Hello,
I'm having some issues with a luxman R-800. When I got it the symptoms were : no fm reception and popping/crackle noises when fm selected accompanied by wild movements of the two meters (centre and signal strength). The auxiliary inputs work fine so it's not the preamp. I replaced two transistors in the front end portion (q102 and 103) with 2n5770 as q103 was an infamous "flying saucer" and actually had a gain of 1 when tested. Now I still haven't got any reception (that is unless I directly connect an fm signal generator to the antenna with its output at max and I only just get enough deflection on the signal strength meter to actually notice it, and with fm muting on it stays dead silent) and the popping sounds are still there although a lot less perceptible. I don't really know where to start although I can already say that the +12v rail is present even if rather at 13.2v.
Thanks in advance for your help
I'm having some issues with a luxman R-800. When I got it the symptoms were : no fm reception and popping/crackle noises when fm selected accompanied by wild movements of the two meters (centre and signal strength). The auxiliary inputs work fine so it's not the preamp. I replaced two transistors in the front end portion (q102 and 103) with 2n5770 as q103 was an infamous "flying saucer" and actually had a gain of 1 when tested. Now I still haven't got any reception (that is unless I directly connect an fm signal generator to the antenna with its output at max and I only just get enough deflection on the signal strength meter to actually notice it, and with fm muting on it stays dead silent) and the popping sounds are still there although a lot less perceptible. I don't really know where to start although I can already say that the +12v rail is present even if rather at 13.2v.
Thanks in advance for your help
Always start at the end. In this case this is the Foster-Seeley discriminator (D204/5), driven by Q204. Touch its pin #6 (input). You should hear some noise. Better yet, set your generator to 10.7 MHz, modulate with 1 kHz, and inject with a small capacitor on pin #6 (or R215). Move to the front of the IF strip (Q203, -2, -1) until you lose signal/gain. Reduce signal level as needed. Popping sounds are worrying. Check DC levels at these four devices. They are marked on the schematic.
Sorry for my late answer, I didn't get the usual notification.
I'll do these tests probably tomorrow and let you know. Is it possible the actual dual fet is the problem here because there is nothing measurable on the output of the front end ?
I did already clean the tuning capacitor (with a dry brush and some compressed air, and put a drop of deoxit on the ground contacts.
I'll do these tests probably tomorrow and let you know. Is it possible the actual dual fet is the problem here because there is nothing measurable on the output of the front end ?
I did already clean the tuning capacitor (with a dry brush and some compressed air, and put a drop of deoxit on the ground contacts.
What I meant by "frontend" is the part in the enclosure directly mounted on to the tuning capacitor.
Lcsaszar, did you ever get a return from this person to know if changing the dual fet changed the problem ?
I don't actually get any reception whatsoever unless I connect my signal generator directly to the antenna input but it does move the tuning meter and only gets through when I'm dialled onto the correct frequency.
Lcsaszar, did you ever get a return from this person to know if changing the dual fet changed the problem ?
I don't actually get any reception whatsoever unless I connect my signal generator directly to the antenna input but it does move the tuning meter and only gets through when I'm dialled onto the correct frequency.
Hello again,
I finally found some time to continue my search. I injected a 10.7 MHz signal into the different if stages and got all the way up to Q201 without losing signal, I am able to get a clean output down to about 50µV, not great but ok I guess. So the issue must be before this right ?
I finally found some time to continue my search. I injected a 10.7 MHz signal into the different if stages and got all the way up to Q201 without losing signal, I am able to get a clean output down to about 50µV, not great but ok I guess. So the issue must be before this right ?
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 .
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 .
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.
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.
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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.
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 !!
Got it, thank you Regenpak for that precious info and all the great guiding !!
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).
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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