Its not impossible for M301 to have an issue. Have you tried tweaking its core to see if the amplitude can be raised? Occasionally (and this isn't really fixable) the tiny caps within the transformer can go bad but tbh I wouldn't say that was the most likely scenario.
Could the amplitude be low going into Q301? The problem here is we don't know what to expect and to what extent the tuned circuit of M301 boosts that.
Could the amplitude be low going into Q301? The problem here is we don't know what to expect and to what extent the tuned circuit of M301 boosts that.
OK, you have the same questions that I have. And unfortunately, tweaks are not possible as the unit is a sealed red plastic box.
I see that user Ylli has offered a solution regarding the failed internal capacitor of the MPX unit of a Sony STR-7035 on the other forum:
https://audiokarma.org/forums/index.php?threads/sony-str-7035-no-fm-stereo.957955/
Not sure Ylli, if you are still following but before trying this, I would like to know your opinion. The user you were helping only had a few millivolts. In my case, I have 250 mV at the output of the diodes.
How about 4.9V at emitter of Q301 vs schematic showing 6V? I have replaced the electrolytics C303 and C304 and verified the resistors going to Q301 base. But no change.
I see that user Ylli has offered a solution regarding the failed internal capacitor of the MPX unit of a Sony STR-7035 on the other forum:
https://audiokarma.org/forums/index.php?threads/sony-str-7035-no-fm-stereo.957955/
Not sure Ylli, if you are still following but before trying this, I would like to know your opinion. The user you were helping only had a few millivolts. In my case, I have 250 mV at the output of the diodes.
Could try adjusting the core of MU301 and see if you can increase the amplitude of the 19KHz at the diodes. Not sure if it would be the top or bottom slug. Before you adjust either, be sure to note the current position and return the tuning to the original position if you see no results. If tuning does increase the level of the signal seen at the diodes (or increased the dc voltage at the junction of R311 & R312) the internal cap could indeed be the problem. I've seen it before. Could start by adding a 33pf cap across R307. If it helps, you will need to try different values of C to get the maximum output. Would expect it to be <150pf.
Looks like there may not be any accessible slugs on MU301. In that case, just try the 33 pf cap across R307.
Hard to say. The emitter voltage will depend on the gain of the transistor. With no transistor fitted you see 7 volts on the base. Of the primary of the coil is very low resistance (you could always measure it I suppose) then you see 7 volts less around 0.6 v on the emitter but you need a high gain transistor to be able to do that because base current is limited.
So 4.9v is not miles out of what might be expected but a bit higher is achievable although 6 volts is right at the top end.
It might be worth investigating.
Thank you Ylli and Mooly. I will need to go get some capacitors from 33pf to 150pf but meanwhile, can I ask you Mooly how you deducted that with no transistor fitted, you would get 7 Volts at the base?
Simply two 68K resistors in a voltage divider from the 14 volt buss. I crunched some numbers, and it looks like you will see 4.9 volts on the emitter of Q301 if the beta of Q301 is about 170. Perfectly adequate beta.
The base bias voltage is derived from R303 and R304 which are two 68k's. 14v supply means there is 7 volts at the junction, however the base also takes current and will pull that voltage lower. How much lower depends on the hFE (current gain) of the transistor. R306 with 6 volts across it means 6/1000 or 6ma is flowing. If the hFE is say '300' then the base current is 0.006/300 which is 20uA and which comes from the 68k's. Current in the two 68k's with no transistor fitted is 103uA, and tapping 20uA from that would reduce the voltage to around 6.3 volts on the base and so you would see about 5.7 on the emitter give or take. A higher gain transistor would see that voltage be a little higher as it takes less base current. I've ignored the 10k as the currents are so small that its effect is negligible.
Wow that's amazing explanations. Thank you for this.
The transistor I have at Q301 was replaced with a modern KSC1815Y which has a measured gain of about 150 hFE. This matches your calculations Ylli. Perhaps, I need to try with something with an higher gain as Mooly calculated.
I have tried to put some capacitors across R307 and thought that it indeed improved the results by about 100 mV but it would be worse on some stations. So I don't think this is conclusive.
I have done more measurements and I'm starting to wonder if the diodes D301 and D302 could be the culprit. The 19 kHz signal goes to the transformer with an amplitude of 2.8V PKPK and goes out with 3.9V PKPK. At the output of the diodes, I only get about 200 mV PKPK. I tested them before and they were okay but could they be contributing to the issue or is this normal?
The transistor I have at Q301 was replaced with a modern KSC1815Y which has a measured gain of about 150 hFE. This matches your calculations Ylli. Perhaps, I need to try with something with an higher gain as Mooly calculated.
I have tried to put some capacitors across R307 and thought that it indeed improved the results by about 100 mV but it would be worse on some stations. So I don't think this is conclusive.
I have done more measurements and I'm starting to wonder if the diodes D301 and D302 could be the culprit. The 19 kHz signal goes to the transformer with an amplitude of 2.8V PKPK and goes out with 3.9V PKPK. At the output of the diodes, I only get about 200 mV PKPK. I tested them before and they were okay but could they be contributing to the issue or is this normal?
