Yamaha CR-640 FM stereo issue

dnnz21

dnnz21
Hi there folks,

I've been reading many articles on this very helpful forum, and it usually helps or directs me enough to solve a problem. But now there is something I cannot solve, not even do I have a clue where to start looking for the answer (although I searched this forum but still didn't find what I was looking for...).
Let's start at the beginning, I love vintage audio gear, especially Denon en Yamaha. Recently I bumped into a CR-640 at a secondhand store and I couldn't resist buying it. when I took it home an plugged it in, it was obvious that none of the lights where functioning. Thanks to this forum (and a multi-meter) I was able to fix this by replacing all the lamps, but one (in the dial). So there it is, in it's full glory, a solid aluminum front with green lights everywhere :)
It was until this moment that I didn't realize the tuner had an issue with stereo reception. The tuner functions normally, automatically switches from DX to local (and back), but although the signal seem quit strong, it never results in a stereo reception. Although sometimes the stereo (and blend) indicator light up, but only with some very weak signals.
Just to be clear, the signal is really mono, so it is not just an issue with the indicator.
So, here's my question, does anybody have an idea what might cause the problem, and preferably, a solution to it as well ;-)

Thanks in advance for your comments.
 
It's either A) broken or B) out of adjustment. Unfortunately you really need a stereo FM generator, frequency counter and THD meter to adjust it properly. I put a summary of the 620 alignment procedure here. The 640 is similar, but check the service manual for differences. My step 9 might be accomplished by hooking a scope to both channels in X-Y mode and adjusting for the "roundmost" display using a good stereo station, assuming the circuitry is otherwise functioning correctly.
 
Hi Conrad,

Thanks for your help and advice.
In case A): it is still a nice receiver and the amplifier part functions without any problem, and I do enjoy the looks of it.
In case B) : Unfortunately I don't own that kind of equipment, maybe I can find someone who could do the job for me (or at least help), I am willing to put the effort in it, but don't feel like spending a lot of money on it. Anyway I am definitely not going to trash the receiver, it can stay in my collection until there's someone who has the ability and opportunity to fix things.

Any other help or suggestions are still welcome.
 
Here is a suggestion that is both highly speculative, and is something I wouldn't usually suggest, because a CR-640 has a very nice tuner section that really deserves a proper alignment.

It sounds to me like it might be primarily the stereo decoder oscillator that is out of adjustment, and it is possible to do a rough adjustment of that, without instruments. Think of it as a "first aid" measure. I don't think there is any appreciable risk of making things any worse.

You would need to locate VR203, a "trim pot" (small adjustable resistor) that is located on the tuner cicuit board. Look a few centimeters rearward of the balance control.

Note the present setting of VR203, so that you can return it to that setting if this trick doesn't work out.

Tune in a strong station that you know is broadcasting stereo. Turn VR203 from one extreme to the other with a small screwdriver, and observe whether the stereo indicator lights at any point. If it does, then try to determine the range of adjustment of VR203 for which it stays lit. Move the adjustment to the middle of that range. That's it.

Again, this is quite speculative, based on my hunch about the symptom. It won't produce the best setting for VR203 (if it works at all), and it won't do anything with regard to any other mis-alignment issues. But it might get you into stereo until a proper alignment is possible.

Cheers,

chazix
 
"Solved" Yamaha CR-640 FM stereo issue

I'm very pleased to tell you that the solution Chazix gave me did indeed solved the stereo problem.
But not completely the way that was suggested:
- I marked the VR203 so I would able to return it to the old position
- I started to turn the VR203 just a tiny little bit, and there it was immediately, the stereo indicator lit up. I suppose it was a dust problem in the trim pot, and just by moving it the issue was solved.
- I turned back the trim pot to the original position and the stereo indicator remained on, and also the signal from the headphone was clearly in stereo.

What I didn't try is what Chazix suggested, moving the trim pot all the way, and locate the center position where there is still a stereo signal, but I am not sure now if I should, maybe it is better to leave it the way it was before.

I believe better results can be obtained by a complete alignment, but I am satisfied with the current result.

Thanks for your help Conrad and Chazix :thmbsp:
 
Super! I think dnn21's theory (that the pot was simply dirty) is plausible. If the stereo capture performance is satisfactory, I think it makes sense to leave the setting where it is.

chazix
 
Well done chazix and dnnz21! Heartwarming when something can be resolved thousands of miles away for $0.
 
I found that when I was still restoring Yamahas I would do exactly what Chazix suggested then for giggles double check with my Triplett DVM amd find the frequency spot on at 19kH. I would not only see the stereo light come on but since my small bench speakers are at ear level I would here it switch from mono to stereo as the sound stage suddenly went wide.
 
I was using headphones, so it was clear the signal changed to stereo. And it is true, thousands of miles away in the Netherlands someone is happy with his €14 receiver :)
Thanks!! (or as we say it: dankjewel )

Regards,

Dennis
 
Hi, merrylander! I'm not surprised by your observation. With this kind of decoder (a phase-locked loop), if the stereo indicator is lit, it means the decoder has locked on to the 19KHz pilot that's being broadcast by a station - and that had better be 19KHz, or the station's engineer would want to know about it. (And in the U.S., the FCC might want to know, too - though who knows if they really care about analog broadcast quality any longer.)

If you were to fiddle with the adjustment after the decoder was locked on, you should find that the frequency stays exactly the same for a fairly broad range of adjustment, and then suddenly goes way off if you adjust beyond the PLL's tracking range.

What will be changing, within the range of adjustment where the decoder can maintain lock, is the phase difference between the broadcast pilot and the decoder's oscillator. You want that phase difference to be zero. The way to accomplish that is by adjusting the decoder so that it oscillates at the right frequency when it's NOT locked. The typical procedure is to make the adjustment with an unmodulated RF signal.

Cheers,

chazix (slacking off even more than usual from the day job...)
 
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