20-07-2020, 09:51 PM
I found a SM with a snapped lead going from R2 33K LO HT feed to 0V. So I removed the decoupler I'd added (usually that point has 20nF to 100nF decoupling and also is DF96 / DF91 screen grid supply. I reconnected that and the OC2 LO died, MW LO died and the part of band where OC1 LO was running died. But the LW LO came to life!
Something is wrong. The R2, 33K, has full length leads and they are bent round to put it on the coil pack terminal. Is it actually at the right place?
I decided to proper map out the coils and switches, so I took off the first IFT unit. I removed some of the SM caps noting value and location. All tested close enough.
Then I decided that was easy and I should take off the second IFT and also open it to find out what was wrong, because it absolutely shouldn't need 180pF added across the secondary. The secondary coil was open circuit! Duh. I should have checked continuity earlier, I was warned about that in about 1967. Nice little pot cores. The mica tuning caps are part of the coil supports. I removed the wire and there seemed to be about 230 turns of 3 or 4 core enamel Litz. My Litz is fatter, so only 120 turns fitted. I found a spool of enamel solid very fine and randomly wound on 240 turns. I didn't cut the wire, but just tinned a small patch. The primary is 1.34mH and 85pF. I unsoldered one coil wire to measure. The secondary capacitor is 56 pF and for a given frequency L x C of one tuned circuit = L x C of another. The inductance was 1.74mH, which seems a bit low for 56 pF so I added a load more turns.
Just random wound by hand. Didn't take long.
I verified by tracing the switches that my assumption where the LW and MW coil wires had come off was correct and I can see the first coil at OC1 position is shared with OC2. Also the RF switch contacts of OC1 and OC2 go to the fourth coil ar LW switch position. The coil at OC2 does indeed connect to MW switch and the coil at MW switch does go to LW switch.
Next time I'll finish tracing out the coil/switch pack and figure out what's wrong. At least the switches now seem OK. The OC1 and OC2 are three pole change over . The MW and LW are two pole change over. I guess the extra poles are to do with the SW coil at the LW switch and using it instead of the ferrite rod aerial.
Something is wrong. The R2, 33K, has full length leads and they are bent round to put it on the coil pack terminal. Is it actually at the right place?
I decided to proper map out the coils and switches, so I took off the first IFT unit. I removed some of the SM caps noting value and location. All tested close enough.
Then I decided that was easy and I should take off the second IFT and also open it to find out what was wrong, because it absolutely shouldn't need 180pF added across the secondary. The secondary coil was open circuit! Duh. I should have checked continuity earlier, I was warned about that in about 1967. Nice little pot cores. The mica tuning caps are part of the coil supports. I removed the wire and there seemed to be about 230 turns of 3 or 4 core enamel Litz. My Litz is fatter, so only 120 turns fitted. I found a spool of enamel solid very fine and randomly wound on 240 turns. I didn't cut the wire, but just tinned a small patch. The primary is 1.34mH and 85pF. I unsoldered one coil wire to measure. The secondary capacitor is 56 pF and for a given frequency L x C of one tuned circuit = L x C of another. The inductance was 1.74mH, which seems a bit low for 56 pF so I added a load more turns.
Just random wound by hand. Didn't take long.
I verified by tracing the switches that my assumption where the LW and MW coil wires had come off was correct and I can see the first coil at OC1 position is shared with OC2. Also the RF switch contacts of OC1 and OC2 go to the fourth coil ar LW switch position. The coil at OC2 does indeed connect to MW switch and the coil at MW switch does go to LW switch.
Next time I'll finish tracing out the coil/switch pack and figure out what's wrong. At least the switches now seem OK. The OC1 and OC2 are three pole change over . The MW and LW are two pole change over. I guess the extra poles are to do with the SW coil at the LW switch and using it instead of the ferrite rod aerial.
