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Pioneer SX-650 Intermittent FM Stereo

LBPete

LBPete

Rolling Along
Subscriber
This SX-650 sometimes receives in stereo and sometimes it doesn't. I don't see a real pattern to it. Turn it on and no stereo light. Tune it to another stereo station, no light, tune it back to the first station, the light comes on. Sometimes playing with the FM mute switch will get it to come on.

Other than that the tuner works well. Strong signal on weak stations and very good selectivity. Any ideas other than a full alignment?

- Pete
 
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I'm familiar with the process. The stereo reception got intermittent after I applied drops of Deoxit D100 to the ground springs on the tuning cap shaft. That was kind of a Hail Mary because the tuner works so well otherwise. In my experience a dirty tuner cap causes the tuner to get very "twitchy" and noisy when tuning across a station. This tuner is rock solid and perfectly balanced when turning across a station. The tuning meter and muting function respond equally on both sides of the station. It's completely silent and the tuning and signal strenth meters respond perfectly.

I'll try cleaning it more thoroughly.

- Pete
 
LBPete said:
The stereo reception got intermittent after I applied drops of Deoxit D100 to the ground springs on the tuning cap shaft.
Click to expand...
if the signal level dropped or if it is now noiser than before, then makes sense, but if it has not changed, then I wonder how it affects the stereo decoder+. does the stereo lamp turn on/off symmetrical to tuning center?
did you check the PLL VCO freq? HA1156 or HA1196, I have to look.
 
To clarify my earlier comment, before applying the deoxit to the ground springs on the tuning cap shaft, it wasn't receiving stereo at all. Mono only and no stereo light. The next day after putting a few drops of D100 on the cap ground springs it would occasionally receive stereo and the stereo light would come on. During that period when tuning across a strong station the stereo light would sometimes come on well before reaching center tuning on the tuning meter and sometimes go off when center tuned.

Today I cleaned the springs with contact cleaner and worked the tuning cap lock to lock multiple times. It's still very intermittent not receiving stereo on most stations. It may be slightly better towards the center of the dial. Another strange quirk is you can tune it to a station and it's receiving in mono, if you turn it off and back on, the stereo light comes on but if you leave it there the signal will drift, the light goes off and doesn't come back on when tuning back to center. Every other aspect is strong and good. No noise, no twitchyness. The signal strength meter increases to the center point of the tuning meter and decreases uniformly after passing the center point. Other than this stereo issue the tuner seems perfect.

- Pete
 
I gave it another cleaning yesterday and when turned on this morning, the stereo function was working fairly consistently. I tuned across the entire dial and the stereo light lit on most of the stations. In some cases it lit on relatively weak stations. In many cases, it took several seconds after center tuning to light. On other stations it lights right away. You can hear the switch from mono to stereo when the light comes on.

After playing for awhile in stereo, the signal drifts and you have to dial it back in. After playing for awhile it stops receiving stereo on stations that did earlier. It goes back to the intermittent operation described earlier in this thread. Since it seems to be temperature sensitive, I hit chips and transistors in the tuner with a little freeze spray but nothing changed.

As far as the AGC circuit, I wouldn't know what to look for.

- Pete
 
In addition to the tuner VC assembly itself, cleaning the circuit board and components in the tuner section may have an effect. Something may be drifting as a result of temperature and/or state of charge, and contamination between circuit traces can affect this.
 
All good advice. I have cleaned the large circuit board that contains the tuner. It does have a lot of long parallel traces. I'll give it another once over.

I played with the antenna yesterday and this tuner is amazing. I live in a pocket with difficult FM reception for some stations and use a small analog TV antenna on the roof of my garage as my test standard. I wasn't concerned about the antenna because on all but the weakest stations the signal strength meter was always near or at full strength. But since a couple comments suggesting checking it, I disconnected it to switch to another antenna. This receiver picks up a whole range of stations without any antenna connected and in some cases received in stereo. Very impressive.

In any case, I played around with a pair of rabbit ears and it did have some effect on stereo reception. At this point I'm convinced that whatever circuit controls the band width of accepting the stereo signal is just set too narrowly. Looking through the alignment procedure in the service manual, I don't see reference to PLL VCO, AGC or stereo light so I'm inclined to leave it alone.

I've recently acquired a Kenwood AM/FM signal generator and HP distortion analyzer both of unknown operating condition. I haven't had a chance to work with them at all but that's a project for a different day.

Thanks everyone for your help.

- Pete
 
The ability to receive stereo is tied to the muting and stereo threshold. These are fixed in the SX-650. So signal level and discriminator zero will affect the muting signal. Muting signal will turn off the MPX decoder / stereo lamp. Detecting the 19Khz MPX carrier can also turn on and off the stereo lamp. If you have so many station could be multipath issues.
 
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LBPete said:
I've recently acquired a Kenwood AM/FM signal generator
Click to expand...
Hook it up, figure out how to turn on-off 19KHz pilot carrier modulation.
I found after I sprayed de-oxit into the var cap it took a day or so for the stuff to dissipate out to bring the var cap back to where it once was.
 
Too late, I returned it on Saturday. I had completed a recap and amp rebuild before I started this thread. I told the owner up front that I wasn't set up to do alignments. He was primarily interested in the amp and phono section and didn't think he would use the tuner section. Frankly, the FM works pretty damn well overall. I showed him it could receive stereo on some stations without an antenna connected.

I've been combing the internet looking for operational information on this Kenwood SG-5110 AM/FM Signal Generator and have been unable to find a manual for it. Most alignment procedures have the RF signal generator connected to the 300 ohm antenna input. The RF out on the Kenwood is marked 50 ohms so I assume I need a 50 to 300 ohm balun to connected to a 300 ohm antenna input. That's a hard beast to find too. Even a 50 to 75 ohm don't seem to be readily available. What are you using?

- Pete
 
I have a passive 50-75 (BNC) from mini-circuits, they are expensive new, but I found a cheap used one on the bay. if there is only 300 ohm balanced i/p then you use a 75-300 balun for that.
but in reality you are not measuring the unit for accurate input quieting sensitivities #'s, you are aligning it for the best quieting sensitivity and the SG o/p # really does not matter too much
 
I have other projects to clear out before diving into this signal generator. It's also going to require a major re-shuffle of equipment to fit it in. I'll certainly be looking for advice when I get there and appreciate your input. Thanks.

- Pete
 
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