No FM stereo on luxman r800 receiver.

johannesnic

johannesnic

Active Member
Hi all,
I have a nice Luxman r800 receiver where everything works fine except I cannot receive FM signal in stereo, it is mono all the time even when the stereo light comes 'on' and the strength signal is way up and centered. When any other function is used I have great stereo except for the tuner part. Is there anything I can do to solve that problem?
I would appreciate any advice and help, thanks......DSCN7017.JPG
 
Briefly looked over the low res service manual they have at Hifiengine. Probably needs an alignment, the one thing you could try without test equipment and knowledge would be the muting adjustment. VR202.
 
I tried VR202 right now, no change in to stereo but it taught me how the muting function cut out point works together with the muting switch on this receiver, and that worked very nice thanks....
I turned the position of VR202 back to where it was visually as I take a photo before I start turning anything.
I was hoping not to bring this unit somewhere, because I have to go very far and that will get very costly.
I am really hoping to get the stereo working on this tuner as this unit sounds really sweet.
Can you tell me more for instance what VR201 does. I found the service manual a bit vague when it comes to what the adjustments stands for.
There are some pots on that board you can adjust but I have no clue what the function of each is.
Thanks very much for your help, I really appreciate it....
 
VR201 is calibrating your signal strength meter to a given signal input. In this case the manual has you adjust it with 7uV input from the generator and your adjust the signal strenght meter to the first mark, but 1/2 is acceptable. Most tuner due just the oposite, put a large signal in and adjust the meter for to the maximum.

If you are getting good signal strength on the meter, and muting is working correctly then VR201 will have no affect on receiving stereo.
Are you sure the lamp is not burned out? Do you hear any change switching from stereo to mono?
 
Hi guys, No I don't hear any change when switching from stereo to mono even though the stereo light and the signal strength meters/centered are working fine. When I tune into a station signal strength/center meters working just like any tuner I have here and when centering the station the stereo light comes 'on' beautifully, just no stereo over the headphones or loudspeakers. The sound is a good quality mono and when I switch to any other function stereo/mixed or reverse no change. Now if I do that on aux/tape or phono I have beautiful stereo, just one of the best sounding receivers on my shelf.
(Is the FM station remain locked?) Well I don't know what you really mean exactly. and that is the manual I have been working with
Thanks.
 
Ok we are a day further and I am trying to figure this out here, I have learned now that when the FM tuning section of the receiver is in 'stereo ' mode and works fine but that the channel separation is so poor that it sounds like mono. Is this something that can be adjusted?
There is another smaller board on the left of the tuner board 'PB 458' with a bunch of adjustment pots.
Is that the multiplex board?
Also did a test with another tuner hooked up on the 'aux' position and really sounds a fair bit louder than the built in R800 tuner.
Could the signal not be strong enough?
 
The Luxman may need more signal strength than average to get clean FM Stereo. Does your tuner behave like this on all stations?
 
Yes, all stations unfortunately, thanks for your help.
I tried hooking up different antenna's without improvement of sound.
Some had a better signal strength result..
I believe now that there is stereo, but a very poor separation making me think it is mono.
I have read something about an oscillator that creates channel separation and something about the 19 KHZ function.
https://groups.google.com/forum/#!topic/sci.electronics.repair/Zd28vL9_AYI
I am just trying to educate myself on what makes a radio work.
I don't know how much abuse this unit received before I became the owner, but I am getting it better than it was.
I finished recapping this unit and replaced all '1345' transistors from Hitachi hoping to solve the stereo problem.
Any help here is appreciated thank you.....
IMG_8657.JPG
 
It looks like you have an earlier type of stereo decoder where there is no frequency adjust pot, all the adjustments are the coils. With the later integrated circuit decoders, you have a 76 KHz oscillator controlled by a pot that is divided by two to get a 38 KHz switching signal and by another two to get a 19 KHz signal that is phase-locked to the pilot signal. With the earlier decoders, you have as a minimum, a 19 KHz pilot tuning signal, a 38 KHz oscillator signal, a bandpass filter for matrix decoding (which is not needed for switching decoders) and an SCA trap for 67 KHz. The nice thing about a decoder is that you have two channels you can compare to each other so if the DC bias is off on one channel, you can check it against the channel that works. You might be able to tune things with a received signal and a scope but a stereo generator may be necessary and it would be good to find a stereo alignment procedure because nothing is obvious from here.
 
