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Luxman R-800 no FM (or very very weak)
Luxman R-800 no FM (or very very weak)
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Old 26th October 2020, 04:30 PM   #11
Depaj is offline Depaj  France
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What kind of voltages should I be getting around Q102 and Q103 ?
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Old 26th October 2020, 06:06 PM   #12
duncan2 is offline duncan2  United Kingdom
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In Radio Communications a "front end " is always the input side -IE- the aerial/antenna input .


The dual fets in the unit are highly sensitive to any low impedance probe which will detune and even lose any signal if directly tested with a multimeter .
It was the habit to test them with a valve (tube ) voltmeter or equivalent solid state tester of 100,s of Meg-ohms even a 1000 Megohms .


A small low wattage soldering iron was used and VERY careful removal of them was required --no bits of excess solder or contact with other components as that would permanently detune the unit .


The soldering iron MUST be very low leakage (AC) .


The injected signal must not be high but in line with the stated sensitivity of the receiver .
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Old 26th October 2020, 06:53 PM   #13
Regenpak is offline Regenpak  Netherlands
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Quote:
Originally Posted by Depaj View Post
What kind of voltages should I be getting around Q102 and Q103?
You can deduct that from the schematic. Across C110 there should be 0 V. This is the AGC lead and goes negative with strong signals. Across R108 (100R) expect about 1 V. This is caused by the drain current of Q101. Q102 has its base set to 4.7/(22+4.7)x12=2.1 V. V(e) should be 0.6 V lower. The 220R resistor in its collector line should have 0.5 V across it. Q103 (oscillator) doesn't need to be measured. Test it with a portable radio tuned to 108 MHz, then tune the Lux to around 99 MHz. You should hear it as a silent carrier.
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Old 27th October 2020, 10:11 AM   #14
Depaj is offline Depaj  France
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Well it looks like something's wrong around q102. its collector is at 13.21v, as is the other side of the 220r resistor so there's no current draw. Its base and emitter are both at 0v (9mv for the base and 0.00v for the emitter. The oscillator seems to work, I tuned my sansui tu-505 to 108Mhz and than got silence with the luxman tuned to about 97.5Mhz, a little off from what you suggested though.

Last edited by Depaj; 27th October 2020 at 10:16 AM.
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Old 27th October 2020, 10:40 AM   #15
Regenpak is offline Regenpak  Netherlands
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Quote:
Originally Posted by Depaj View Post
Well it looks like something's wrong around q102
Then you have found your issue. Fix the base voltage and you're done.



A propos offset: as you know, the IF of your receiver is 10.7 MHz. This means that the local oscillator is 10.7 MHz above the receive frequency. If you have your TU-505 on 108 MHz this means it receives between 107.5 and 108.5 MHz. Your Lux should be tuned to 97.3 MHz +/- 0.5 MHz. So 97.5 MHz is smack on.
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Old 27th October 2020, 06:31 PM   #16
Depaj is offline Depaj  France
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Easier said then done, I checked all individual components and everything tested fine, everything until I took a magnifying glass and checked the soldering and my eyes caught something strange : there are some disc capacitor inserted into the circuit board (two plates separated by something that looks like a piece of circuit board. Well turns out one of these started corroding and it actually put a bridge between the two plates creating a very low resistance to ground (50R). I removed the disc, put a ceramic cap in place and... the Stereo light came on tuned into my favourite station


Got it, thank you Regenpak for that precious info and all the great guiding !!
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Old 27th October 2020, 09:09 PM   #17
Regenpak is offline Regenpak  Netherlands
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Chapeau!
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Old 28th October 2020, 11:05 AM   #18
Depaj is offline Depaj  France
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Talked too soon, the crackle is back, although reception is still good. It does seem to come from the front end because it's present at the base of q201. I wonder if maybe another one of these disc caps is starting to corrode through. it's not easy to determine which caps these are and the value (not always indicated on the schematic).
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Old 28th October 2020, 12:21 PM   #19
Depaj is offline Depaj  France
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The answer to my question is yes, two more were all black on top, one of them actually disintegrated when I touched it. I replaced both with same value caps, on of them polysterene 15pF and the other 5000pF wima mks2. I soldered both as closely to the board and now I have crackle free, stable and great reception. Quite the journey to get here but very satisfying !!
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Old 28th October 2020, 12:37 PM   #20
Regenpak is offline Regenpak  Netherlands
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How is it present on Q201's base? All caps in the front end are marked in the schematic except for the trimmers and the tuning C sections.


Edit: looked like you already solved it!
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