Part number needed - sound issue

[burn_654]

Hi, burn_654 here!

If you recall I was trying to fix up a pc engine duo not too long ago, I can say that there was some success thanks to you fine folks! (and a lot of patience)

I need a part number for Q604 shown here, pretty sure it 's a transistor:
The underside of the board.


As you can see it's pretty blown.

I did a partial recap and as part of that, I did some cleaning with swabs and rubbing alcohol beforehand.
As I wiped around this component (as it showed some nastiness) a bit of plastic broke off of it! And now you see my concern.

I marched forward, tested after one section of caps and got my games and cds to read. Great luck. ADPCM works (Ooga bonk!) and all tracks from both a pressed music cd and music cdr work.
The problem is that the cd-audio is GREATLY distorted. Still recognizable to me (familiar cds) and not skipping though. As far as I can remember, before the cd-audio sounded ok for the one or two tracks it could play before I did the cleaning/recap. Maybe some very slight noise but not this ridiculous.

Pretty sure it's this component given the circumstances. Almost as if it's not giving proper voltage for the cd-audio anymore. Anyone got a match? Is this indeed part of an audio circuit?
Testing on headphones, everything else sounds great.

edit: derp. image is up now.

[thesteve]

its part of the anti thump circuit.
any NPN will work
check capacitors C610 and C670 for shorts.
power applied to left AV jack is a possible cause as well

[burn_654]

Thanks for the quick response thesteve.

I thought the anti-thump only applied for the av jack? (ie, stopping a stereo pop when the console turns on) I'm using headphones. But any faulty component in the audio circuit could cause other problems I guess.

Any NPN? I'm really unfamiliar with them, I could have sworn there were different types/voltages I should adhere to.

[thesteve]

yes they come in a multitude of types, but that circuit will work with any of the correct polarity (NPN).
it applies to AV and headphones

[burn_654]

Well, I replaced it with a 2N4401 from radioshack, stated npn. Used the packaging and the circuit board notes to get it lined up correctly.

Still the problem remains, highly distorted cd audio.
I replaced some of the pre-and post amp caps, no difference.

The rest of the audio sounds good on both headphone and av (though I admit my av lead is a little cobbled, from an old keyboard cable).
The only thing of note was that when removing the old transistor it lifted a pad, quite easily. I traced it to the next spot and soldered there.

I had replaced pretty much all of the caps south of the cd-rom drive on my first cap-replacement spree. While this got things working, could any of them have caused the distortion that I'm experiencing? It's literally like an overdrive of the cd-tracks.

[burn_654]

Update:

Double-checked my cap replacements, no apparent issues there...

Took the transistor completely out of the circuit, there was still crazy amounts of distortion on the cd audio.

Put in a 'MPS2222A' transistor, same thing.

Any ideas?

[thesteve]

if the distorsion is only on the CDA you will need to check for bad via's near the amps at the back of the board (buttom side)

[burn_654]

Holy crap! I got it working!

While I couldn't locate bad via's on the backside, I undid a few of my re-done caps to sanity check. In undoing them, some bad traces were definitely apparent...I found the nearest points to that and jumped them with small bits of wire. The transistor is back in place. Put my caps back in, little more pot tweaking and bingo!

My PC-Engine Duo fully lives and Gates of Thunder never sounded sweeter

Thanks for your help thesteve! And thanks to this forum for the handy capacitor replacement/sound/duo maintenance threads!


Time to get more games...

[roflmao]

Awesome!