Sony ST-S390 does not stop when autotuning

[PE9ZZ]

I got this nice RDS tuner as working but the autoscan feature does not work most of the time. I cannot fathom what the criteria for the microcontroller are for when to stop. This is what happens: briefly turn the tuning knob to initiate autoscan. When a station is found it should stop. I monitored the AUTOSTOP signal (top trace) to the micro which comes from the IF chip LA1235 and goes low when a signal is received. I also had my scope trigger from the CLOCK signal to the synthesizer chip which is only present when scanning. If the AUTOSTOP works OK the CLOCK pulses (faintly visible in the lower trace at the bottom) go away:



When it trudges on, it briefly stops, even says TUNED in the display and the pulses continue:



The scale is 0.2 s/div, the tuning is from 97.2 MHz to 98.0 MHz.

I cannot figure out why the micro does that. Unfortunately a quick search did not produce a user manual. The SM is mum about this. What other criteria must be satisfied for the micro to stop?

 

[PE9ZZ]

"It's not a bug, it's a feature!" Figured it out. In order for the autoscan to work the tuner's RDS feature must be switched off. Even signals with RDS are skipped most of the time. Apparently the RDS decoder cannot lock quickly enough before the micro already has decided to continue tuning. The only way off-air differs from my test signal is the presence of RDS. That's why it would sometimes stop with an off-air signal but never with my test signal. I could improve scan performance somewhat by tweaking RV801 and observing the recovered clock signal on pin #18 of IC801 but the micro is just too quick rejecting a signal. This is something I cannot fix. I could not find a service bulletin. Well done Sony!