Guys - many thanks for taking the time to respond!
Steve P - A known 'good' 6C9 would be an ideal tool to help determine the cause of this fault. Alas, they don't seem to be that common. I've tried bunging in three different ECH42s that I had lying around, but this resulted in no change on SW.
As regards the LO coils and caps, beyond testing for the presence/absence of continuity and capacitance, there's not much else that can be done with these, is there? I am also resisting delving into all that RF circuitry, which ain't all that easy to get to.
I was interested to see that you mentioned the antenna circuitry - can this have an effect on local oscillator function?
Al. - the 'scope probe bore the details "10X 100MHz 13pF 10MΩ", the last figure being the impedance, I believe. It's unlikely that this could've stopped the oscillator, isn't it? I took up your suggestion and checked the voltage on the triode grid, with respect to cathode, using a DMM. The results were:
LW: ~ -10V
MW: ~ -5V
SW: -0.3V
There was variation as one tuned across MW and LW, but not SW. I take it that this suggests that the LO is not functioning on SW.
The set covers the usual range on SW (6-18MHz) and I'm using about 40 foot of wire strung horizontally around my attic as an aerial.
Alan - Thanks for the info about your 75A. It's always good to know that the set one is working on is at least capable of receiving stuff on SW. Does yours have a substantial aerial?
Rob T - I suspect we may be on different service sheets - there's no L7 on mine and C29 is the audio coupling cap. I'm using the Pye sheet for a P75A - I haven't been able to find any other sort!
I think what I'll do is pass over this fault for now, and carry on with some other issues that this set has (faulty volume pot, some other waxies to check/replace).
I'd still be interested in any opinions about my fault-finding logic, though, especially w.r.t. the questions in my original post.
What about the bypass capacitor on the cathode of the 6C9? I haven't changed this (a 0.04µF tubular job) as MW and LW function suggested to me that it was OK.
Steve P - A known 'good' 6C9 would be an ideal tool to help determine the cause of this fault. Alas, they don't seem to be that common. I've tried bunging in three different ECH42s that I had lying around, but this resulted in no change on SW.
As regards the LO coils and caps, beyond testing for the presence/absence of continuity and capacitance, there's not much else that can be done with these, is there? I am also resisting delving into all that RF circuitry, which ain't all that easy to get to.
I was interested to see that you mentioned the antenna circuitry - can this have an effect on local oscillator function?
Al. - the 'scope probe bore the details "10X 100MHz 13pF 10MΩ", the last figure being the impedance, I believe. It's unlikely that this could've stopped the oscillator, isn't it? I took up your suggestion and checked the voltage on the triode grid, with respect to cathode, using a DMM. The results were:
LW: ~ -10V
MW: ~ -5V
SW: -0.3V
There was variation as one tuned across MW and LW, but not SW. I take it that this suggests that the LO is not functioning on SW.
The set covers the usual range on SW (6-18MHz) and I'm using about 40 foot of wire strung horizontally around my attic as an aerial.
Alan - Thanks for the info about your 75A. It's always good to know that the set one is working on is at least capable of receiving stuff on SW. Does yours have a substantial aerial?
Rob T - I suspect we may be on different service sheets - there's no L7 on mine and C29 is the audio coupling cap. I'm using the Pye sheet for a P75A - I haven't been able to find any other sort!
I think what I'll do is pass over this fault for now, and carry on with some other issues that this set has (faulty volume pot, some other waxies to check/replace).
I'd still be interested in any opinions about my fault-finding logic, though, especially w.r.t. the questions in my original post.
What about the bypass capacitor on the cathode of the 6C9? I haven't changed this (a 0.04µF tubular job) as MW and LW function suggested to me that it was OK.
