 04-02-2021
Ok sounds like a generator problem. Would check by listening for the 2nd harmonic of 460kc (920kc) on a modern digital receiver. Don't know what you are using but could have a dirty band switch or out of alignment on that band.
RE: RF but no IF on 610 - Patrick - 04-02-2021
Nope. I'm using an HP 8657A. Tried it on other chassis. Works fine. 'Tis a mystery.
RE: RF but no IF on 610 - morzh - 04-02-2021
Are you sure 6A7 is good?
Increase your IF signal a whole lot if possible.
Make sure your cap on your 6A7 soldered well.
Check IF coils for continuity.
Try bypassing 6A7: inject the 460kHz right into the IF xfmr (6A7 plate) via a capacitor and put a Zener in parallel to the generator output to protect it from initial surge of Plate voltage.
PS. Have you UNPLUGGED the Grid Cap wire from 6A7 Grid Cap? If not, it should be unplugged.
RE: RF but no IF on 610 - Patrick - 04-02-2021
Yeah, I tried 2 other 6A7's. First thing I did was check the IF coils. One thing that was puzzling, is that the primary of the second IF should be 11.5Ω, but it measures 39Ω. Pulled the can off, and everything LOOKS fine, but still 39Ω. All the other coils are spot on. Haven't bypassed the 6A7 yet.
RE: RF but no IF on 610 - morzh - 04-02-2021
But you did disconnect the wire to the 6A7's cap when applying the signal to the cap?
RE: RF but no IF on 610 - Patrick - 04-02-2021
Absolutely.
RE: RF but no IF on 610 - morzh - 04-02-2021
I cannot explain it.
If it were not disconnected, there is the wavetrap. It attenuates 460kHz.
If you used good high levels of 460kHz and it does not show at he output, I would disconnect the filters and put the signal directly through them, trying to align them to it.
Keep the input is under the plate voltage, so if using a solid state generator, use a capacitor and a zener in parallel to the generator's output (i use it all the time, I described it a few years ago.)
RE: RF but no IF on 610 - morzh - 04-02-2021
How sure are you your generator is good?
I am asking because if you used it to align other radios it will work with them then.
Do you have a counter that could confirm the accuracy of 460kHz?
Also, is it possible you simply have not found the spot in the IF cans where the signal would go through? It could be simply really badly out of IF alignment, so one of the filters would always block it.
I think now even more, it is time to disconnect the filters and align them one by one; start from 27 first, then solder it back and align the 21.
then try to inject the 460kHz via the 6A7's cap.