Tuner meter and stereo signal light on Sony STR-6045

[Mooly]

Well spotted. It kind of looks like R at a distance (laptop) but you are right.

 

[jrbastien]

OK, I understand now. This is just there to complete the circuit. It is not necessarily getting 14V either.

With the corrected R312 resistor, I get similar values to mine on the entire circuit. So working backward to get 3.3V at the emitter of Q302, I would need a signal of 14 volt PKPK amplified by MU301 to 20 Volt PKPK. Is it even possible?

I have attached the circuit for the review by the expert. 🙂

 

[Ylli]

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.

That is strange. You are dealing with a fixed frequency of 19 KHz here, and what station you are tuned to should not make any difference - as long as it is a stereo station.

 

[jrbastien]

Not sure if I was measuring at the place you intended me to but DC voltage definitely varies at the junctions of the diodes weather you are in the middle of strong station or in between stations.

Now I have some great news to report:

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.

This is what lead me to the solution. I installed a transistor with a higher gain at Q301 (KSC1845 in replacement of KSC1815Y that already had replaced a 2SC633A) and got 5.5 Volts instead of 4.9 Volts at the emitter. It produces only 1.0 Volt at the emitter of Q302 instead of 3.3 volts but it looks like it is enough to activate Q302. Perhaps the diodes need to be replaced to get more but I am satisfied with it.

I have updated the simulation with the real value at V1 and it is still pretty close to what I have now. I have attached it here.

I think we can call it a success and close this post!

Beyond fixing the problem, I've learned so much here. Mooly and Ylli, I can't thank you enough for your guidance and patience.

 

[Mooly]

Now I have some great news to report

Well I see a little red stereo light lit up...

That's great news 👍 Perhaps experimenting with more RF oriented transistors for Q301 might get you more signal, ones with low junction capacitance and high gain but it would be trial and error. Or swap with Q303 which only works as a DC switch.

Or leave it alone and put the top on 😉

 

[jrbastien]

The top is back on the unit. I still have to monitor this for a while. If you remember, the beginning of this post was that I was loosing the signal meter and the stereo light after a day or two with the cover on. I keep my finger crossed. 🤞 🤞

 

[Mooly]

Fair enough 🙂 I'd forgotten all that, its like a zillion posts in the past for me 😱

 

[Ylli]

but DC voltage definitely varies at the junctions of the diodes weather you are in the middle of strong station or in between stations.

. I thought you meant it was substantially different from one station to the next. Yes, it would drop between stations. Glad you got it working to your satisfaction!

 

[jrbastien]

Just to close the loop, this is a complete success. After 3 weeks of monitoring the original problem is completely gone. Thank you all again.

 

[Mooly]

Pleased to hear it is all and thanks for the update.