Sega Genesis 1 - No mono audio output

[ohmmy]

Re: Sega Genesis 1 - No mono audio output

Sounds like it if you do not even hear anything at all. Another test is to check the DC voltage at the output pin of the LM358, it should be around 1/2 of the Vcc power supply voltage.

Ok vcc is 4.8v and output of lm358 is 2.48v

So now I'm confused. Is it bad?

[budm]

Re: Sega Genesis 1 - No mono audio output

That is correct voltage, the op-amp is being used in single power supply instead of bi-polar (VCC +, VCC -). To use OP-AMP in single power supply, they have to bias the + input pin at 1/2 of the VCC so the output will be set at 1/2 VCC, so as far as the DC voltage is concerned, it is OK.

[ohmmy]

Re: Sega Genesis 1 - No mono audio output

That is correct voltage, the op-amp is being used in single power supply instead of bi-polar (VCC +, VCC -). To use OP-AMP in single power supply, they have to bias the + input pin at 1/2 of the VCC so the output will be set at 1/2 VCC, so as far as the DC voltage is concerned, it is OK.

I see. So can it still be bad even if the voltage is correct? Should I replace it or?

[budm]

Re: Sega Genesis 1 - No mono audio output

You can have correct DC but can still be bad IC.
Another thing you can do is to take the Left and right channel outputs from the hedphone jack pin 3 and pin2 by using 1k resistor on pin 3, 1k on pin 2, the ends of those 1K will be joined together to make it into mono signal, then feed that into C30 negative leg, the negative will have to be lifted off the board, put the other caps back in place. The volume of the headphone will have to be adjusted. That should work to verify the circuit.

[ohmmy]

Re: Sega Genesis 1 - No mono audio output

You can have correct DC but can still be bad IC.
Another thing you can do is to take the Left and right channel outputs from the hedphone jack pin 3 and pin2 by using 1k resistor on pin 3, 1k on pin 2, the ends of those 1K will be joined together to make it into mono signal, then feed that into C30 negative leg, the negative will have to be lifted off the board, put the other caps back in place. The volume of the headphone will have to be adjusted. That should work to verify the circuit.

Do I have to use the resistors? Don't have any lying around

[budm]

Re: Sega Genesis 1 - No mono audio output

Yes, you do, they are needed for isolation.

[ohmmy]

Re: Sega Genesis 1 - No mono audio output

Ok and this is bypassing the lm358 chip right?

[budm]

Re: Sega Genesis 1 - No mono audio output

We will be using the headphone audio outputs to drive pin 8 input pin of IC13 instead of 358, just be sure to lift the negative leg of the cap which will be the leg we will put the signal to.

[ohmmy]

Re: Sega Genesis 1 - No mono audio output

Ok, picked up some 1K resistors and did what you suggested. Still no audio. Does this conclude ic13 is at fault?

Weird that I didn't get any audio when tying caps c30 c31 together like you instructed me to do previously. Maybe I didn't have volume up loud enough on tv, I don't know

[budm]

Re: Sega Genesis 1 - No mono audio output

Something is not making sense here at all, we did that cap tying together, I have you do this headphone mode just to make sure you did not do the first experiment wrong. Well, we can also use the headphone amp to simulate the buffer amp inside IC13.
We also did bypass IC13 output by feeding the signal from 358 to the cap that feed the scart and you still did not get the sound, you can try that using the output of the headphone to feed the cap C31 again.
You did turn the volume of the headphone up, right?

[ohmmy]

Re: Sega Genesis 1 - No mono audio output

Something is not making sense here at all, we did that cap tying together, I have you do this headphone mode just to make sure you did not do the first experiment wrong. Well, we can also use the headphone amp to simulate the buffer amp inside IC13.
We also did bypass IC13 output by feeding the signal from 358 to the cap that feed the scart and you still did not get the sound, you can try that using the output of the headphone to feed the cap C31 again.
You did turn the volume of the headphone up, right?

Oops! volume was down on headphone out! Good thing I still had it wired. Ok yes, now there's audio out

[budm]

Re: Sega Genesis 1 - No mono audio output

Whew!, Ok, so we use the headphone amp to drive the input of the IC13 and you get sound, that means the 358 is not working.

[ohmmy]

Re: Sega Genesis 1 - No mono audio output

Sweet I can get that cheap on eBay.

I'm curious if you don't mind explaining, why were 1k resistors used instead of other values? Also, for future reference how could the headphone amp be used to simulate the buffer amp in ic13?

[budm]

Re: Sega Genesis 1 - No mono audio output

1K give enough isolation of the Left and Right headphone power amplifiers so the output of one amplifier will not affect the output of the another amplifier. If signal of one amplifier is out of phase, one output will try to go high, another output will try to go low, then the amps will buck each other and can damage the output stage. With 1K, the current will be very low not enough to cause problem. If you tie two outputs of the amplifier directly together, if the signals feedings were out of phase, you will damage the amplifiers.
Btw, if you can live with this setup, I would leave it in case you do the damage to the board during removing/installing the IC.

[ohmmy]

Re: Sega Genesis 1 - No mono audio output

Ah ok. Thanks. So if we wanted to bypass the LM358 and ic13 using headphone amplifier would we just tie in to positive of c31?

[budm]

Re: Sega Genesis 1 - No mono audio output

OK, let get this correct, we use the L and R output of the haedphone amp and feed the INPUT PIN 8 of buffer amp through cap C30 negative pin, at first you indicate that you still do not get any sound, but it turns out to be that the headphone volume was not turned up.
So then I was going to suggest you to bypass the IC 13 buffer amp and use the headphone to drive SCART coupling cap C31 to see if you will get the sound.
But now we know that the IC13 is good but the 358 is not putting out any sound to pin 8 of IC13, so 358 is bad.

[ohmmy]

Re: Sega Genesis 1 - No mono audio output

OK, let get this correct, we use the L and R output of the haedphone amp and feed the INPUT PIN 8 of buffer amp through cap C30 negative pin, at first you indicate that you still do not get any sound, but it turns out to be that the headphone volume was not turned up.
So then I was going to suggest you to bypass the IC 13 buffer amp and use the headphone to drive SCART coupling cap C31 to see if you will get the sound.
But now we know that the IC13 is good but the 358 is not putting out any sound to pin 8 of IC13, so 358 is bad.

Yes correct and I just ordered a 358, but I was just curious for reference/knowledge if I ever encounter a similar problem.

Would I bypass ic13 same exact way I bypassed 358 using the two 1k resistors except this time feeding into c31's positive leg lifted off board?

[budm]

Re: Sega Genesis 1 - No mono audio output

Right now we bypass the 358 only.
But lets say that the IC C13 buffer is bad, then you will use the Headphone amp outputs (through the isolation resistors) to drive the SCART (C31 +) and the RF modulator (you have to lift R72/ C87 junction off the board and tie it to C31 +). This will bypass both 358 and the IC 13 buffer amp, we cannot use the 358 to directly drive the SCART (which we did) because it does not have enough power to drive the SCART/MODULATOR), so the headphone amp become both a mixer amp (358) and the buffer amp (IC 13 audio buffer), in this case the resistors may have to be drop down to 500 Ohms or so.

[ohmmy]

Re: Sega Genesis 1 - No mono audio output

Thanks so much, I appreciate the help and patience in putting up with my inquisitions. Great help. I'll post back results of changing the 358 when it comes in.

[budm]

Re: Sega Genesis 1 - No mono audio output

No problem, glad to be able help.