Hi! I'm considering building a replica of a small direction finding receiver from the 1960's, link here:
http://www.cryptomuseum.com/df/fu303/index.htmIt's a fairly straightforward design, a tuned circuit with a myriad of plug-in coils for bands, and some AF stages, with the ability to inject 1000Hz into the rx, to listen for direction (assume sound dies out OR becomes loudest there).
Here's the issue: the nice schematic that's available from the manual is missing some values for some of the components. See attached.
![](https://www.eevblog.com/forum/projects/missing-info-on-schematic-could-you-wing-it/?action=dlattach;attach=262709;image)
Specifically, two transformers between AF stages, an output coil (choke?) to gnd, a coil in the 1000Hz gen, and potentiometer P3 in the 1000Hz gen.
The output choke is probably just filtering, that might be less critical. The unmarked potentiometer is just attenuating the output from the 1kHz osc. so that's probably not critical either. maybe 10K or 100K would do..
The trouble is the AF transformers. They look to be standard hi/lo impedance transformers, like the ones you see in telephone equipment. Could I just wing it, and use some of those? I've never seen it before, so I don't know how critical it is.
Maybe some home-wound 10:1 transformers on a bit of ferrite rod would do..
I think I'll settle on AC128's instead of the OC75 transistors. They're pretty similar, they're not made from glass, and I have some already..
Thanks in advance!