Sony ST242L tuner works on only half the frequency range - UK Vintage Radio Repair and Restoration Discussion Forum

jmcilkley · 28th Sep 2020, 5:10 pm

I'm trying to resurrect a Sony ST242L. When I started it didn't tune to anything at all but liberal use of de-ox on the band selector brought it back to life. It now works perfectly but only on half the range. 98MHz up is fine but below 98 it stays on the same channel.
This is an ususual design in that the tuning is done inside a program sensor unit so no visible tuning condenser. I have the service manual (attached) but it gives no clue as to what goes on inside this module so i don't know where to start looking. I wonder if anyone has experience with this unusual method and can give any pointers?
Sorry, can't upload the manual as it is too big but it's available on that wonderful site :
https://elektrotanya.com/sony_st-242l.pdf/download.html

Herald1360 · 28th Sep 2020, 7:50 pm

The program sensor doesn't do the actual tuning, that's done by the varicap diodes in the FM front end. However, they get their variable tuning voltage via that mysterious block.

Is it a sealed unit or are its components accessible?

Have you checked that the high and low limit tuning voltages fed to the program sensor are correct?

jmcilkley · 28th Sep 2020, 10:56 pm

No - that was going to be my first check unless someone had a better idea. The sensor can be removed quite easily but needs to be disconnected from the tuning dial string. I can't tell if it's accessible until I take it out.

jmcilkley · 29th Sep 2020, 10:18 am

OK, checked the voltages and the low end is way too high and the variable resistor that is supposed to adjust it has no effect (but the vr checks out ok for resistance). The top end and middle show the correct voltages. I guess that means that something is wrong in the program sensor unit so that will be the next step to remove that and see what's inside.

28th Sep 2020, 5:10 pm	#1
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Sony ST242L tuner works on only half the frequency range I'm trying to resurrect a Sony ST242L. When I started it didn't tune to anything at all but liberal use of de-ox on the band selector brought it back to life. It now works perfectly but only on half the range. 98MHz up is fine but below 98 it stays on the same channel. This is an ususual design in that the tuning is done inside a program sensor unit so no visible tuning condenser. I have the service manual (attached) but it gives no clue as to what goes on inside this module so i don't know where to start looking. I wonder if anyone has experience with this unusual method and can give any pointers? Sorry, can't upload the manual as it is too big but it's available on that wonderful site : https://elektrotanya.com/sony_st-242l.pdf/download.html

28th Sep 2020, 7:50 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Sony ST242L tuner works on only half the frequency range The program sensor doesn't do the actual tuning, that's done by the varicap diodes in the FM front end. However, they get their variable tuning voltage via that mysterious block. Is it a sealed unit or are its components accessible? Have you checked that the high and low limit tuning voltages fed to the program sensor are correct? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

28th Sep 2020, 10:56 pm	#3
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: Sony ST242L tuner works on only half the frequency range No - that was going to be my first check unless someone had a better idea. The sensor can be removed quite easily but needs to be disconnected from the tuning dial string. I can't tell if it's accessible until I take it out.

29th Sep 2020, 10:18 am	#4
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: Sony ST242L tuner works on only half the frequency range OK, checked the voltages and the low end is way too high and the variable resistor that is supposed to adjust it has no effect (but the vr checks out ok for resistance). The top end and middle show the correct voltages. I guess that means that something is wrong in the program sensor unit so that will be the next step to remove that and see what's inside.