Lynx II - Only one side of the headphone works

[Norrecito]

Hi,

 

A friend has sent me an Atari Lynx II that had a "no power" issue to see whether I can fix it for him.

 

At first I didn't had much hope for this Lynx because as soon as I have put some batteries in it, smoke started coming out at the bottom.

Things turned around when I have managed to restore the power circuit (the previous person working on it made a mess and left big solder blobs that must have caused a short).

 

After fixing this, the Lynx had booted up just fine and even played games normally but there was two issue left:

• The speaker was very quiet even on the maximum volume
• When a headphone is plugged in, you can only hear sound on the right side. There are some weird noises coming from the left side but I can barely hear them and they don't sound normal at all.

 

I came across some articles which suggests that this is the classic case of putting power into the headphone socket which most likely damages the speaker as well.

 

Once I have replaced the speaker my first issue was fixed immediately however I can't seem to fix the headphone problem even after doing the re-cap of the board. I have basically reflowed all components in the sound circuit.

 

I only have two ideas left:

• The headphone jack itself has been damaged internally (I don't see any damage and didn't find anything interesting with my multimeter)
• One of the "LM386" chips gone faulty

 

If I had to choose, then I would put my bet on the "LM386" however I'm not that familiar with the Lynx and I'm not sure whether I came to the right conclusion. 

 

Fortunately "LM386" is available locally so I can always get them, however I would like to hear your opinion first  

 

The motherboard is "C104471-001 Rev.A 1". I don't have a power supply for the Lynx but I do have a spare board that is the same revision so I can compare measurements and switch out components.

[+karri]

I had a similar sound problem where the wires from Mikey to the amplifier chip were corroded away. It was also some strange model as the schematics did not match the board. In any case I had to solder two wires directly from the Mikey to the amplifier to get the sound to work.

 

[Norrecito]

Thanks karri,

 

That's a pretty good idea, so far I have avoided taking the shielding off but I could possibly check these connections.

 

Is each "LM386" responsible for one side of the headphone, meaning that there could be a bad (or no connection) to one of the chips from Mikey (Hayato in my case)?

[+karri]

Yes.

[+Stephen Moss]

11 hours ago, Norrecito said:

Is each "LM386" responsible for one side of the headphone, meaning that there could be a bad (or no connection) to one of the chips from Mikey (Hayato in my case)?

Not sure what schematic you are looking at but the one I use shows the dual op-amp NJM2072 (U12) as the amplifier between Mikey and the headphone Jack, then after the headphone Jack the L & R channel signals are summed together at the input of the LM386 (U5) to convert to Mono for the speaker.

I suggest working on the headphone issue first as any problems there would also affect the speaker, also if the headphone jack is operating correctly then with headphones inserted that will isolate the output of U12 from the speaker circuit so that no issues from the speaker circuit will adversely affect your investigation into the headphone issues. 

 

I would start by trying to measure the voltage on pins 1 & 3 of U12 relative to pin 4, the measurement may be small and varying but if they are approximately the same (not sure if Mikey outputs the same signal on both channels for mono games, so best to test with a game that outputs stereo sound if possible) as opposed to one varying and one sitting at 0V or 5V then it may be OK, so look at a possible open circuit of caps  C21/C64 or inductors L21/L20

[Norrecito]

43 minutes ago, Stephen Moss said:

Not sure what schematic you are looking at but the one I use shows the dual op-amp NJM2072 (U12) as the amplifier between Mikey and the headphone Jack, then after the headphone Jack the L & R channel signals are summed together at the input of the LM386 (U5) to convert to Mono for the speaker.

I'm a little bit confused here because both U5 and U12 looks LM386 to me.

 

They have the same text:

"386

21458

JRC"

 

Originally, I was trying to follow these schematics however, I have noticed that the audio circuit is somewhat different:

https://atariage.com/forums/topic/237290-atari-lynx-ii-codename-hayato-schematic/

[Norrecito]

FIXED!!!

 

It was a bad "LM386". Still can't believe that I have managed to guess this.

 

I have replaced the "LM386" at U12 (with the one from my spare Lynx II board) and now I can hear sound from both side of the headphone.

 

Thanks for the help guys!