Sony St-S370 tuner problem

[atsirik]

Hello, I have a St-S370 Sony tuner, which works, however the tuned frequency is not the one displayed on screen. For example to tune to one station with frequency 100.30MHz you have to select 100.40MHz (at which point both TUNED and stereo become lit, and RDS starts working for that spoecific station). This deviation of .1MHz is present to the whole band. I downloaded the service manual, but I do not seem to find if there is any way of re-calibrating the frequency... As I do not have experience with digital tuners (and do not own special calibration equipment as some mentioned in the service manual - only the basics, scopes etc.), is there something I can try?
Thank you

 

[davidsrsb]

Either the IF frequency is off and 100kHz is just about possible, or the PLL crystal is 100ppm low

 

[atsirik]

Thank you very much for the
I will try this weekend to check the frequencies and let's see what I will find...

 

[MarcelvdG]

Have you got suitable equipment? With an ordinary scope it will be very difficult to measure a 1 % error in the IF (assuming that the nominal IF is the traditional 10.7 MHz) and it will be impossible to measure a 1000 ppm crystal oscillator frequency error (which is a really large error for a crystal oscillator, by the way). If you own a frequency counter or something, please keep in mind that any capacitive load on the crystal will detune it a bit.

Murata used to sell ceramic IF filters that were right on the target 10.7 MHz centre frequency and (presumably cheaper) versions that were a bit off. The ones that were off had some special marking on them. If the ST-S370 uses ceramic filters, the datasheet might tell you what centre frequency they have.

 

[atsirik]

Thank you all for your replies,
Yesterday night I took a look and to my surprise initially it was tuning to the correct frequency which started to drift to that +0.1 after a while. I also noticed that when cooling the LA1335 IC which is labeled "FM IF det", the tuner would return to the normal frequency and then start drifting again (even more than +0.1). So I guess there is something wrong there, but I want to check the power supply first because I took some measurements and also didn't like the level on the +5V line.

 

[atsirik]

So... changed LA1335, frequency was still a bit off, adjusted using a radio with strong signal (I know... I shouldn't without fm generator but it did the job...) the NULL setting to the lowest I could - almost 0 - (didn't touch distortion adjustment as I do not have the corresponding equipment) and now the tuner seems to work fine. Tried this morning (before work) for 1 hour, no frequency shift, all stations I tried (covering the full FM spectrum) seem to tune fine.
Thank you all for your advices!

 

[Mooly]

Good to know you have sorted the problem 🙂

For information... ceramic filters are usually marked with a colour code (dab of paint, white, yellow, red etc) which group them into similar centre frequency bands.

It doesn't normally matter which are fitted to a tuner as long as all match. They must all have the same colour spot marking.

 

[davidsrsb]

My ST-S361 is very similar and has always suffered from the IF detector coil ferrite slug dropping down inside the coil over time. I think Sony must have used a glue that vanished with age.