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FM Muting/OTS No Stereo CR-820

M

MiKokomojoe

New Member
After a successful output transistor replacement, all seemed well.
The only issue I note is that I have no stereo from the tuner.
The FM tuner only works in mono mode - FM Muting off.
I get AM signal fine, signal strength meter goes up to 40-50.
With FM Muting On, I get no FM sound at all; signal meter at 9.
With FM muting Off, I get FM sound, no stereo light, signal meter at 9.

What can I check?
 
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Muting usually kills stereo even if it is off. So if muting active the signal is mono even though stereo would work just fine. Looks like the muting detector doesn't work anymore. Was muting OK before the transistor replacement? Sometimes blanket replacement (if that is the case here) introduces hard to solve problems. Stereo light will be off for sure.
 
Did it work before that? The signal level circuit of this Yamaha is unusual as it detects the amount of ultrasonic noise in the demodulated audio (TR304/5). The less noise the higher the signal level meter. I looked at the schematic but can't quickly figure out how the muting works. Need some more time to study...
 
As far as I can tell, it sounds mono.
No stereo light.
I don't know if it worked before. It has been down for 15 years. Belonged to my Dad.
I played a cd thru Aux., but can't tell if it sounds stereo or not.
 
MiKokomojoe said:
I played a cd thru Aux., but can't tell if it sounds stereo or not.
Click to expand...
There is a MODE mono /stereo button. Toggle it on and off. There should be a big difference in sound quality between mono and stereo.
 
I don't get it. Your meter works. It is connected to TR305 emitter. From its collector it goes to a flakey circuit with TR306-9 and TR201. The latter's collector goes to pint#11 of IC201, which is the MPX demod chip. And I have no datasheet of this LA3350. Pin#6 is the stereo indicator. If you tune through a strong station, does its voltage change?
 
Oh, wait a minute! TR307 and 8 are muting for when off-center. It is connected to IC103 which is the discriminator buffer (opamp). I guess that pin#11 of IC201 is the mono signal when high. So, what does that pin do?
 
PE9ZZ,
I appreciate your responses, but you are talking 3 steps over my head. I am not that experienced in electronics.
I have a decent multimeter, and can follow step-by-step instructions if you have the patience.

As for strong stations, are you referring to the signal meter on the faceplate? It barely moves between 0 and 10.
I have a cr-800 in the same room, and the signal meter goes up to 40-50 on the same stations.
The tuning needle does swing widely when moving thru a strong station.
 
Oh, I thought you had almost full deflection. Should learn to read better. Then it's definitely a matter of the signal strength circuit not being right. As I pointed out, Yamaha doesn't use a straight forward signal level criterion but a more elaborate "signal quality". Lemme sleep on it to come up with a step by step plan to troubleshoot it. Or maybe someone else chimes in. The mono signal you receive is not noisy at all is it?

Edit: and one more thing you can try: while tuned to a strong station, does changing VR301 do anything on the meter? And maybe unmute the tuner?
 
It sounds like poor gain either in the frontend or IF. At minimum you need an oscilloscope with >100MHz bandwidth to troubleshoot this. Plus rf knowledge.
Take it to a technician with rf experience. RadioX is in Michigan. Don't know if he is accepting work with this virus. I'm not.
 
New observations....

I moved the tuner fully across the entire dial slowly, and discovered five local stations that came in with FM Muting on.
The Stereo light was on, and the DTS light was on. Muting appears to work properly.
Two of those stations showed about 60 on the Signal meter. The rest were 20-30.
So, it appears that the OTS system is working, it's just not pulling in many stations. I live in suburban Detroit, so there is no shortage of stations on the dial; and my CR-800 in the same room picks up a great many more with FM Muting on. And most of those are 60-70 on the Signal strength meter.

I will check my wire antenna; maybe swap it with the one on the 800 so I can rule out a bad antenna.
 
You should be able to adjust it with VR301. But now it's confirmed that it does unmute it indeed looks like lack of sensitivity. Or bad antenna.
 
I have found this to be a common problem with Yamaha tuners. Bad ceramic caps in the frontend are usually the cause. You have to be able to go through the circuits with a special low capacitance probe and a spectrum analyzer to see where the gain drops off. Then it has to be aligned which requires special equipment. This is not for a beginner.
 
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