Very odd Mega CD Model 1 Sound issue (Sounds out of tune)

So turned on my Mega CD 1 today and straight away heard something wasn't right. At the Bios boot screen the music plays but sounds weird and out of tune..

Games play fine and anything with CD audio sounds normal, but when I tried sensible soccer which has some via sections with what I'm guessing is not CD audio that sounds weird too...

Here's a video of the boot screen;

http://i7.photobucket.com/albums/y27...psxguetaow.mp4

Sorry the audio is a bit low it's late and don't want to wake the misses :p

I've spent loads of time searching the interwebs with no results matching my issue...

Hoping the awesome peeps on Sega-16 can help me out.

I've opened up the unit and cleaned all the contacts.

My first thought is that some of the caps may need replacing, but I yield to the superior knowledge on the forum.

If the caps haven't been done before best to get them done they leak like a sift on these. have u looked on the motherboard for any signs? or trace damage?

I couldn't see any leakage on the boards, but I've ordered new caps any way and will do a full cap replacement and report back my findings.

Well, I'll rule out caps right here. Yes, they tend to go bad after all those years, but they never cause glitches of these kinds.
Bad caps will result in moving/rolling picture, quiet or dead audio, color problems.. All kinds of analogue misbehaviour.

This problem is hard to nail down.
I'd start tests by using a different Mega Drive, cleaning the MCD/MD connector, inspecting the mainboard for obvious problems.
And if that doesn't produce anything, a better recording of the problem, please ;)

Tried all of the above, no change. Will post some better video in a few mins :)

Ok some much better video evidence of the offending Mega CD who can't play a song in tune...

https://www.dropbox.com/s/24qdp1o5q2...15720.mp4?dl=0

Sounds like when you touch fuzzy in SMW2.

Quote:

---

Originally Posted by Speedle

Ok some much better video evidence of the offending Mega CD who can't play a song in tune...

https://www.dropbox.com/s/24qdp1o5q2...15720.mp4?dl=0

---

Wow, its like its stuck between a carnival and a horror movie.

Tempo is right and all sounds seem to play. Pitch is weirdly off :p

Sorry, I've no idea how this could happen. You've got to debug it. Somehow :p

Hrm, I wonder if one of the lines to the Ricoh PCM chip could be messed up, causing corrupted data writes.

So some additional fault finding I've done. I've swapped the main board out with a known working one and it works fine with another board. Moving this faulty board to another unit the fault follows the board so it's definitely the main board at fault.

I did a full cap replacement (as I had spares ready anyway) no difference as suspected.

So out of ideas really... no idea what components on the board control the pitch. Perhaps replacing the crystal might be worth a shot? I have a spare pretty dead Mega CD for donor parts.

No, the crystal would have to run so far out of spec (to produce those pitch shifts) that it wouldn't work at all.
I suspect it's a bad IC.

Fair enough, I changed the BIOS chip out, so I know it's not that particular Chip... Wonder which one it could be... Huff!

I'd wager it's the chip marked "Sega 315-5476A". It's in charge of the PCM sample playback.

Quote:

---

Originally Posted by superandroidtron

I'd wager it's the chip marked "Sega 315-5476A". It's in charge of the PCM sample playback.

---

Thanks dude that's a big help, I will check the traces in that area as there is some cap leakage that could have damaged a trace. Failing that I'll replace the chip with the spares I have :)

Quote:

---

Originally Posted by Speedle

I will check the traces in that area as there is some cap leakage that could have damaged a trace. Failing that I'll replace the chip with the spares I have :)

---

In my case, having had a similar problem, the problem could be solved by reflowing the solder connections of IC3, IC13 and IC14.

This strange PCM sound symptom seems to happen when one of the RAM ICs (13/14) has a bad connection to the PCM chip (IC3).

Some PCM music plays well, if the PCM samples are so small as to fit in one RAM IC only.
The garbling only starts when both RAM chips are utilized.

Quote:

---

Originally Posted by Eidolon

In my case, having had a similar problem, the problem could be solved by reflowing the solder connections of IC3, IC13 and IC14.

This strange PCM sound symptom seems to happen when one of the RAM ICs (13/14) has a bad connection to the PCM chip (IC3).

Some PCM music plays well, if the PCM samples are so small as to fit in one RAM IC only.
The garbling only starts when both RAM chips are utilized.

---

I had a similar problem recently and I was able to repair it. Yup, it was related to PCM RAM. I describe the repair in detail here:
https://youtu.be/y8Xw1pkEHeA

In my case it was a very odd issue indeed: the PCM RAM was missing a connecton on !OE, for no apparent reason..