12-06-2022, 07:49 PM
(12-06-2022, 05:53 PM)Mike Watterson Wrote: Great!
Well, the FM only uses the chip for IF. Likely the VHF LO coil got moved/bashed a bit, maybe RF coils too. You can't connect direct, but the LO does leak out the aerial in, so you can use a 76 MHz to 120 MHz set to sniff LO. Likely 10.7 IF though 6.5MHz has been used. Possibly LO is 10.7 below RF so might be 76.8 to 97.3 for 87.5 to 108. If so tube the DUT to 100 MHz and a 2nd working set should get a signal at 89.3. A set with tuning meter, or put DVM on discrimator helps. Tune around on test radio to find LO from your faulty set.
Compressing LO coil lowers frequency.
Obviously a spectrum analyser makes it easy. The transistor nearer aerial will be buffer/preamp and 2nd transistor is LO, feeding an IFT feeding IC.
Got all that. It looks like 3 transistors in the FM front end. Thomson diagram shows an oscillator module feeding into the chip, I think this one is different.
There is significant damage around the oscillator coil and the nearest transistor. I have straightened out the transistor legs, there was a short there. There was a coil spread out flat on the board, it had obviously been stood vertical originally as it was full of wax that sort of went back to shape as I stood the coil up.
This board is horrible, its a shame because the cabinet is very good and soundly made. Would be much better with a valve set in it!
Thanks for the help so far.
Boater Sam.
