Share your knowledge on these two classic MPCs
By MPCHunter Mon Dec 21, 2009 2:52 pm
Any idea on what the correct checksum would be for the Voice Chips?

Before doing anything else, I would recommend changing all the electrolytic capacitors in the machine. These caps are at least 20 years old, and are probably near the end of their lifespan.

There aren't a whole lot of them in the machine, but if any of these are bad/failing they may be letting noise in to the signal paths.
By skylab001 Wed Dec 23, 2009 5:47 am
Yeah I'm definitely recapping it, thats a given before I set out on a hunt like this I already found a few electrolytic caps that had blown and a few cracked ceramics. I should have it all done by this weekend. At that point I'll be on to double checking the machines circuits. I may have found one short already, but I want to be sure it was from a bad cap before I make any assumptions to anything else.
By skylab001 Wed Dec 30, 2009 3:42 am
So I have an update, I have recapped the entire machine with new electrolytics and tantalums. The ghosting is still there. I didn't think it would fix it, but one could hope. The sound is slightly cleaner now, but I believe that is only because I had 3 popped caps on the stereo outs and direct output 1. The other thing is that the clock noise I was having on my machine is now more noticable than before. I had hoped recapping would fix this particular issue. I have found that it is the 36mhz clock on the voice board that is intermittantly bleeding into my audio path, I just haven't figured out where it it could be bleeding into my audio signal.

*** Can somebody confirm something for me? IC25 on the voice board(near the clock) in the MPC60 service manual calls for a 74AC74P, but my MPC60II has a socketed 74HC74P. Is this just a difference between the 60 and 60II? The chips seem to have the same function and be pin compatible....but are they socketed on all machines? This is the only other chip in the whole unit other than the OS, VOICE, and CPU that is socketed. I was curious if mine was maybe like this from a previous repair or is there some reason this chip has a socket?

I have also ruled out the voice chips as the cause of the note ghosting. After recapping and double checking for shorts and cold solder joints I was excited to get it back together and see if it made a difference. In my haste the one thing I forgot to re-install were the voice chips. In the end this may have been a good thing because I was able to clearly hear the ghosting notes and noises I had heard in the past....but quite clearly. Can anybody comment on exactly what job the voice chips perform? From the way my machine worked without the voice chips, it seems they work to amplify the sounds? I could still hear the sounds I was triggering faintly and slightly distorted at the same level of the ghost notes----low.
By mpc3000le Fri Jan 01, 2010 2:23 am
skylab001 wrote:*** Can somebody confirm something for me? IC25 on the voice board(near the clock) in the MPC60 service manual calls for a 74AC74P, but my MPC60II has a socketed 74HC74P. Is this just a difference between the 60 and 60II? The chips seem to have the same function and be pin compatible....but are they socketed on all machines? This is the only other chip in the whole unit other than the OS, VOICE, and CPU that is socketed. I was curious if mine was maybe like this from a previous repair or is there some reason this chip has a socket?-


I have serviced 3 MPC60 MKII in the last week.

In addition to my own.

IC25 in ALL the machines I worked on is a:

74F74 ( BI-POLAR FAST SCHOTTKY similar to a 74AS74 but faster )

This chip is not socket fit.

The presence of a socket tends to indicate a previous repair.

Circuit diagrams are often different from final spec.

AFAIK that a 74HC74 is function but not logic level compatible to a 74F74 suggests you could try switching it out with a 74F74 or a 74HCT74.

All I can think of right now.

Rohan
By skylab001 Thu Jan 07, 2010 4:59 am
Thanks for the chip info Rohan. Since it was socket I decided to try 3 different chips, the 74F74, 74AS74, 74HC74. The only difference is the HC causes a higher noise floor. My unit seemed to have a little bit lower noise floor with the F and AS. I still had the ghosting and clock noise problem though. I've noticed with my original factory RAM I have the clock noise issue and note ghosting, and with my new RAM chips I get a high pitched squeal that comes and goes, which seems like it could be filtered out, but it still doesn't seem to be functioning quite right. I still think I may be having some issues with the RAM but I can't tell for sure until I find a test disk which I'm finding difficult to source. I've also checked all the logic chips on the south side of voice board and they are not the issue for the ghosting or my high pitched squeal. Back to trying to figure this out.
By skylab001 Thu Jan 14, 2010 1:58 am
Can anybody tell me how hot the Main CPU chip should be getting during normal operation, maybe after about an hour of being on? Should it be getting hot enough to burn you if you touch it? I was checking out the mainboard and noticed that the CPU was HOT.....ever touch a non heat sinked voltage regulator? It doesn't seem right to me, and the machine was being very problematic at the time.
By skylab001 Sat Jan 16, 2010 10:50 pm
Can somebody help me out with a few voltage readings? I'm reading -13.8 and 13.94 on the 12v rails of the psu. Is this normal and the change to 14v just not reflected in the schematics? I hadn't thought it was an issue before, I think I had read the 12v for 14v originally :-( when I was going over the schematic. Also I'm getting a variable few(2-11) millivolts on the 0v rail, might be nothing but I'd like to have something to compare it too. Feel free to PM me or post it here. If you could tell me how hot your CPU is at the time would be great too.
By skylab001 Wed Feb 17, 2010 5:24 am
Is there anybody that can get a binary read from each of the Voice EPROMS? I ended up taking my mpc to an old EE who has given me some answers and rebuilt my unit. But he needs to rewrite the voice chips for me before I get it back and wants to be sure the read comes from a clean working machine. Any capable volunteers??