I've received my UHF converter box and finished recapping it (also replaced the selenium with a modern diode+100 ohm dropper).
It appears to work, but not particularly well. Incoming signal should be strong (fed via cable). Schematic is available on last page of Sams 259-3, which is on the ETF page here (
http://www.earlytelevision.org/tv_sc...s_postwar.html) - Tip of the hat to Tom Albrecht for that!
Symptoms are as follows, all resistors check OK but haven't tested the capacitors. Electrolytics replaced, others are ceramic and can't be measured in-circuit. 6AF4 is also unknown state.
1. The entire tuning range of the converter spans about 8 channels. (62-64, got 70 once).
2. Some regions on the dial pick up FM radio stations. I attribute this to not having a proper tube shield on the 6AF4 since it is much less if I temporarily wrap it in grounded tinfoil, and said regions are likely multiples of the station's frequency. (e.g. 88.9 FM when Ch55 (~716Mhz) is onscreen).
So far there are a few things I suspect it could be, and I was hoping someone here could point me in the correct direction. As I'm interested in learning, even if you don't have an answer but just some basic circuit theory explaining how the setup works, I'm happy to listen. I do get the higher-level principle of superhet down conversion/how an oscillator works but I don't think I grasp all of the circuit blocks quite yet (e.g what the intent of each of the traps are).
I gather from the tuning range that the oscillator isn't spanning to the correct frequencies (based on the channel seems to be stuck in a narrow band of them.
B+ measurements seem reasonable. They do seem to vary a bit depending on the tuner, from about 115V to 135 as called for on the schematic. Voltage drop across the dropper resistor indicates we're drawing about 25mA B+.
The 6AF4 plate voltage (at 80v in the Sams) varies from 70-90 in a similar way as the B+. Grid voltage I have seen go from near zero up to -10 as I tune the dial.
1. Bad 6AF4? I don't have a replacement on hand, but that might be the most logical, but not the cheapest place to start... They're plentiful and can be had for a few $ but since they come from the US I pay upwards of $10 for shipping a single one... which isn't particularly cost-effective.
2. Bad UHF diode? I really don't suspect that's the case given I do get reception, but I'm curious what kind of symptoms a bad crystal mixer (1N82) would cause. I remember reading about it in a thread either here or elsewhere, but can't find it any more.
3. The tuning 'gimmick' (labelled as such below L3 in the schematic). It's comprised of two wires, one insulated, twisted together. Any idea how sensitive that is to misalignment?