Tube receiver FM separation issue

[mustang guy]

I am going to troubleshoot a problem with my Pioneer SX-800, where the FM reception only sounds good in mono, even though the signal is strong, and the FM Stereo light is lit. In the service manual, there is a troubleshooting section for FM poor sensitivity, and there is another section for a variety of Multiplex issues.

When I connect a stereo source to the AUX input or the PHONO input, there is little perceivable trouble with stereo seperation.

My question is this. Is the multiplex circuitry typically involved with AUX and PHONO inputs? If so, then the FM poor sensitivity area is where I will start. If not, I really think it is the multiplex circuitry.

Any additional advice is welcomed. I have the SM in PDF, but I can't post it online publically.

[Groomlakearea51]

The MPX circuitry is prone to interference, etc. on older tube tuners. It's difficult to get around it unless you use a really good, isolated FM antenna. Noted this on many FM tuners, including TU-99, 919, 9900. 719, 719, and the Macs. I live in the sticks, and until I isolated the antenna it was an issue.

Just a

thought.

[tube fanatic]

The problem is not in the preamp or amplifier sections. To troubleshoot and properly align the front end and multiplex circuitry you need to have some fairly sophisticated test equipment. Unless you are experienced in doing this, I suggest that you refer it to a qualified repair person. It's easy to mess things up very quickly! That said, you could try replacing the 6CW4 and 6AQ8 in the front end. Even though the stereo light is illuminated, you may still not have sufficient signal to get a really good lock on the station.

[Guest " "]

if your tuner uses exposed variable capacitors (the evenly spaced aluminum fins that move as you tune stations ) you could air blast them with canned air to blow off all the dust.

[T.H.E. Droid]

Sensitivity has more to do with the ability to pick up the station, not the quality of FM decoding.

The stereo FM signal is not a simple RF analog of a stereo audio signal. The main FM signal is actually the L+R channel signals combined. That makes it compatible with older monaural FM demodulators. To get stereo FM, a difference signal (Left minus Right) is modulated onto the sidebands of the main signal. You can't hear them directly. The FM tuner demodulates (removes the FM carrier) this sideband signal, then takes the mono (L+R) signal and combines with the difference signal (L-R) which elimintes the right channel info. It also inverts the difference signal and combines with the mono signal (L+R) -(L-R) which negates the left channel. Those two derived channels become the Left and Right analog audio.

Since you're receiving the monoaural signal OK, and the stereo light is on (which means the tuner is seeing the sidebands), somewhere in the tuner there is something going wrong with the inversion and summing process. On older equipment, the tuner is usually made of discrete components, meaning that something may be broken or mialigned that can be corrected. On newer equipment, FM demodulation is usually performed in a single integrated circuit that is not repairable.

A technician with the proper FM alignment equipment can probably repair and/or realign your tuner.

[mustang guy]

The MPX circuitry is prone to interference, etc. on older tube tuners. It's difficult to get around it unless you use a really good, isolated FM antenna. Noted this on many FM tuners, including TU-99, 919, 9900. 719, 719, and the Macs. I live in the sticks, and until I isolated the antenna it was an issue.

I had planned on a better antenna. I will make sure I buy one with good isolation charicteristics.

The problem is not in the preamp or amplifier sections. To troubleshoot and properly align the front end and multiplex circuitry you need to have some fairly sophisticated test equipment. Unless you are experienced in doing this, I suggest that you refer it to a qualified repair person. It's easy to mess things up very quickly! That said, you could try replacing the 6CW4 and 6AQ8 in the front end. Even though the stereo light is illuminated, you may still not have sufficient signal to get a really good lock on the station.

I was pretty sure it was either the FM front end or the multiplex circuitry. I am the type of guy who really enjoys learning. I learn by doing. I am content listening to mono, but I really want to know why she's only fireing on 7 cylinders. I had an electronics repair shop test all the tubes and they tested OK. I don't know how good the tester he used was, or if it was properly calibrated. He said an old timer gave it to him, and he only uses it occasionally. I am going to get a better antenna, and we can see if that extra signal helps.

if your tuner uses exposed variable capacitors (the evenly spaced aluminum fins that move as you tune stations ) you could air blast them with canned air to blow off all the dust.

While I was cleaning potentiometers, I sprayed those fins. They didn't look at all dusty. This receiver is actually very clean.

Sensitivity has more to do with the ability to pick up the station, not the quality of FM decoding.

