25-05-2011, 07:41 PM
Rob and Steve - thanks for your further inputs.
Rob - indeed, the coil does only have three connections, as you say.
Steve - I haven't carried out your recommendations above (changing R3, R4, R6 and C16) because I can't understand how it would help, for the following reasons:
As regards L5, the circuit diagram indicates that it can be successfully isolated by moving the wavechange switch to either MW or LW, and this is what I did to get my resistance measurements (I also disconnected the trimmer C10). The core looks untouched, and I know better than to start twiddling those things!
So anyhow, it looks like L5 is the culprit, and as I'm not prepared to conduct surgery in that area at the moment, this set can remain sans SW.
(As an aside, I can't see how that service sheet value of 27V at the oscillator anode can possibly be correct. The valve museum page for the 6C9 says 80V, and I'm getting 67V on MW and 73V on SW, whch seems in order.)
Rob - indeed, the coil does only have three connections, as you say.
Steve - I haven't carried out your recommendations above (changing R3, R4, R6 and C16) because I can't understand how it would help, for the following reasons:
- R3 is bypassed by the wavechange switch on SW, so that shouldn't have an effect.
- R4 is high (at 17.5kΩ), but I tried bridging this with a 100kΩ resistor (to get 15kΩ), and this failed to induce oscillation.
- R6 measures OK at 50kΩ.
- C16 is common to all wavebands - how could this component affect SW only?
As regards L5, the circuit diagram indicates that it can be successfully isolated by moving the wavechange switch to either MW or LW, and this is what I did to get my resistance measurements (I also disconnected the trimmer C10). The core looks untouched, and I know better than to start twiddling those things!
So anyhow, it looks like L5 is the culprit, and as I'm not prepared to conduct surgery in that area at the moment, this set can remain sans SW.
(As an aside, I can't see how that service sheet value of 27V at the oscillator anode can possibly be correct. The valve museum page for the 6C9 says 80V, and I'm getting 67V on MW and 73V on SW, whch seems in order.)
