Hello,
I have a Pioneer Lx-34 receiver, but having some weird (to me) problems with alignment. I have a schematic, but could not find a service manual, so I am using the general guidelines from a book (Gordon J. King F.M Radio Servicing Handbook)
I managed to make the IF trace look almost like it looks in the book, and the sensitivity of the tuner increased - now a station that looks weak before almost makes the eye tube close. So far so good, however:
1. The noise level has decreased only a little for any particular station, especially in stereo. It is better than before, but considering that the indicator is almost closed, there should be little noise on that station. A station that makes the indicator overlap has lower noise (cannot really hear it).
2. IF 10.7MHz is present at the ratio detector output. It's not very high level, but makes the scope trace fuzzy, hopefully it does not affect anything.
3. The tubes seem to test good.
Does anyone with more experience know why the noise did not reduce as much as I hoped? I'm probably missing something obvious, but I guess that's the learning process 🙂
I have a Pioneer Lx-34 receiver, but having some weird (to me) problems with alignment. I have a schematic, but could not find a service manual, so I am using the general guidelines from a book (Gordon J. King F.M Radio Servicing Handbook)
I managed to make the IF trace look almost like it looks in the book, and the sensitivity of the tuner increased - now a station that looks weak before almost makes the eye tube close. So far so good, however:
1. The noise level has decreased only a little for any particular station, especially in stereo. It is better than before, but considering that the indicator is almost closed, there should be little noise on that station. A station that makes the indicator overlap has lower noise (cannot really hear it).
2. IF 10.7MHz is present at the ratio detector output. It's not very high level, but makes the scope trace fuzzy, hopefully it does not affect anything.
3. The tubes seem to test good.
Does anyone with more experience know why the noise did not reduce as much as I hoped? I'm probably missing something obvious, but I guess that's the learning process 🙂
Maybe just Friis formula at work? Friis formulas for noise - Wikipedia
Improving the adjustment of an IF filter somewhere halfway the IF chain will increase the gain of that stage at the IF frequency and will therefore reduce the impact of everything behind that filter on the noise factor. However, normally the noise factor is dominated by the first few stages of the receiver, so that may not be very noticeable.
In any case, an FM receiver with properly adjusted IF filter distorts less and has a better channel separation than one with an improperly adjusted IF filter, so it is very good that you managed to get it properly adjusted.
Might be perfectly normal, I don't know.
Another way of saying the same: after adjustment the gain for the desired signal has gone up, but so has the gain for the atmospheric and thermal noise received by the antenna, the noise of the RF stage and the noise of the mixer stage, so the signal-to-noise ratio at the detector may not have improved much. In any case, you will have less distortion, better channel separation and possibly less out-of-channel noise affecting the detector (lower FM threshold, so better reception of the really weak stations that you can barely receive in mono).
Link to the schematic:
PIONEER LX34 AM-FM STEREO VALVES RECEIVER SCH Service Manual download, schematics, eeprom, repair info for electronics experts
I guess it may just be that the receiver picks up more noise from the antenna now that I adjusted the filters.
What I have not figured out yet is one adjustment on the stereo decoder - L501 (see schematic). Adjusting that with either 19kHz or 38kHz input does not result in an obvious minimum. Adjusting it while listening to a station makes the noise different (more or less high frequency noise), but also, I have not noticed any minimum.
The way it is arranged it is supposed to be a notch filter, probably not allowing 19kHz pilot tone to go to T502 or not allowing 38kHz going back to V501a.
PIONEER LX34 AM-FM STEREO VALVES RECEIVER SCH Service Manual download, schematics, eeprom, repair info for electronics experts
I guess it may just be that the receiver picks up more noise from the antenna now that I adjusted the filters.
What I have not figured out yet is one adjustment on the stereo decoder - L501 (see schematic). Adjusting that with either 19kHz or 38kHz input does not result in an obvious minimum. Adjusting it while listening to a station makes the noise different (more or less high frequency noise), but also, I have not noticed any minimum.
The way it is arranged it is supposed to be a notch filter, probably not allowing 19kHz pilot tone to go to T502 or not allowing 38kHz going back to V501a.
You seem to be overlooking the role of the limiter. The stronger the signal the more noise will be cut out by the limiter before the detector sees it.
Assuming the signal is strong enough for the limiter to limit, why would making the signal and the noise at the limiter's input stronger by the same factor make any difference? Or are you thinking about signal levels where the limiter is still sort of soft clipping rather than cutting of amplitude variations altogether?
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Yes. I can equally ask you how could amplifying the RF noise possibly make an FM tuner's audio noisier? Only under the same conditions.
True, but since the thread starter referred specifically to stereo noise, I assumed that the levels would be high enough for the limiter to limit. At very low levels, you won't get stereo. Then again, I'm used to tuners with integrated circuits and plenty of gain, maybe valve tuners limit more softly.
When the limiter works perfectly (perfect sign function and no noise of its own), RF noise still gets converted to audio noise because some of the RF noise shifts the zero crossings of the IF signal. Amplifying both the desired signal and the noise at the limiter's input then doesn't change the locations of the zero crossings, so it doesn't affect the audio noise.
When the limiter limits perfectly (perfect sign function and no noise of its own) and there is no desired signal but only noise, then amplifying the noise at the limiter's input still doesn't change anything because the zero crossings stay where they are. When it doesn't limit perfectly, then amplifying the RF noise may indeed increase audio noise. (The same holds for a tuner with nearly perfect limiter and a softmute circuit; integrated circuit tuners often have a softmute circuit to reduce interstation noise.)
When there is a desired signal, but the signal to noise ratio is quite poor and you widen the IF bandwidth by tuning the IF filter incorrectly, there will be a wider band of noise at the detector input and it will produce more click noise.
When the limiter works perfectly (perfect sign function and no noise of its own), RF noise still gets converted to audio noise because some of the RF noise shifts the zero crossings of the IF signal. Amplifying both the desired signal and the noise at the limiter's input then doesn't change the locations of the zero crossings, so it doesn't affect the audio noise.
When the limiter limits perfectly (perfect sign function and no noise of its own) and there is no desired signal but only noise, then amplifying the noise at the limiter's input still doesn't change anything because the zero crossings stay where they are. When it doesn't limit perfectly, then amplifying the RF noise may indeed increase audio noise. (The same holds for a tuner with nearly perfect limiter and a softmute circuit; integrated circuit tuners often have a softmute circuit to reduce interstation noise.)
When there is a desired signal, but the signal to noise ratio is quite poor and you widen the IF bandwidth by tuning the IF filter incorrectly, there will be a wider band of noise at the detector input and it will produce more click noise.
I agree that it looks like a notch filter for the 19 kHz pilot tone. No idea why you don't see a sharp minimum. Is C506 OK?
The capacitor seems OK. I tried replacing it and got the same result. Anyway, It looks like I adjusted everything as best as I can (but not perfectly), time to start using this receiver and working on something else 🙂
Thank you for your advice.
Thank you for your advice.
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