The stereo FM signal is not a simple RF analog of a stereo audio signal. The main FM signal is actually the L+R channel signals combined. That makes it compatible with older monaural FM demodulators. To get stereo FM, a difference signal (Left minus Right) is modulated onto the sidebands of the main signal. You can't hear them directly. The FM tuner demodulates (removes the FM carrier) this sideband signal, then takes the mono (L+R) signal and combines with the difference signal (L-R) which elimintes the right channel info. It also inverts the difference signal and combines with the mono signal (L+R) -(L-R) which negates the left channel. Those two derived channels become the Left and Right analog audio.

Since you're receiving the monoaural signal OK, and the stereo light is on (which means the tuner is seeing the sidebands), somewhere in the tuner there is something going wrong with the inversion and summing process. On older equipment, the tuner is usually made of discrete components, meaning that something may be broken or mialigned that can be corrected. On newer equipment, FM demodulation is usually performed in a single integrated circuit that is not repairable.

A technician with the proper FM alignment equipment can probably repair and/or realign your tuner.

Nicely said. In answer to my question above, the consensus seems to be signal improvement( in case the side bands aren't quite strong enough), FM front end alignment, and then multiplex alignment. I can easily do the antenna part. That may be the only problem. As for the rest, I would have to build, buy or borrow an FM signal generator, an Oscilloscope and a VTVM at the very least. Or try to find a qualified tech in my little town.

[tube fanatic]

The MPX circuitry is prone to interference, etc. on older tube tuners. It's difficult to get around it unless you use a really good, isolated FM antenna. Noted this on many FM tuners, including TU-99, 919, 9900. 719, 719, and the Macs. I live in the sticks, and until I isolated the antenna it was an issue.

I had planned on a better antenna. I will make sure I buy one with good isolation charicteristics.

The problem is not in the preamp or amplifier sections. To troubleshoot and properly align the front end and multiplex circuitry you need to have some fairly sophisticated test equipment. Unless you are experienced in doing this, I suggest that you refer it to a qualified repair person. It's easy to mess things up very quickly! That said, you could try replacing the 6CW4 and 6AQ8 in the front end. Even though the stereo light is illuminated, you may still not have sufficient signal to get a really good lock on the station.

I was pretty sure it was either the FM front end or the multiplex circuitry. I am the type of guy who really enjoys learning. I learn by doing. I am content listening to mono, but I really want to know why she's only fireing on 7 cylinders. I had an electronics repair shop test all the tubes and they tested OK. I don't know how good the tester he used was, or if it was properly calibrated. He said an old timer gave it to him, and he only uses it occasionally. I am going to get a better antenna, and we can see if that extra signal helps.

if your tuner uses exposed variable capacitors (the evenly spaced aluminum fins that move as you tune stations ) you could air blast them with canned air to blow off all the dust.

While I was cleaning potentiometers, I sprayed those fins. They didn't look at all dusty. This receiver is actually very clean.

Sensitivity has more to do with the ability to pick up the station, not the quality of FM decoding.

The stereo FM signal is not a simple RF analog of a stereo audio signal. The main FM signal is actually the L+R channel signals combined. That makes it compatible with older monaural FM demodulators. To get stereo FM, a difference signal (Left minus Right) is modulated onto the sidebands of the main signal. You can't hear them directly. The FM tuner demodulates (removes the FM carrier) this sideband signal, then takes the mono (L+R) signal and combines with the difference signal (L-R) which elimintes the right channel info. It also inverts the difference signal and combines with the mono signal (L+R) -(L-R) which negates the left channel. Those two derived channels become the Left and Right analog audio.

Since you're receiving the monoaural signal OK, and the stereo light is on (which means the tuner is seeing the sidebands), somewhere in the tuner there is something going wrong with the inversion and summing process. On older equipment, the tuner is usually made of discrete components, meaning that something may be broken or mialigned that can be corrected. On newer equipment, FM demodulation is usually performed in a single integrated circuit that is not repairable.

A technician with the proper FM alignment equipment can probably repair and/or realign your tuner.

Nicely said. In answer to my question above, the consensus seems to be signal improvement( in case the side bands aren't quite strong enough), FM front end alignment, and then multiplex alignment. I can easily do the antenna part. That may be the only problem. As for the rest, I would have to build, buy or borrow an FM signal generator, an Oscilloscope and a VTVM at the very least. Or try to find a qualified tech in my little town.

This is not the type of equipment on which to learn how to do an FM stereo alignment! You will need to buy (building one is an extremely advanced project) an FM multiplex/sweep generator. That isn't going to be inexpensive. In addition, you will need to have a good understanding of how to use/interpret what you see on the 'scope. Where are you located? I may be able to help you find someone qualified to do this for you.

