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KR-4070 no FM

c.coyle

c.coyle

Fighting the Dunning-Kruger effect.
I picked up a Kenwood KR-4070 receiver. All inputs and outputs work fine, except for FM. All controls, switches, and jacks have been cleaned. I haven't found anything open, broken, burnt, or bulging.

On FM, when I tune across the dial and hit a strong local station that I know is broadcasting stereo, both meters deflect, but I get no audio and no stereo light. Instead, I get intermittent, volume-dependent, metallic crackling (headphones, haven't hooked up speakers yet), anywhere on the dial, signal or no. Otherwise, silence. I can't hear any actual station or get the stereo light, no matter how strong the signal.

1. There are 2 2SA640 PNP's near the MPX chip. That along with the crackling led me to replace them with 2SA970BL's, but no change.

2. I deoxited VR1, the VCO pot, which looked a little corroded. No difference.

3. I went through the signal path step by step with a signal tracer, and it traces all the way from antenna terminal to speaker output.

I don't like replacing part after part on a hunch. Something tells me it's the MPX chip or something in the detector circuitry upstream from it. I am, unfortunately, scopeless.

Thoughts or suggestions?

EDIT: Added schematic clip.
 

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stupid question - did you try hooking up an external antenna?

Sometimes these tuners are uber sensitive and need help.

Had a friend who's 2220B pulled in very good FM signals but the STEREO light never came on. Used an antenna and light came on - was getting great reception but not good enough to trigger the stereo light.
 
I took the stereo bulb out of the circuit, leaving Test Points 8 and 9 open. The bulb tests GOOD. To my surprise, without the bulb in the circuit, I could briefly hear local FM stations, very low volume, one channel only. I could hear white noise between stations. Hard to tell if I was hearing stereo or mono since the volume was so low.

After about 5 minutes it all went away and I was back to the original silence with intermittent crackling.

Pulling the bulb and then hearing stations could be a coincidence, but in the words of the great detective, Harry Bosch, "in a case like this, there are no coincidences."

I replaced all the electrolytics on the tuner board, no change. Next step is reflowing every connection on the tuner board.

Help me out here, I'm stumped. Bad MPX chip? Transistor? Bad passive component? Something obvious that I'm missing?
 
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Gettin' there

Well I replaced the Sanyo LA3350 MPX chip, and now I have a stereo light and plenty of good sounding FM audio in the right channel, only. I have both channels on AM and Aux inputs, so I am thinking this is an alignment problem.
 
Alignment may help, but since the STEREO lamp is on the tuner is evidently receiving and decoding a signal in two channels. Getting those two channels out of the FM section will require the wafer function switches to be clean and connections intact. Those two channels also pass through the FM muting section, including Q6 and Q7.
 
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