Advice Needed Technics st-gt650 fm tuning issue

Captain Jack

Captain Jack

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I recently acquired the above-mentioned radio tuner and I am having a small but annoying issue with it. It will not auto-find or lock FM stations, decode RDS or give me stereo sound.

The signal strength is good and audio is nice and clear (but in mono). AM band works OK - it locks onto stations when using automatic search.

I took a quick look around the main PCB and it looks clean with no cracked joints. There is a service manual floating around but I don't want to start replacing various components nilly-willy on the FM side in hope that one of them will resolve the issue. Sadly, no oscilloscope to start probing around.

Any ideas?
 
Channel Hopper

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FM Mode selector button fault/disconnected ?
 
Captain Jack

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Well... no. As FM signals come in loud and clear, just not locking on properly or decoding RDS.

I suspect either an issue with MPX filter circuit or somewhere before, like the discriminator not set to zero properly.
 
Captain Jack

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Welp, the discriminator was the culprit. Instead of reading 0mV, it was reading 1.4V! I am hoping that it's the dirty pot and not some other component that has caused to be so far out.
 
Terryl

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Terryl

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Captain Jack

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Ouch. I can buy an actual oscilloscope for that!
 
Llew

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@Captain Jack

I have a Technics ST-G70L tuner, which, checking the schematic Terry points to, appears to have similar FM/AM circuitry (without RDS).

I had a problem that seemed to be something like yours, with stereo going down completely some time after the stereo indicator started flickering on and off.
Before checking voltages around the IF Amp/Detector IC102, I tweaked the offset voltage adjustment coil off the IC T101 a little, which restored the stereo O/P. I didn't actually check voltages afterwards - if it happens again I will do so.

The problem occurred after several years' use of the tuner, which leads me to think it may have been due to premature ageing of some components, as the tuner sat above an amp.

The tuner is very well designed, with ample space inside betweem the (linear) PS and main circuit board. I suppose I was unlucky with positioning the tuner immediately above the amp. OK now anyway.
 
Terryl

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Ouch. I can buy an actual oscilloscope for that!
Yah I know, I had a really nice HP scope, the main CPU went FUBAR and they (HP) wanted 3K to fix it, I sold it as non working for 400$ on fleabay, cant afford a new one these days.
 
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