Stargate CMOS RAM Problem?

[cpyne]

Thanks! OK, here's more detail with the door closed, then open, then book keeping mode

----------Closed--------Open-------Open/Bookkeeping
4G-7------High-----------High-----Brief Pulse then High
5A-2------Low------------Low------Brief Pulse then Low
IC1-5-----High------------High------High
IC1-6-----High------------High------Brief Pulse then High
R65-------4.8v------------4.8v------4.8v

 

[andykmv]

do the same again, and this time also include 1C pin 8 as well (pin 8 is the Chip Select input, normally high, active low pulse) and the Write Enable pin 10.

from what you have shown above, the chain of chips 4g, 5a, IC1 (3,4,5&6) are working ok, just need to check Q1 by checking that the CMOS RAM /CS input changes from HIGH to LOW and back again (HIGH, pulses briefly, HIGH).

 

[cpyne]

As expected on 1C pin 8 > HIGH, HIGH, Brief Pulse-HIGH

Aslo, IC pin 10 is pulsing in all three situations

 

[andykmv]

As expected on 1C pin 8 > HIGH, HIGH, Brief Pulse-HIGH

hopefully the logic probe is reading high/low levels correctly. this can be checked by using a voltage divider or variable voltage supply.
below .8= low,
above 2v=high
in between .8 and 2v - not sure what your probe will or should read.

Yellowdogs suggestions...
(1) the CMOS is indeed bad and needs to be replaced.
(2) the memory protect switch (front left side of the coin door) might be bad.
(3) the chips that sense the memory protect switch might be bad.

i reckon we have exhausted items 2 & 3, and no 1 replace the cmos ram is the next step as we need to eliminate it as a source of problems. make sure you install a new a double wipe IC socket! not sure what your soldering/desoldering skills are like but as ken says u need to be very careful removing ics as te ic pads on the pcb are very delicate. too much heat can lift the pads as can too much force. if you are not good at desoldering, once you get your replacement ic, cut all the legs of the existing one to remove it then desolder the leftover pins and use a handheld desolder pump to remove the solder from the holes. then solder in new socket and fit new ram IC!

after that the documented diagnostics suggest retesting then if still faulting starting to replace the relevant ics in the CMOS ram circduit and then retesting.

 

[cpyne]

Thanks for all your help with this!!! I've got some new cmos ram on order along with some good quality sockets. I'll let you know how I make out.

Thanks
Charlie

 

[andykmv]

cool! man i hope it works first off - thinkin positive here!

 

[cpyne]

Andy,

New ram installed and the machine works great! Thanks for all the help with this. I've probably increased my logic probe skills from "clueless" to "knows enough to be dangerous", but it's a huge start. Thanks again!

Charlie

 

[andykmv]

whoo hoo! enjoy!

 

[baritonomarc]

Andy,

New ram installed and the machine works great!

Charlie, did you use the same CMOS RAM chip specified on the stargate manual, or something more recent but working? Where did you found it? Thanks

 

[cpyne]

I used a 6114 ram chip from Great Plains Electronics. They have it listed as a replacement for the 5114

http://www.greatplainselectronics.com/search.asp?pg=1&stext=5114&sprice=&stype=&scat=

 

[YellowDog]

Congrats! Sorry for making you check all the logic circuits first. I have found that people immediately swap out the CMOS chip, which is fine if it is already socketed. But when it is not, a lot of people find that the very small traces leading to the CMOS chip are very easy to damage or break. So I usually make sure there is nothing else wrong before just willynilly swapping a new CMOS chip in. You wouldn't believe how many boards I've had sent to me where I have had to repair damage from bad solder/desolder jobs.

The 6114 is a viable replacement. I normally get them from GPE as well.

ken