The 200mv pk/pk you see on the diodes is 'ripple'. What matters is the DC voltage that is riding on. The DC voltage out of the diodes should equal the peak of the AC less the diode volt drop and that should be enough to turn on Q302 and Q303 turn the stereo light on.
I get 1.23 VDC at the output of the diodes. 3.7 VAC - 0.3 VDC (maybe time 2?) should gives me about 3 VDC not 1.23 VDC.
My other teacher ChatGPT (that is not as good as you Mooly), tells me that 1T40 diodes are germanium diodes that should only drop about 0.2-0.3 Volts. I measured them and they drop respectively 0.592 volts and 0.561 volts.
We may have found the culprit! Now, my local store may not have those in stock. It will take a long time for me to check this assumption.
Hmmm. It would be odd for both diodes to be faulty...
Its an interesting one for sure and so I'm just playing around here. Simulation. Schottky diodes which are low forward voltage (you could always try IN4148's you know).
Anyhow 🙂 Look at the arbitrary points circled here. I missed labelling the base of Q303.
This shows input amplitude and output from MU301. 2.8 v pk/pk going in and around 4vpk/pk out.
This shows the output after the diodes and what I labelled V_Bias. So the diodes are losing little. The peak reaches almost to 4 volts.
This shows Vlamp bias and V lamp and also the base of Q303. So the lamp (LED here) is on with Q303 collector down at near zero volts.
When you look at the real circuit there are other complications such as feed offs to other parts of the decoder but the diode voltages and waveforms should all be similar. If you have the volts going in then you should see something expected coming out. D303 is also an IT40 on the diagram so could be used to compare... odds of both being duff though... nah
I'll attach the sim as it will just click and run in LTspice.
Its an interesting one for sure and so I'm just playing around here. Simulation. Schottky diodes which are low forward voltage (you could always try IN4148's you know).
Anyhow 🙂 Look at the arbitrary points circled here. I missed labelling the base of Q303.
This shows input amplitude and output from MU301. 2.8 v pk/pk going in and around 4vpk/pk out.
This shows the output after the diodes and what I labelled V_Bias. So the diodes are losing little. The peak reaches almost to 4 volts.
This shows Vlamp bias and V lamp and also the base of Q303. So the lamp (LED here) is on with Q303 collector down at near zero volts.
When you look at the real circuit there are other complications such as feed offs to other parts of the decoder but the diode voltages and waveforms should all be similar. If you have the volts going in then you should see something expected coming out. D303 is also an IT40 on the diagram so could be used to compare... odds of both being duff though... nah
I'll attach the sim as it will just click and run in LTspice.
First, again, thank you for this.
Regarding the 1T40 diode, all 1T40 on the board show a forward voltage drop of about 0.57 volt. This may be normal or may be they have quite drifted. I can't find anything on the web to confirm this. Would you happen to have some manuals that has their specifications?
Your last simulation shows that Q302 Vbe is about 0.63 volt. With 1N4148, I only get 0.56 volt. It looks like it is still enough to turn on Q302 and Q303 but this is very close.
Where the value on the real circuit differs is at the junction of R312 and R315. I only get 0.7 volt while the simulation shows 1.3 volt. That explains why Q302 is off.
Maybe we need to draw more of the circuit to be more realistic? I'm going to give it a try. Meanwhile, here is the circuit with 1N4148 diodes and R315 labeled.
Regarding the 1T40 diode, all 1T40 on the board show a forward voltage drop of about 0.57 volt. This may be normal or may be they have quite drifted. I can't find anything on the web to confirm this. Would you happen to have some manuals that has their specifications?
Your last simulation shows that Q302 Vbe is about 0.63 volt. With 1N4148, I only get 0.56 volt. It looks like it is still enough to turn on Q302 and Q303 but this is very close.
Where the value on the real circuit differs is at the junction of R312 and R315. I only get 0.7 volt while the simulation shows 1.3 volt. That explains why Q302 is off.
Maybe we need to draw more of the circuit to be more realistic? I'm going to give it a try. Meanwhile, here is the circuit with 1N4148 diodes and R315 labeled.
You're welcome. Its a bit of an interesting one 🙂 I'll have a closer look again later.
I've no paper stuff now on diode equivalents I'm afraid but I'll see what I can turn up.
This could all be further forward toward the input. The circuit shows 3.3 volts on the emitter of Q302 and we can achieve that with a higher input voltage to the doubler. We don't know the signal level on the base of Q301. Also we come back to MU301 as any issue there could reduce the expected output from that stage. Without a known good one to compare against its becoming difficult. Ideally we need to be able to say for sure that the level going into Q301 is correct to at least split the circuit into two as to where the fault lies.
If you increase the input voltage on the sim to 4 volts we get the 3.3 volts on Q302.
Thank you Ylli. This now much closer to the real circuit as it gives us 800 mV compared to 700 mV. But is the representation of the light circuit okay? Shouldn't we have R319 (5.6k) in there?