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Old 21st Apr 2021, 3:58 pm   #1
chartz2
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Default Luxman T-102L (1987) no stereo, no signal meter

Hi,

Not exactly vintage, sorry about that, but my A76 and my three FM4's work just fine!

My T102 has lost its stereo ability, as well as its signal led meter.

I can see the 19 kHz mpx vco and I can adjust it with VR303. I checked everything and voltages are as per SM. I obviously checked the 'force mono' circuit (my first idea).

The IF signal is loud and clear, I checked after every amp and filter.

I get fine audio, with no hiss or birdies, from my usual stations.

So, no stereo, no signal strength meter. Also, the auto scan doesn't find anything even though I can tune in any station I want manually and memorise them just fine.

Any leads please?

Thanks for looking.
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Old 22nd Apr 2021, 1:22 pm   #2
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

Hi,
I’ve checked the complete PCB for bad solder joints. I found a few doubtful ones but no cigar.
The tuner works just fine but no stereo.
Yesterday I listened to the same mono station all day. Before switching off, I selected a stereo station, and I saw for two seconds the signal meter fully lit and the stereo beacon too.
Then it went back to failure mode.
Odd.
What can I try next please?
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Old 24th Apr 2021, 1:31 pm   #3
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

This morning... it lasted 2 seconds before returning rot previous condition. Mono station.

Anyone?
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Old 24th Apr 2021, 2:13 pm   #4
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

The only suggestion I can give to your problem is it quite likely to be electolytic's either open circuit or high esr. I have a old analogue esr tester which I built 30 years ago that will check most capacitors in circuit, If you don't own something that will test capacitors in circuit you will have a mamouth task.
John
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Old 24th Apr 2021, 2:28 pm   #5
Mr 1936
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

Hi chartz2

The stereo decoder is based on the AN7471S IC. This has a VCO kill input on Pin 17, which appears to be linked back via diodes to the "Force Mono" part of the circuit.

Some tuners automatically switch to Mono when the signal level is low. It sounds as though your tuner "thinks" the level is too low, even if it isn't. I think the lack of signal level meter and the inability of the Scan circuit to detect the presence of signals is all part of the same problem.

I guess you need to try and monitor the DC voltages in the Force Mono section, even if it means soldering thin wires on so that you can extend your meter probes without needing to hold them in place. Then you can continuously check whilst it is operating and "catch it in the act".
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Old 24th Apr 2021, 2:49 pm   #6
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

I don't know this specific tuner, but look at the IF/detector chip. There will be a RSSI detector looking at the levels in the stages of the limiter amplifier IF. This gives a roughly logarithmic indication of signal strength.

It is used to drive the signal strength meter

It is used with the comparator which detects signals and stops scans

It is used to drive the stereo killer, to default the decoder into mono if the signal level appears too low.

It is the common denominator in all the problems you have described.

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Old 24th Apr 2021, 3:45 pm   #7
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

Thank you gents for your ideas and thoughts.
I’ll have another look and get back to you.
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Old 24th Apr 2021, 4:13 pm   #8
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

Update: Ic201 pin 5: 0 V. Cap ok.
Other voltages as per SM.
Voltages in the force mono section don’t move, as per SM values too. Voltage at pin 17 does goes up to about 5 V when switching over to AM.

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Old 24th Apr 2021, 4:48 pm   #9
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

Everything leads to what David says.
The lack of sensitivity is not real, so what makes it believe it is is a mystery.
The force mono voltage rises up to 5.5 V when switching over to AM but is about 100 mV when FM is engaged.
I can see pin 7 (meter drive) voltage rise up (to a low 2.6 V) when I move the dipole around (I know its correct orientation) or go down.

Last edited by chartz2; 24th Apr 2021 at 5:18 pm.
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Old 24th Apr 2021, 7:38 pm   #10
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

Edit: I cleaned the muting pot and I now see the led meter fully lit - on only one station! The other stations from the same (very powerful) transmitter don't light the signal meter at all.
Odd.
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Old 25th Apr 2021, 9:09 am   #11
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

Oh, and the stereo beacon blinks, albeit very faintly so.
Still one one station only, not even the most powerful.
Go figure.
David?
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Old 25th Apr 2021, 9:47 am   #12
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Default Re: Luxman T-102L (1987) no stereo, no signal meter





Only one station, loud and clear, no hiss. Quite distant too (120 miles away).
Stable right now.
All other stations (including powerful ones) show no deflection of the signal meter, let alone stereo.
But the hiss-free sound is here.
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Old 25th Apr 2021, 11:18 am   #13
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

It looks like you are on the correct path, but you have not yet got to the end.
Progress!

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Old 25th Apr 2021, 11:25 am   #14
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

Thanks.

How do you understand that?

What does the 4066 do?
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Old 25th Apr 2021, 12:26 pm   #15
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

Hi

It looks like your tuner has two selectable FM detectors. The phased lock loop type (PLL) will work best with weaker signals. The pulse counting type will have even lower distortion but works best with stronger signals.

The CD4066 is used as a four phase switching mixer (multiplier), to carry out the stereo demultiplexing. All four analogue inputs are driven in parallel by Pin 7 of IC303 (AN7471S). The four digital sampling pulse trains to operate the gates also come from IC303, and these will have phases of 0, 90, 180 and 270 degrees. The analogue outputs from the four gates are combined in IC301 (AN7275S) to give the left and right audio signals.
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Old 25th Apr 2021, 12:48 pm   #16
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Default Re: Luxman T-102L (1987) no stereo, no signal meter

Thanks. What operates the selection between the two FM detectors then?

There is no manual switch to bo that.
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