That should be just the opposite. Background random noise should increase in stereo, at least on weaker stations.
Mono reception is usually significantly more quiet than in stereo. The pop causing a sound change seems a key clue.
I would poke around with a wooden or plastic tool to see if you can make the pop happen. Maybe a bad solder joint or part.
Mono reception is usually significantly more quiet than in stereo. The pop causing a sound change seems a key clue.
I would poke around with a wooden or plastic tool to see if you can make the pop happen. Maybe a bad solder joint or part.
I would agree with rayma on this, the pop sound suggests a possible poor connection, and possibly an intermittent component. Heat and freezer spray applied carefully may help narrow it down. Do not alter any alignment settings.
Just want to point out that this receiver uses a 'noise gate' to enable/disable stereo. A noisy signal (or the receiver inducing noise into the signal) can cause the gate to disable stereo.
Many tuners switch to mono when the stereo signal is weak and noisy.
Do you have an FM antenna connected while testing?
Do you have an FM antenna connected while testing?
Looking the schematic at #24, the current flowing through the bulb, also is used to bias D307 and D310 via R318 and Q303 (Off) forward in order to leave the ring demodulator out of service, and give mono output when no stereo pilot is detected (or is too weak). Si, if led (plus any other R shunting it) is insufficient, the ring demodulator will not operate properly.
Observe that D301 & 2 are the 19KHz to 38K frequency doubler, and in absence of pilot Q302 is permanently off and with it also Q303, and part of the lamp (off) current is deviated with R318 to the demod'tor.
Observe that D301 & 2 are the 19KHz to 38K frequency doubler, and in absence of pilot Q302 is permanently off and with it also Q303, and part of the lamp (off) current is deviated with R318 to the demod'tor.
Well, it looks like I will have to look at this issue sooner than expected. FM stereo is completely gone now although the signal meter is going strong. I'm wondering if by trying to save £65 of shipping for a £2 light bulb (cannot find this lamp other than in Europe), I managed to break the multiplexer.
I don't think this is the issue as I had about 33mA in this led circuit. Original bulb was 40mA. And also R319 is probably there to keep the multiplexer working when the bulb is out. Just in case, I tried replacing my led circuit with a 60mA lamp and no success. But you are right, I have seen Sony's on another forum needing a bulb to have stereo working. Their design is slightly different.
For those interested, taken from the service manual, the MPX circuit works like this:
I don't have an oscilloscope so I can't verify the frequency at Q301 and Q302. But here are the measured voltages:
My questions:
I don't think this is the issue as I had about 33mA in this led circuit. Original bulb was 40mA. And also R319 is probably there to keep the multiplexer working when the bulb is out. Just in case, I tried replacing my led circuit with a 60mA lamp and no success. But you are right, I have seen Sony's on another forum needing a bulb to have stereo working. Their design is slightly different.
For those interested, taken from the service manual, the MPX circuit works like this:
I don't have an oscilloscope so I can't verify the frequency at Q301 and Q302. But here are the measured voltages:
My questions:
- What would the oscilloscope allow me to do other than checking the frequency doubler?
- Is between 1.1 and 1.3V high enough? At least it seems MU301 is not dead.
- Anything significant can be measured to determine if T301 is working?
An oscilloscope would let you look at the base voltage driving Q303. 7.5 volts shows a problem but is it a DC issue or is it because there is AC present and giving readings that confuse a DVM.
Are the small electrolytic caps OK around there such as the 0.47uF and 1uF. The 0.47uF looks like it works in combination with the B/E junction of Q303 (which behaves like a diode) to rectify the AC of the oscillator and give a more constant drive current to Q303. If its open circuit the transistor may not be in a defined state of conduction... which your 7.5 volts shows. Either the transistor is duff or the drive is wrong, most likely the latter.
Any problem with a totally unsuitable bulb would at worst (lets say it drew way to much current) only cause possible failure of Q303 or load the supply too heavily but R316 prevents anything to extreme.
Are the small electrolytic caps OK around there such as the 0.47uF and 1uF. The 0.47uF looks like it works in combination with the B/E junction of Q303 (which behaves like a diode) to rectify the AC of the oscillator and give a more constant drive current to Q303. If its open circuit the transistor may not be in a defined state of conduction... which your 7.5 volts shows. Either the transistor is duff or the drive is wrong, most likely the latter.
