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Sansui 5000A FM stereo problem

B

Bart Eddy

AK Subscriber
Subscriber
I have a Sansui 5000A that works very well on everything except FM stereo. When I select the FM stereo switch, there is no sound. Without it selected, it will play FM in mono. On most receivers, I would replace the multiplexer chip. However, the 5000A multiplexer unit is all discrete components. I'll get in and start testing individual components, but does anyone have any advice on the likely culprit? Thanks for your help.
 
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No. I tried the muting switch on and off several times and no sound when the FM stereo only button is engaged. I checked all eleven diodes and eight transistors on the multiplexer board and couldn't find a smoking gun. After the stereo button is pushed on, the voltage to both sides of the stereo bulb starts very low and steadily climbs over the span of a minute or two to 6 VDC. Transistor TR-408 doesn't open to allow the bulb to go to ground so it can turn on. I have not back-tracked from there yet.
 
The 'stereo only' button will filter out any station not in stereo. The 'mute' will filter out weak stations. For some reason it's not getting a strong enough signal, so you'll hear nothing. For troubleshooting purposes, leave both of these 'off'.
 
Thanks, but I'm confused. I can't get any station to come in on stereo even with a strong signal on several local channels I know well. I use a dipole antenna for testing. The signal strength is 4+. The bulb is fine. I'll do as you suggest to see if anything comes out differently or not. Dumb question - does the FM stereo only button only filter out mono stations or does it need to be turned on for stereo stations to play in stereo instead of mono?
 
It may take a while to find but I believe there are threads on AK about issues with the 5000A FM. Didn't bookmark them as I don't care about FM, haven't for decades.
 
I found the problem. It was the diode supplying DC power to the stereo bulb. As stated earlier, the voltage to the bulb would start at zero and steadily climb to 6 volts after a few minutes. I disconnected the bulb from the power supply, yet the power supply still acted the same. I replaced the power supply capacitor, no change. I added a diode in parallel to the existing diode on top of the board by wrapping its leads around the leads of the existing diode leads, then soldering them because it was difficult to take out the old diode. Problem solved, voltage instantly jumps to 7 VDC and the stereo light comes on.
 
Bart Eddy said:
I found the problem. It was the diode supplying DC power to the stereo bulb. As stated earlier, the voltage to the bulb would start at zero and steadily climb to 6 volts after a few minutes. I disconnected the bulb from the power supply, yet the power supply still acted the same. I replaced the power supply capacitor, no change. I added a diode in parallel to the existing diode on top of the board by wrapping its leads around the leads of the existing diode leads, then soldering them because it was difficult to take out the old diode. Problem solved, voltage instantly jumps to 7 VDC and the stereo light comes on.
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Hi, I was just going to start a thread with the same "no stereo "issues on a nicely benched/tested Sansui 5000A I snagged last week but I cannot get any stations, strong or local, to come in true STEREO (only mono) and the STEREO light does not come on. It seems you have solved this problem with diode and getting the broadcast in STEREO, correct?. Is there any chance you could share a photo or diagram of the update you did I would really appreciate that. Thanks!
 
Do you have a service manual? If not, you can get one for the 5000A from elektrotanya.com. They are safe to use. Search for sansui 5000a sm2. It is 36 pages. On page 34 (the 3rd from last page), you will see the schematic that has the power supply circuitry to the stereo light. It is immediately below the label for F-1029 and to the left of the label for F-1042. It is a simple circuit that takes 5.7 VAC from the transformer and converts it to 7.8 VDC with a single diode (D-001) and capacitor (C-018). I had to search for the small board that contains this circuitry to measure the voltage out of the power supply. You can find it in the middle left of the unit between the rectifying capacitors. Look for the green capacitor (C-018) in the pictures. Your's will probably be gray. To the left of the green capacitor is a round black diode. If you look closely at the close-up picture, you can see a small diode installed next to and in parallel with the round black diode. You need to meaure the voltage on the left (positive) lead of the capacitor to see if you have the same problem that I had. Before it was fixed, the voltage started at zero and slowly climbed over a few minutes to 6 VDC. After replacing the diode, it comes up immediately to 7 VDC and lights the stereo light. This needs to be done when you have a strong signal from a FM station that broadcasts in stereo. Good luck. 20250904_210013.jpg
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Bart Eddy said:
Do you have a service manual? If not, you can get one for the 5000A from elektrotanya.com. They are safe to use. Search for sansui 5000a sm2. It is 36 pages. On page 34 (the 3rd from last page), you will see the schematic that has the power supply circuitry to the stereo light. It is immediately below the label for F-1029 and to the left of the label for F-1042. It is a simple circuit that takes 5.7 VAC from the transformer and converts it to 7.8 VDC with a single diode (D-001) and capacitor (C-018). I had to search for the small board that contains this circuitry to measure the voltage out of the power supply. You can find it in the middle left of the unit between the rectifying capacitors. Look for the green capacitor (C-018) in the pictures. Your's will probably be gray. To the left of the green capacitor is a round black diode. If you look closely at the close-up picture, you can see a small diode installed next to and in parallel with the round black diode. You need to meaure the voltage on the left (positive) lead of the capacitor to see if you have the same problem that I had. Before it was fixed, the voltage started at zero and slowly climbed over a few minutes to 6 VDC. After replacing the diode, it comes up immediately to 7 VDC and lights the stereo light. This needs to be done when you have a strong signal from a FM station that broadcasts in stereo. Good luck. View attachment 3588854
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Wow, thanks for the detailed response and photos. What is the brand/model# of the diode that replaced D-001 and where did you find it? Thanks for all of the valuable info.
 
Bart Eddy said:
These are what I use. Most any diode will do since the power and voltage are low. Did you measure the voltage to prove that is your problem?

www.ebay.com

20pcs - 1N4007 1000V 1A General Purpose Rectifier Diode - DO-41 - Goodwork | eBay

This item is sold as a lot of 20 pieces.
www.ebay.com
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Bart-sorry for the delay but wanted to get back to you on the FM light/stereo issue. I do not have voltage reader but did the diode replacement (soldered to the original legs) you did on D-001. In the mean time, I applied Deoxit to the MPX separation "screw" and function selector switch multiple times to clean up the contacts and made adjustments until the separation on the mpx was distinct. Afterward, I was able to get true stereo (checked on headphones vs mono) on fm but no FM Stereo light yet the reception was good. Today (as I was doing pushups on the floor) I noticed the FM Stereo light was flashing on and off. Since it is a local station I listen too, I moved the FM switch on the back to Local and boom-red light stays on, reception is much improved the the stereo separation is very distinct. So at this point I'm a happy camper!
 
Bart Eddy said:
Great to hear. Learn something new every day.
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Yes, and something else I noticed is the more I play/power on the Sansui (it sat in storage for a long time before I bought it ) the better it sounds. Better detail in sound nuances and better bass. Thanks again Bart for all of the detailed photos and instructions on your end.
 
I wonder if the capacitors are reforming, though I'm not sure this is a thing with electrolytics.
 
Guido57 said:
I wonder if the capacitors are reforming, though I'm not sure this is a thing with electrolytics.
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Guido57-Not really sure but I swear the sound is coming around; esp now that I have solid stereo reception/separation on the FM. BTW, I like your avatar; assuming you are a cycling enthusiast?
 
As I assume you are. Not sure where I found that picture but as the years roll on the hills start to seem less friendly.
 
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