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Old 15th Jun 2019, 11:18 pm   #31
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 949
Default Re: No LW/MW receive - Sony ST-S311

Hmmm, think I may have found the problem! (or at least one of them!)

Careful comparison of the voltages on the IC against the quiescent values given in the data sheet showed almost everything to be within a couple dozen mV of where expected, except two pins, which were closer to a volt out. One was supposed to be 0v and was 0.9v - that was the signal meter line. The other was the IF amp input pin, which was about 1v too low. This connects to one side of the ceramic filter...

Checking the filter was none too easy - the IF filtering is a Toko module consisting of a tuned IFT and a ceramic filter, electrically separate but packaged in the same plastic housing. Had to unsolder the entire thing and remove it to test.

The leg of the filter that goes to the IF amp input pin reads 3k to ground! The other leg infinite at DC.

Bypassing the IF filtering entirely with a 10n cap, and the set now indicates tuned with a signal greater than 150uV (-65dBm), and I can trace the demodulated audio tone as far as the input of the LA3401 IC. Ive no audio output on AM though, there seems to be another issue with the AM/FM select voltage to that IC.

Im somewhat stumped now though. To go further I need to replace the ceramic filter, but the filter module seems to be unobtainable. Its a Toko part marked 4047 1311 93507. I could probably remove the IFT from it and refit that on its own, and fit a separate ceramic filter if I can find one. Ive none in my junk though, all mine are 455kHz and this is 450kHz.
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