Pioneer SX-3700 Quartz-Locked Tuner Target +10 High?

[riverty]

Imagine you have an SX-3700 and a target FM radio station frequency of 93.30 Mhz. Also imagine that you have the FM radio in "Mono" Mode.

You tune to 93.30 Mhz and the target station comes in fine, audio is good and signal strength is +4 bars. You then switch from "Mono" to "Stereo" Mode. Right away the frequency climbs +.10Mhz to 93.40, the audio now cycles in and out, and the quartz light and stereo light alternate in sync with the cycles. If you slowly dial back and "force" the tuner back down to 93.30, you get full stereo audio and the tuner seems stable.

Now let's say you flip the FM Mode switch from "Stereo" back to "Mono", OR the signal fades a bit and the quartz signal lock is broken, the tuner frequency immediately falls all the way down to 93.05.

It seems the FM tuner would work fine if it locked on the target frequency but this one tries to lock +.10Mhz high.

The unit looks unmodified and original. While it *could* happen, I don't think the FM tuner has been jacked with which leads me to think something has drifted out of spec rather than an adjustment (alignment) is needed. AM works great. AUX in is good. Seems to run fine and sound good otherwise.

I've never seen a tuner target the wrong frequency (+.10Mhz) before. Does anyone have suggestions I could target? What would cause this behavior?

Thanks all!

 

[rcs16]

Interesting problem. I have never looked at that design before. I see that it has some sort of AFC control circuit, yet another Pioneer quartz locked marvel design.
maybe if you adjust the tuner per the service manual instructions it will fix the problem. Easier said than done, if you do not have the experience nor test equipment. I could be wrong but that is were I usually start for a tuner, read the service manual to make sense how it works,check voltages, go thru the adj procedure.
p7 of the service manual describes the quartz lock circuit. The design counts/displays the Local Oscillator freq and the quartz lock steers it to center, so if your discriminator zero is off it could cause this issue.
I had a similar issue with a sx-1980 that i have on the bench, I just adjusted the quad detector zero (center tune meter) and it made the quartz lock work properly. this sx-3700 design is done differently than in the sx-1980 however.
After I read the service manual the FM tuner instructions are missing information as it does not say how to zero the quad coil T2 and other important adjustments = bunch of idiots. So I would have to basically research and fool around to figure how to adjust this tuner.
If you disconnect the antenna, put it in mono/muting off and tune to a quiet area of the dial so all you hear is white noise, what does the center tuning display show? it should show centered ,if not adjust the lower core of T2
From what I see, the mono sw position will disable the quartz lock feature.
Wish i could help more, good luck

 

[glend123]

I know this is an old post, but my SX-3700 does this exact same thing, it's hard to tune in stereo, in mono it locks in pretty good.
I just picked up another 3700 and this one tunes in stations just fine.