[mustang guy]

This is not the type of equipment on which to learn how to do an FM stereo alignment! You will need to buy (building one is an extremely advanced project) an FM multiplex/sweep generator. That isn't going to be inexpensive. In addition, you will need to have a good understanding of how to use/interpret what you see on the 'scope. Where are you located? I may be able to help you find someone qualified to do this for you.

I am in Parkersburg, WV.

I will likely just ship it off to http://www.radioxtuners.com/. They were recomended by forum member USNRET in another thread. He has been very satisfied with his results.

[tube fanatic]

This is not the type of equipment on which to learn how to do an FM stereo alignment! You will need to buy (building one is an extremely advanced project) an FM multiplex/sweep generator. That isn't going to be inexpensive. In addition, you will need to have a good understanding of how to use/interpret what you see on the 'scope. Where are you located? I may be able to help you find someone qualified to do this for you.

I am in Parkersburg, WV.

I will likely just ship it off to http://www.radioxtuners.com/. They were recomended by forum member USNRET in another thread. He has been very satisfied with his results.

I'm not familiar with them but looking at their site, they're just what you need. Another alternative is Bob Eslinger:

http://www.oldradiodoc.com/main.html

He's an ace at doing this stuff also and has a lifetime of experience to draw from. Good luck and be sure to post the results after you get it back!

[boom3]

Although all the advice posted is accurate, we've collectively overlooked the fact that stereo FM sensitivity is less than mono, i.e., it takes a stronger signal to get a stereo output with acceptably low noise. I would try a RF preamp ahead of your tuner. They used to be very common, but I guess now can be found your favorite online auction site. Try to find one whose gain is adjustable. Another fact is, of course, the antenna. My experience has been that older FM tube tuners-with certain exceptions-do not have the sensitivity of modern solid-state tuners. I'd also see if anyone in your area has the same model receiver or a tube tuner of equal quality and swap it out with yours to hear any difference. I suspect your problem is a mixture of inadequate antenna and a RF front end that is just not strong enough for your location.

[tube fanatic]

Although all the advice posted is accurate, we've collectively overlooked the fact that stereo FM sensitivity is less than mono, i.e., it takes a stronger signal to get a stereo output with acceptably low noise. I would try a RF preamp ahead of your tuner. They used to be very common, but I guess now can be found your favorite online auction site. Try to find one whose gain is adjustable. Another fact is, of course, the antenna. My experience has been that older FM tube tuners-with certain exceptions-do not have the sensitivity of modern solid-state tuners. I'd also see if anyone in your area has the same model receiver or a tube tuner of equal quality and swap it out with yours to hear any difference. I suspect your problem is a mixture of inadequate antenna and a RF front end that is just not strong enough for your location.

Your point is well taken which is why I recommended trying a new 6CW4 and 6AQ8. In my experience nuvistors didn't have an incredibly long service life and sometimes tested "normal" but were inadequate for the job. Mixer tubes also can test ok but need replacement. Regrettably, both of these tubes are a bit costly.

[mustang guy]

Good information. I will perform the steps below until the FM stereo is working satisfactorily.

1. Mount a good FM antenna as talked about in the recent FM Antenna thread.
2. Find a variable gain FM booster, and attach it as close to the antenna as feasable.
3. Buy the tubes you recommend.
4. Ship the receiver off.

Thanks again guys. As usual, great help in this forum!

[Groomlakearea51]

I wanted to reserve comments on the RF pre route as they can get expensive. But that is a solution that, all else being equal, works. Insubstance, the most cost effetive way to approach the problem is to assume that the receiver is otherwise ok, and the issue can be solved with a good antenna. That usually works; if it does not, the other solutions discussed will probably solve the problem, and if solved, or at least resolved to some degree, a good antenna may be the "missing" part of the puzzle.

Just a thought.

[H]

[WMcD]

I suspect the stereo light comes on when the receiver detects the pilot tone which is used in connnection with the broadcast of the L-R signal. That signal is in the sideband but detection of the pilot tone does not mean the L-R signal is strong enough tp kick the multiplex decoder into operation (IIRC).

I too think this is an antenna problem in that a better antenna will deliver a stronger and less noisy signal. The connection of the additional feed to aux or phono is providing a bit of a ground which is assisting with reception.

Of course a tuning up by a professional can't hurt. OTOH, you might find there is no improvement.

WMcD

[Groomlakearea51]

WFGMD pointed out something interesting; the use of another "circuit". Since the tuners generally send a nominal 15--250mV signal, where it's actually sent as far as "high level" inputs should make no difference. The issue could be resolved from that aspect by simply trying "aux" or a tape circuit just to see if that makes a difference.