Thanks for your answer, I am going to study what you just said here.
I appreciate it very much.
If I have to buy the equipment to do this repair myself I will and so want to learn how this tuner works, that is all what this hobby means to me.
I love to learn as I enjoy this hobby of old receivers, nothing better in live for me as it has consumed me now for more than 30 years listening and comparing vintage HIFI audio equipment.
 
PB-458 appears and verified by looking at the schematic.. That is your AM board.. No affect on your FM stereo at all.

MPX would be L205,6 and 7. Separation adjustment is R203. At lease according to the alignment procedure. Which as you peak 19khz with L206 and adjust for maximium separation with R203.

Since you light the stereo lamp makes me think L206 is good, and probably does need adjusting. L205 and 206 may need some adjustment. Mark it and wing it. With a signal and MPX generator it would be simple, but without it will really never be correct. You might be able to put a scope in X-Y mode and watch for good separation.
 
Wow this is serious info I didn't have before. Thanks so much.
I could not find that info in my manual, I guess you looked at the schematic and understood what is what or have a better manual.
DSCN7679.JPG
What I found on this board was T206, T207 and T205 instead of the "L" you mentioned.
T206 is yellow and T207/T205 are black.
Notice VR203 is right above it.

What I did was first took a photo of all the positions than adjusted or wiggled T 205 and T 206 and put them back in position but when I moved VR203 counterclockwise by about almost a quarter turn I had 'stereo' on the head phones immediately.
I know it is not perfect but, now I definitely know it is not a defect but an adjustment it needs.
I'll see if I can tweak this on ear a bit better, and thanks for making me understand.
I am going to buy a scope and signal generator and learn some more.
Thanks so much.
I have STEREO!!!!!

Johannes.....
 
johannesnic said:
I am going to buy a scope and signal generator and learn some more.
Click to expand...
For one fix? could cost you quite a few $, just the shipping costs get expensive.
Hi Johannes,

Congrats, you have the Wolf in stereo again. I live relatively close to you and can do an alignment for you if you want one and are in my area.
The old stereo decoder, uPC554c is a first gen IC decoder, before the improved PLL later designs, so it could be improved with a better chip such as a LM4500A or HA11223. there is a 4-ch output you can use for this.

Rick
 
Not for one fix, I have about fifteen receivers on my shelf, and a few could use a proper tuner alignment, so it doesn't hurt to learn and buy the equipment necessary.
I have had receivers that could have sounded much nicer if they had an alignment before I had decided to part with them.
However you got me very curious and interested with the improvement possibilities on the stereo decoder of this receiver.
Just googled your location, but 200+km is too far and too costly to consider and I am not shipping my babies anymore, but thank you very much for the offer.
That was very nice.....
 
2+ hr drive is too far, too bad, good luck with your equipment search.
I know your area, go there on weekends for fishing and recreation. I see the bar/restaurant on Moira Lake is gone, too bad
 
Johannesnic and I have been having a 'conversation' for a bit. To be honest, I neglected to notice it was a 'Conversation' and not open forum posts. For review... He replaced the demux chip (uPC554) with an NTE 1142, which did not solve the problem. I had him lift C245 & C246 alternately, and the signal on the opened channel went away, indicating the problem was in the demux area and not after it. Took a look at test point CP-5 and only a 19 KHz negative going pulse can be seen. As the signal across this LC feeds the double balance mixer internal to the mux chip, I would have expected 38 KHz at that point.
DSCN5497.JPG
The 19 KHz impulse should be causing T206 to 'ring' at 38 KHz. I am suspecting an issue with T206, specifically with the internal capacitor. I found a similar circuit using this chip in a Grundig RR900, and there the capacitor is external to the coil and is listed at 2.2 nF. That would make the coil about 8 mH. Doesn't mean those are the values here, but it is a starting point. The layout of the board shows an unconnected pin on the T206 footprint, which I am hoping is the high side of the coil/top of the capacitor.
Annotation 2020-06-23 194325.png
Resistance measured between the two marked leads measures 1.8 ohms, so I think that is likely so we may have access to both ends of the coil. Plan is to try adding external capacitance and see if we can get any kind of 38KHz response at the TP. Last resort will be to remove the part and do a careful disassembly.

Any comments from the vast knowledge base of this forum more than welcome.
 
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