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Major title repair

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Lurch666 View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Lurch666 Quote  Post ReplyReply Direct Link To This Post Topic: Major title repair
    Posted: 04 Dec 2018 at 4:50pm
I'm attempting to fix a major title board for stegrey but I'm not getting very far.
When I first got it it just booted to a white screen.
After wobbling some of the jumpers I got a white screen with two big vertical light brown bars.
Checking with my scope I found activity with the cpu,program roms and ram as expected if it was booting.
No sounds or signs of life though.
I replaced an ls373 at D6 on a hunch but no difference.
I then went to my usual path of "it must either be the ram or cpu".
Got some replacement CPUs coming from America but got some ram today.
Now the ram is tms 2063 but found the equivalent is cxk5064 that was in a gals panic 2 that I have with a graphics fault so I pulled them from that board (but one didn't survive so I had to wait for replacements) and tried it in the major title but still no difference but the rams from the major title don't work in the gals panic 2 so I'm assuming they were bad.

Now I'm back to the blank white screen.Still seeing activity in the circuit but wondering if I'm missing something.

Lack of documentation or schematics is slowing me down and there doesn't seem to be much info I can find so if anyone could give me a pointer that would be great.

I will keep on trying though as I can never leave these things alone.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Flinnster Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 2018 at 5:41pm
Oh Confused Good luck. Can't say I've seen one of these whitescreen, but for sure it's probably not getting as far as a rom / ram check then Cry


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Post Options Post Options   Thanks (0) Thanks(0)   Quote TheDaddy Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 2018 at 6:05pm
I have 2 with just a brown screen and 1 with a with white screen. No audio but cpu/mem seem fine, both are receiving and sending info as it should ( as far as i can tell ). I feel your pain i really do.

Good luck !

Dave.

Edited by TheDaddy - 04 Dec 2018 at 6:05pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lurch666 Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 2018 at 6:27pm
Forgot to say I also checked the program roms and they matched mame so they are ok.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote caius Quote  Post ReplyReply Direct Link To This Post Posted: 07 Dec 2018 at 3:07pm
First of all you should narrow the fault(s) in a the CPU or VIDEO board, also the bottom board can prevent the boot.After done it you can focus your troubleshooting in the specific board.But I guess you have no working set.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lurch666 Quote  Post ReplyReply Direct Link To This Post Posted: 11 Dec 2018 at 12:25pm
Tried looking at the bottom board and I'm seeing activity there as well so something is going on.

Noticed the cpu is NOT watchdogging so either there is no watchdog or the system is running ok but I'm still getting no video output apart from a white screen.

Waiting for a replacement cpu just in case but I doubt it's that.

Next I'm going to check all the 74ls chips to see if any of them are giving the wrong outputs.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Equites Quote  Post ReplyReply Direct Link To This Post Posted: 11 Dec 2018 at 12:33pm
Originally posted by Lurch666 Lurch666 wrote:

Waiting for a replacement cpu just in case but I doubt it's that.
 

Is the CPU warm?  A stone-cold CPU usually points to a dead one.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lurch666 Quote  Post ReplyReply Direct Link To This Post Posted: 19 Dec 2018 at 7:52pm
CPU gets a little warm and I'm getting data from it.
Anyway STILL waiting for the replacement CPUs and decided to have an initial poke around the 74ls chips tonight.
Not very revealing.Checking the ones near the CPU and there is not much activity.
There are three 74LS191s that have no clock signal going to pin 14 and various or & and gates with no data going to them.
Don't know if they are supposed to be active or maybe they don't do anything till the board is running.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Flinnster Quote  Post ReplyReply Direct Link To This Post Posted: 19 Dec 2018 at 8:19pm
Can you check the timing crystals?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lurch666 Quote  Post ReplyReply Direct Link To This Post Posted: 19 Dec 2018 at 8:58pm
Well the CPU clock is present but until you mentioned crystals I didn't realise there is one on the video board as well.
Too late for me to check it (early shift tomorrow) but will have a look when I get the chance.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Equites Quote  Post ReplyReply Direct Link To This Post Posted: 19 Dec 2018 at 9:06pm
If the PCB is watchdogging you won’t be able to check signals because they PCB is constantly resetting. Have you checked for clock and reset at the CPU?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Shinydaz Quote  Post ReplyReply Direct Link To This Post Posted: 19 Dec 2018 at 11:14pm
Mine turned out to be a dead fajitsu near the cpu, surprise surprise.
Out of room but always room for one more !
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lurch666 Quote  Post ReplyReply Direct Link To This Post Posted: 20 Dec 2018 at 5:06am
@Equites:It's not watchdogging.
Don't know if it's a fault with the watchdog or it's actually running code.
I do get a brief high to reset the CPU on startup then it stays low.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lurch666 Quote  Post ReplyReply Direct Link To This Post Posted: 20 Dec 2018 at 3:16pm
Just checked the crystal on the video board and I'm getting a signal.
it's normal for the output of these things to just be about 1 volt peak to peak isn't it?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote favouredson Quote  Post ReplyReply Direct Link To This Post Posted: 21 Dec 2018 at 9:45pm
Originally posted by Lurch666 Lurch666 wrote:

......I do get a brief high to reset the CPU on startup then it stays low.


Is that statement correct?

Surely the CPU reset line should show a low pulse and then constant high.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote favouredson Quote  Post ReplyReply Direct Link To This Post Posted: 21 Dec 2018 at 9:48pm
Originally posted by Lurch666 Lurch666 wrote:

Just checked the crystal on the video board and I'm getting a signal.
it's normal for the output of these things to just be about 1 volt peak to peak isn't it?


No idea about that.
I only ever check for activity and then whether the frequency is correct.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lurch666 Quote  Post ReplyReply Direct Link To This Post Posted: 21 Dec 2018 at 10:09pm
The cpu is a 70116 that has reset on high.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lurch666 Quote  Post ReplyReply Direct Link To This Post Posted: 23 Dec 2018 at 11:32am
I'm beginning to think this might be beyond me.
Apart from the lack of schematics and half the chips being unidentified even the standard 74ls chips are hard to check.
I have found that there seems to be no chip select signals going to any of the ram chips near the CPU.
Looking at the 74ls245 chips there is one that had no direction signal going into pin 1.
I have traced this to a 74ls139 and that has a high enable so it's inactive.
The enable comes from a square surface mount chip with no markings.Checking other pins on that I can see various signals but have no way of telling what's supposed to be doing what.

The white blank screen is causing problems as well.
On other boards you usually get a screen of garbage that can tell you if any changes are making any difference but here all I have ever had is the white screen.
Still waiting on the CPU replacement but I'm not holding out much hope that it's the problem.

Going to keep poking around but it's going to be more luck than judgment that fixes this one.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lurch666 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Dec 2018 at 1:37pm
CPUs turned up at last.
As I thought it didn't make any difference but at least I could eliminate that and get on with properly checking the other parts of the circuit.
Found the single 2018 ram on the cpu board was faulty so have replaced that.
Also found that the set of eight 2018 rams on the video board right hand side (four near the bottom and four near the top) all have inactive chip selects. The other 2018 rams on the video board have active chip selects.
The chip select for them seem to go to a pal (IC23) so I used the image from jammarcade to make a new pal but still am getting no activity on the chip select for those ram chips.
Will see if I can figure out where the signals going to that pal come from.
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