Any problem with a totally unsuitable bulb would at worst (lets say it drew way to much current) only cause possible failure of Q303 or load the supply too heavily but R316 prevents anything to extreme.
1.1-1.3 volts at the top of C313 is not nearly enough. Q304 is a noise gate - it disables the stereo decoder if the signal is too noisy. It is off, so that indicates you have a clean signal at the emitter of Q301, although it may not be strong enough*. I would check C314, C316, D301, D302, in that order. If these parts test good, then I would lift one end of R320 and D303 and then recheck the voltage developed at the junction of D301, D302 when tuned to a strong stereo station. It should be at least 4 volts.
*The signal for the signal strength meter is taken from the output of Q204. There are two additional IF/Limiter stages after that takeoff point. It is possible one of these is bad. Check the voltages on Q205 and Q206.
It is also possible that the FM detector is misaligned. When tuned to a strong station (tune as close as you can to the center of the station) measure the voltage at the junction of R238 and C215.
*The signal for the signal strength meter is taken from the output of Q204. There are two additional IF/Limiter stages after that takeoff point. It is possible one of these is bad. Check the voltages on Q205 and Q206.
It is also possible that the FM detector is misaligned. When tuned to a strong station (tune as close as you can to the center of the station) measure the voltage at the junction of R238 and C215.
Mooly: C318 = 1.0uF, C319 = 0.508uF (nominal is 0.47+-20%). Both are good. Q303 tests good on board (I did not de-solder to test collector to base or emitter)
Ylii: See the results below:
Q205: Good (VC=6.9V, VE=5.4V)
Q204: Good (VC=6.1V, VE=4.3V, VB=4.8V)
Q203: Good (VC=4.7V, VE1.8V, VB=2.5V)
Ylii: See the results below:
C314 = 0.355uF, C316 = 0.370uF (nominal is 0.33 +-20%). Both are good. D301, D302 test good on board (do their ends need to be lifted?)
Unfortunately, still 1.3V.
Q206: Good (VC=13.1V, VE=2.5V)
Q205: Good (VC=6.9V, VE=5.4V)
Q204: Good (VC=6.1V, VE=4.3V, VB=4.8V)
Q203: Good (VC=4.7V, VE1.8V, VB=2.5V)
Voltage at the junction of R238 and C215 is 1.5V
I would just dab another cap across these to 100% prove them OK. The e.s.r. of the cap is very important. Same applies to them all really, just measuring value isn't enough, the e.s.r. is equally as important.
My concern on these two caps is leakage. Use your ohmmeter on them - should read 1meg, Yes, you would need to lift one end of either D301 or D302 to test the onboard - otherwise they are in parallel DC wise and an open diode would not be detected.
This is the output of the FM detector (ratio detector). When tuned to the center of a station, this voltage should be zero. 1.5V indicates the ratio detector is not tuned to the center of the IF. The procedure in the manual calls for test equipment I don't think you have, but we can get close just by tuning to a strong station (be sure you are tuned as close as you can get to the center of the station). Monitor the voltage at the junction of R238 and C215. Tune to the center of a strong station. You will need to tune the top core of T201 (see fig 3-5 in the manual) for zero volts. Note where T201 is currently tuned so you can return it to that position if you don't get the expected results. Adjust T201 top core no more than one turn in either direction with the goal of zero volts at R238/C215. If you can't get zero volts in that +/- one turn range, let me know what you do see.
Mooly and Ylli, I will do more testing this weekend following your suggestions. I'm just wondering, can the discriminator alignment give me back stereo? How can it become misaligned just by replacing a stereo light with a led?
And lastely, is it safe to recap the MPX board with new electrolytics or will it make the alignment worse?
And lastely, is it safe to recap the MPX board with new electrolytics or will it make the alignment worse?
Replacing electrolytics should not change the alignment but stick to the same values.
The bulb is important in that it places 14v on the collector of Q303 when in mono and that voltage is fed elsewhere via R318. When in stereo Q303 should be zero volts.
Why not link out Q303 (short C to E) and see what happens. See if the audio seems to be in stereo.
The bulb is important in that it places 14v on the collector of Q303 when in mono and that voltage is fed elsewhere via R318. When in stereo Q303 should be zero volts.
Why not link out Q303 (short C to E) and see what happens. See if the audio seems to be in stereo.
