Sansui TU-X711 stereo loses when it heats up

[kleinbus]

Hello friends, I recently acquired this digital tuner, highly regarded by Sansui fans. The problem is that after being on for a while, the stereo light starts flashing until it goes out, leaving it playing mono. I can only assume the problem occurs with increased temperature. I would appreciate any advice or information. I have some electronics knowledge; I have a multimeter and once replaced the electrolytic capacitors on an amplifier. Perhaps I could try to fix the problem if it's not too complicated.

 

[rcs16]

Does it do it on all stations, esp ones that have strong signals?
Id have to look at the schematic and service manual to give more advise.

 

[wjcjr]

Check the MPX VCO pot. Mark current position then ever so slightly move a degree or so in one direction, if stereo light doesn't come on do the same in the other direction.

 

[Bob@FM]

Also double check the detector zero adjustment if the VCO Pot adjustment does not fix it.

 

[PE9ZZ]

MPX dIC2 has a resonator (dX01) and no pot. Maybe it gets forced into MONO. Check pin#11. Discriminator zero could also be an issue, check that too (TP1/2).

Wait a minute, am looking at the TU-X111... The X711 is elusive, found a scan with missing pages. Did find the TU-X701 which seems identical. The MPX chip is dIC7 and also has a resonator. As for the demodulator, I have no idea how it works. Pretty complex.

 

[PE9ZZ]

As for the demodulator, I have no idea how it works. Pretty complex.

Looked at the schematic some more. It's a PLL demodulator. Pretty unusual.