Sega CDX woes, CD player works but not games?

I've got an old CDX that has undergone some repairs over the years and I'm determined to get it fully functioning somehow. It loads the BIOS fine, has the Sega logo and music, but when it searches and begins to play Track 01 (Data) It emits a terrible sound and wont go to the produced/licensed screen.
CD playback is perfect though, and it will play music in the BIOS, search tracks, etc.

I though the LC78835 was the culprit at first when I got no audio, but then I isolated pin 3 (mute) and when that was logic low it outputted the audio fine. So I'm wondering where to look now. I was thinking it would be the CD controller or the main microcontroller, or an I/O error between the SEGA 315 chip and the CD board? Not sure what to look at / replace.

-Segasonicfan

 

x

 

oh riiiight, that makes perfect sense! Its only reading tracks as audio. weird, i wonder what would cause this?

 

The laser would be a good start, otherwise inspect the capacitors.

 

nah its not that. Ive done those repairs, this is something different with the I/O

 

You've replaced 100% of all caps with brand new ones of quality and you've replaced the laser? I avoid using Chinese caps from eBay as they could be counterfeit in branding or way off spec from what is on the label. These days I buy Panasonic caps from Digi-Key rated to handle up to 105°C and occasionally they may have two otherwise identical caps with one having a longer rated lifespan of a thousand or two more hours.

As for the laser I'm working on a CDX now that just won't cooperate. I'm assuming I need to adjust the focus/gain/tracking POTs but I don't have access to an oscilloscope at the moment.

 

I have a thread on laser adjustment over here:
http://www.sega-16.com/forum/showth...ti-Mega-laser-replacement&highlight=multimega

Its very possible to do without a scope. Just more time consuming and involves a lot of tweaking. However, I recommend you get a new laser off ebay before messing with those pots. They are about $20.

The reason I don' think my issue is caps is because of all the crazy poking around I did 10 years ago inside it when I was learning to solder. This is my oldest broken unit and I do remember shorting something out. Just trying to figure out what I shorted. I did a cap fix in another CDX unit this month that repaired it, and it had very different symptoms.

-Segasonicfan

 

There's a logic signal which comes from the DRIVE DSP or MCU which tell the CDD chip about the track mode flag in the TOC of the disc. If that signal doesn't arrive to the chip it will deal with it as a AUDIO track. The drive is supposed to mute any track which is flagged as DATA.

Get some magnifying lens and go pursue that broken trace. Good luck mate. That system look pretty repairable.

 

THANK YOU for this post. Finally getting someone with a lot of knowledge! Unfortunately, I think one of the chips is fried due to some poking around years ago. Any idea what pin that logic signal might come from or what it would be labelled as? The MCU and DSP pinouts are here:

http://console5.com/wiki/LC66306A
http://console5.com/wiki/LC78681E

I found a supplier in china that will sell me ONE of those chips for $20+$10 ship. Kind of dodgy but worth a shot. I just need to find out which one to replace.
-Segasonicfan

EDIT: looking at the datasheet, perhaps it is the "ROM out" pin on the image attached? I'm leaning towards the LC78681E being the culprit since it also controls the muting function (apparently)

 

Well the MCU is "generic" and the pins functions do vary based on the code programmed on it.

About the DSP, see where the pins C2FCLK, LASY, C2F and ROMOUT are going to.. I "guess" they go to the CDC chip (LC89513) but that's me ... lol

 

Thanks for the fast reply!

Sadly, they go nowhere Only C2F (pin 42) is connected and it goes to pin 15 of a 74HC245 tranceiver.

 

They certainly run under the chip package then. These pins are required for the proper operation of the thing lol

 

It has to be configured some other way or the datasheet pinout has to be wrong. The datasheet is LC78681E while the chip says LC78681, so perhaps there was a revision. I triple checked with a flashlight under the PCB and can clearly see no traces are connected under the package and there are no through-hole spots they could be going to either. I can take some pics a little later.

-Segasonicfan

EDIT: there are different revisions, tho pinouts are the same on these:
http://www.alldatasheet.net/view.jsp?Searchword=LC78681

 

Here are some pics to show what I mean. I am quite sure those pins are unused:

 

Look that 3 pin device with shorted out pins ...

 

You mean the MCU pin 3? I traced the mute function of the D/A there and I desoldered it at one point so it looks like that. Its not shorted though it looks like it in the pic :/

 

The D/M signal that indicates if current track is audio or mode-1 data is actually output by the CDD micro-processor (LC66306A apparently on a CDX ?) to Sega ASIC.

According to Mega CD Service Manual, it is labelled as L-MUTE and is ouput on P32 programmable output. This might some other issue though if you can't boot into game, I think this signal is only used by the Sega ASIC to pass (or not) serial data coming from DSP to CD-DA DAC/Fader (LC7883x).

 

ah, thanks for the post!
I see what you mean on the service manual, coming from the uPD75P008GB on page 3. Yeah, the CDX uses the LC66306A in this case. P32 is pin 11 on that, not sure if it would be completely compatible. Ill give it a try though!

 

ah, no dice. When P32 was disconnected I got no laser beam Its looking like my microcontroller is toast, which if its custom programmed, may be sad news for this CDX.

 

Not sure what you are doing but disconnecting pins is not going to fix anything, you are only supposed to check if the connection with Sega ASIC is fine. It won't tell you anything either since that pin must be pulled high to indicate when data track is being played.

To see if it still P32 with this chip, just trace the according pin from Sega ASIC (pin 87 on 315-5632 according to Mega CD 2 Service Manual)

If I were you, I would also check the connections between the LC8951x chip (CD Data Decoder) and the Sega ASIC on one side then with DSP chip on other side (SDATA, BCLK and LRCLK only), again using Service Manual schematics as reference.