Since sometimes even the point of view of a non-technical person (me) can even have its small usefulness, this is my modest one.
In my opinion it's strange enough that you can't tell the difference between stereo and mono, but you probably haven't deepened this aspect enough.
Furthermore, it could be that the Sony engineers who AFAIK have sometimes demonstrated particularly singular projects have inserted that light bulb into a scheme in which its function is not immediately guessable even for experts of the highest level like here.
If the receiver were mine I would go back to the bulb without regrets.
By the way, could you link which bulb is?
Edit to add that I just found the following ones
https://www.ebay.com/itm/144361609723
https://www.ebay.com/itm/144361619471
not sure if exactly the same replacement though.
In my opinion it's strange enough that you can't tell the difference between stereo and mono, but you probably haven't deepened this aspect enough.
Furthermore, it could be that the Sony engineers who AFAIK have sometimes demonstrated particularly singular projects have inserted that light bulb into a scheme in which its function is not immediately guessable even for experts of the highest level like here.
If the receiver were mine I would go back to the bulb without regrets.
By the way, could you link which bulb is?
Edit to add that I just found the following ones
https://www.ebay.com/itm/144361609723
https://www.ebay.com/itm/144361619471
not sure if exactly the same replacement though.
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I can tell now that it is definitely mono. It was confusing at first when stereo was on and off.
The bulb is 4.5V 40mA. I had tried to fit a 5V 60mA originally and it was quite dim.
The second link you gave me from ebay, may be the right one even if specifications are not provided but it says it may not ship to Canada and when $20 to $30 USD will be added to shipping it will come at $45 USD for a bulb.
Plenty of vendors in Europe sell them when you search for "T1 1/4 4.5V 40mA". Unfortunately the shipping costs are prohibitive.
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The ratio detector adjustment may bring back stereo. It might be far enough off that the 19 KHz pilot is not getting though with enough strength to drive the de-mux. Changing the lamp should have had NO effect on any other operation. If the bulb was burned out (open), R319 is still there to provide the current to the ring demod to switch it into mono mode. If the bulb were shorted, Q303 would have to sink a bit more current than it expects, but certainly not enough to damage Q303 (50ma).
Changing the electrolytic caps will have no effect on tuning. As Mooly suggests ground the collector of Q303 and see if it sounds stereo.
Changing the electrolytic caps will have no effect on tuning. As Mooly suggests ground the collector of Q303 and see if it sounds stereo.
Yes, the vendor from the 1st link sells 6 lamps (he says it's the complete kit) for USD 22.00 + shipping costs for Canada other USD 22.00: I realize that they are always USD 44.00 for 6 lamps though...
The following are other two chances, they do international shipping, but you have to ask for a quote:
https://banquedelampes.fr/en/t1-1-4-we-4-5x11-7-4-5v-40ma-c-2r-10khrs-clear-8718739000544
https://www.verlichtingnl.nl/schiefer-1-4-wire-ended-4-5x11-7mm-4-5v-40ma-c-2r-clear-10-000hrs
Hoping you solve in a way.
Ylli, I did lift the legs of C314 and C316 to verify leakage with ohmmeter. They both shows OL in each direction. Also, I lifted one leg of D301 and D302 to test them. They tested good out of the circuit.
Mooly, as for shorting C to E on Q303, it brought back the stereo light but did not sound better. It even sounded worst.
I did not attempt the FM discriminator alignment yet as I'm wondering what kind of tool I need to turn the top core of 201. It looks like an Hex key could work but not sure it is safe to use metal on these. The kits I see on the web are all plastic.
In the process, I also discovered that I lost AM! Is it the result of all my manipulations or is it related to my FM stereo issue? I need to dig more into this.
Mooly, as for shorting C to E on Q303, it brought back the stereo light but did not sound better. It even sounded worst.
I did not attempt the FM discriminator alignment yet as I'm wondering what kind of tool I need to turn the top core of 201. It looks like an Hex key could work but not sure it is safe to use metal on these. The kits I see on the web are all plastic.
In the process, I also discovered that I lost AM! Is it the result of all my manipulations or is it related to my FM stereo issue? I need to dig more into this.
No, you need plastic tools for these fragile iron-dust cores, you used to be able to buy a whole set of such for different styles, getting harder to find now.