FM3 Stereo Light

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25-Jan-2014 06:19 PM

Bristol603

Bristol603

Posts: 6


I have a newly acquired FM3 (issue 3 pcb). It sounds very good, but a niggling problem is that the stereo light normally doesn't come on even though I am listening to a solid stereo sound. It will behave properly on some occasions, but only rarely. This is even with a strong FM signal from a roof mounted antenna. I have checked the bulb, bulb holder, wiring and continuity via a resistor to the pin (6) on the MC1310P. I am running out of ideas. Any suggestions what further I can check? I could swap in a replacement IC, but I tend to feel this either would or would not work.

The two tuning lights work fine and I suppose I don't really need the stereo light, but something in me wants the unit to work 100%.

Thanks.

Winchester, UK
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26-Jan-2014 12:10 AM

EJP

EJP

Posts: 1690

Check the voltage at pin 6 when you think you have stereo. It should be zero. If it isn't, you don't have stereo at all, in which case you should tweak the adjacent trimpot until you do: find the extremes at which the voltage remains zero and set to the midpoint. (Don't touch any other trimpots.)

If that doesn't fix it, it must be the lamp, or the resistor it is fed through (unlikely). Or else the MC1310, also very unlikely. I have stocks if you need one,

Dada Electronics Australia
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10-Feb-2014 10:23 PM

Bristol603

Bristol603

Posts: 6

Still not getting the stereo light to come on. I have another sympton to report. Occasionally when I switch on the FM3 (via the 33), the stereo light will come on and I get nice stereo sound on all stations (makes me realise I wasn't getting stereo before when I thought I was). If I then switch off and straight back on (no setup changes whatsoever), then the stereo light doesn't come on and I am back where I started. Any ideas?

Winchester, UK
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19-Feb-2014 12:31 AM

EJP

EJP

Posts: 1690

You might need a new MC1310 but first I would renovate the tuner: replace all three electrolytics, and the Zener diodes in the PSU. Check that you have plus and minus about 14V at the red and black leads on the underside referenced to earth.

Dada Electronics Australia
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20-Feb-2014 10:41 AM

Bristol603

Bristol603

Posts: 6

Thanks. I traced the problem to a bad joint on the tuning circuit for the 19kHz pilot signal.

Winchester, UK
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