15-05-2011, 08:13 PM
Thanks for your additional comments, everyone.
Steve - I did the aerial-to-signal-grid test and got no signal. This is what led me to believe the local oscillator (LO) was at fault. The HT on the triode anode is OK as far as I can tell, since the P75A service sheet says it should be 27V, which seems absurdly low. You can see my voltage readings in post #21. In the same post there are some test results for the coils and caps in the SW LO circuit.
From these, I concluded that one of the coils was at fault, specifically the one in the grid side of the LO. The Pye sheet for the P75A doesn't give a resistance value for this, but the trader sheet for the P75 does: 43Ω. The coil in my P75A measures at 1.3Ω - so it seems that there is a short somewhere in this coil.
Jamie - thanks for the offer, but I have the manufacturer's service sheet for this set (serial no. 612594).
Rob T - thanks for answering my question on whether the SW LO coil on the P75A would be the same as that on the P75. As described above, it looks like the coil is the source of the trouble.
Now getting at this coil seems to involve some major surgery (removal of tuning capacitor, probably lots of unsoldering around the wavechange switch, if not removal) which I'm not prepared to go into at the moment. For the time being, the set can remain without SW. But can someone help me out with a couple of questions:
Thanks for your help with this.
Steve - I did the aerial-to-signal-grid test and got no signal. This is what led me to believe the local oscillator (LO) was at fault. The HT on the triode anode is OK as far as I can tell, since the P75A service sheet says it should be 27V, which seems absurdly low. You can see my voltage readings in post #21. In the same post there are some test results for the coils and caps in the SW LO circuit.
From these, I concluded that one of the coils was at fault, specifically the one in the grid side of the LO. The Pye sheet for the P75A doesn't give a resistance value for this, but the trader sheet for the P75 does: 43Ω. The coil in my P75A measures at 1.3Ω - so it seems that there is a short somewhere in this coil.
Jamie - thanks for the offer, but I have the manufacturer's service sheet for this set (serial no. 612594).
Rob T - thanks for answering my question on whether the SW LO coil on the P75A would be the same as that on the P75. As described above, it looks like the coil is the source of the trouble.
Now getting at this coil seems to involve some major surgery (removal of tuning capacitor, probably lots of unsoldering around the wavechange switch, if not removal) which I'm not prepared to go into at the moment. For the time being, the set can remain without SW. But can someone help me out with a couple of questions:
- Do you think my diagnosis is correct? I can't quite believe that a coil should be at fault. Is this likely?
- How is the SW LO coil constructed? I can see a coil of quite thick copper wire, and possibly one of very fine wispy wire wound over it. Does this sound right?
Thanks for your help with this.
