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Sega Model 2A fixed - Random missing polygons

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lix View Drop Down
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    Posted: 09 Dec 2018 at 12:42am
Bought a faulty Sega Rally twin which had graphics glitches on one side. The fault manifested itself with random polygons disappearing from view, and when switching between viewpoints sometimes a few of the polygons from the external car camera would be left in the 1st person camera.

First I noted that the boardset in the working side had had a previous repair on the video board, a 256kBit static ram at IC2 (TC55328AP-20), had been replaced, so I duplicated this repair on the faulty board, but alas it had no effect on the fault.

The boards were heavily endowed with dust from the cage fan, which I know from Daytona USA repairs usually indicates one or more of the surface mount memories has gone, or some of the main customs need reflowing. I decided against reflowing all the chips in the area and tried replicating the fault by taking the known working boardset I had, and grounding the databus pins on the memory chips spread around the video board. While I don't recommend this as you can easily hit a 5v line or might cause damage, I have a history of poking probes where I shouldn't so I went on a rampage.

I found that grounding certain databus pins on the 64Kbit static ram chip at IC4 (CY7C185 25ns) caused similar loss of polygons, so I took the same chip off a known dead model 2A video board and fitted it to the faulty board, and hey presto, fixed!

Here's a video of the fault in action, apologies for the low bitrate.


IC4 near the middle here:




And the end result (the bench monitor needs recapping):

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Post Options Post Options   Thanks (0) Thanks(0)   Quote robotech Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2018 at 4:47am
Well done
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Post Options Post Options   Thanks (0) Thanks(0)   Quote channel27 Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2018 at 8:49am


Is that ic checked as part of the memory test in the service menu?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote yorkshire_spam Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2018 at 10:42am
Nice one!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tb2000 Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2018 at 11:21am
Nice fix! I have a model 2b board with missing graphics and a flickering screen, I wonder if it has a similar cause to this? I always thought it was probably a custom but it's worth some investigation!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lix Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2018 at 12:08pm
Originally posted by channel27 channel27 wrote:

  

Is that ic checked as part of the memory test in the service menu?

Unfortunately no, there's quite a few memory chips on the video board that are not included in the memory test and they all relate to the 3D geometry processing hardware. The IC references corresponding to the video board I believe in the Sega Rally memory test are IC45 to 50 which handle the 2D tile layer, and IC54 & IC55 handle the final output palette. All the other ICs are the program and data roms being checked.


Edited by lix - 09 Dec 2018 at 12:09pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote channel27 Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2018 at 12:35pm
It is very odd which chips they decide to leave out...!

Can I ask where you got the info on which chips do what, please? I’d like to put a wiki together for all the Sega Model 1,2,3 board sets containing ic info and fix logs. Any info would be excellent, please

I’m guessing the Mame drivers are a good place to start?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote John Bennett Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2018 at 1:44pm
Nice fix. Wet finger is my tool-of-choice for open-circuit IC bus locating

Originally posted by channel27 channel27 wrote:

It is very odd which chips they decide to leave out...!

Can I ask where you got the info on which chips do what, please? I’d like to put a wiki together for all the Sega Model 1,2,3 board sets containing ic info and fix logs. Any info would be excellent, please

I’m guessing the Mame drivers are a good place to start?


Often RAM and ROM isn’t tested when the CPU can’t see it - so it’s on the other side of a custom chip and only custom chips manipulate it - they’d have to add loopback hardware at expense to self-test it.

Emulation accuracy also tends to tail-off at the point where the CPU dumps graphics codes into RAM - after that, they dont model the individual IC behaviour - they just do the best interpretation with modern computer hardware to make things look correct. Sometimes they don’t even know what some of the ICs do - it’s just considered a black box - codes and ROM in and graphics out. It’s gets you something close enough in most cases and it’d be 1000x harder to model devices down to the gate level.

Edited by John Bennett - 09 Dec 2018 at 1:46pm
http://www.philwip.com -sibling 'fix-off' (I’m losing by miles).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kev A Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2018 at 10:14am
Cracking work fella, just one question, with the video board how do you actually probe around what with it being on the bottom of the stack? is it a case of soldering tiny wires and building the stack each time.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote lix Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2018 at 11:11am
Originally posted by Kev A Kev A wrote:

Cracking work fella, just one question, with the video board how do you actually probe around what with it being on the bottom of the stack? is it a case of soldering tiny wires and building the stack each time.

Fortunately on the 2A there's quite a few non-surface mount memory chips, so I have the boardset standing vertically and I can probe the pins that way. I have a good knowledge of pin functions on most memory and logic chips from other board repairs. Obviously I can't get access to the surface mount chips with the stacks together, but I've done a fair amount of mental tracing of vias and I have a dead board for reference which I can run continuity tests on to trace what via goes to what chip. Resting the tip of the probe on a via is usually good enough to get a signal.

I also use a video probe, so I can see on the screen what the data looks like, and from that get a rough idea as to what the chips function is.

My next project is a Daytona boardset which had the video board absolutely mangled by someone who seemed to be more adept at plumbing copper pipes with a blowtorch and plumbing solder. The large customs had 5% of their legs ripped off or bent and the memory chips are swimming in flux! However, I tried the model 2 CPU board in my 2A stack between a working 2A video board and 2A rom board with Sega Rally and to my surprise it booted and ran the demo sequence.

Shall type up some more notes from my discoveries when I get a chance!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robotech Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2018 at 2:17pm
Yes mate any information at all regarding repair of the sega model boards will help someone out
As there dont seem to be much info online

I will post some of my repairs also when i get chance

Edited by robotech - 10 Dec 2018 at 2:19pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Hurray Banana Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2018 at 5:55pm
Great fixing
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kev A Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2018 at 7:24pm
Thanks for explaining dude :-)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote channel27 Quote  Post ReplyReply Direct Link To This Post Posted: 16 Nov 2021 at 9:11am
I've got a couple of video boards with a palette RAM problem at ICs 54 & 55. 

Apologies for bumping an old thread but can I replace the 25ns CY7C185's with 20ns? (CY7C185-20PC rather than CY7C185-25PC)

Thanks again for the write-up Thumbs Up


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Post Options Post Options   Thanks (0) Thanks(0)   Quote lix Quote  Post ReplyReply Direct Link To This Post Posted: 16 Nov 2021 at 12:55pm
Originally posted by channel27 channel27 wrote:

Apologies for bumping an old thread but can I replace the 25ns CY7C185's with 20ns? (CY7C185-20PC rather than CY7C185-25PC)
I shouldn't see there being an issue with using a slightly faster chip. Some of the palette issues I've had in the past were these chips going bad but also the custom Sega multiplexer IC connected to the address and data bus of these palette ram chips. So if changing the rams doesn't help then it's likely the multiplexer. I can't remember the IC off the top of my head, but it's got very fine pitch pins and needed solder reflowing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote channel27 Quote  Post ReplyReply Direct Link To This Post Posted: 16 Nov 2021 at 2:00pm
Thats great. Many thanks for the info Thumbs Up


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