FM MPX Ulcers ! (blinking stereo lamp)

Jun 26, 2007 4 Replies
S

I'm restoring a vintage Kenwood Model Nine GX receiver (cir. 1970's)no schematic.



I have gotten the FM working finally and receiving stereo but the stereo indicator lamp thumps on and off with the modulated signal. If it's tuned to a station with a dead carrier, it holds stereo. The audio does "garble" a little when the thing is flopping in and out of stereo lock.



So far, I've replaced the LA3350 MPX chip, the 1500 pico cap associated with the oscillator, Adjusted the VCO for 19Khz(which it does), I Aligned the IF stages and tuned the front end coils with a scope (At first the tuner could not receive stereo until I did that !)



When I scope the VCO output, it's a stable square wave but it does shift a bit in step with the audio.



There are 2 adjustable coils in the area on it's own small board. I'm assuming maybe they are quadrature coils. I have tried swinging them a bit but there's no effect. I snapped a pic of this board

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if anyone is interested in viewing it.



This is starting to frustrate me and I could use some advice. TIA Sparkey



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M

I don't have a clue, but when I saw the subject header, my first thought was "the signal is too weak". That doesn't seem the case, but your focus on the stereo decoder may be false. What if the decoder is fine, but something in the FM demodulator is bad or misadjusted? If the decoder doesn't get a good signal, it won't work.

Michael

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H

Maybe some of the electrolytics in the power area are going west. Check the power sully at all pointss. That would possibly explain the variations in [erformance with varying audio.

H. R. (Bob) Hofmann

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S

I got to the bottom of this problem today!!

When I pulled the RX board out again yet another time, I did notice a discoloration on a couple of the traces near the edge of the board. I'll be damned, looks like the remnants of a liquid spill ! Looking at the top side of the board it hardly showed but the spill must have followed gravity to the underside of the pcb, where it dried. I chased down the traces and sure enough ! It dried shorting pin 15 (loop filter) to pin 2 (composite signal in) I scratched it off and cleaned it with acetone I had to re-tune the 19khz VCO but I had beautiful and STABLE FM stereo ! Thanks for the repies. write this one down guys, I was going nuts !

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M

That's 'cuz in the 70's everybody drank while listening to "underground" rock stations so you had a high probability of spills...

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