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Sega/Gremlin Frogger

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GadgetFreak View Drop Down

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    Posted: 06 Oct 2018 at 7:12pm
Got this board in a trade a while ago, it was missing Gfx roms, Sound roms & all five 74s201 rams.
So dug out my Super Cobra to use the edge connector. On first fire up I got no frogs on the title screen and the Frogger title had lines through it.

So hooked up my tester and very quickly found a bad 2114 @ IC51.

This brought back the frogs on the title page though they didn't look right and were the wrong colour. But I still had the lines through the Frogger title so started checking the 74ls194's. Two of the four were bad @ IC103 & IC105.

At this point I decided I may have an issue with the 74S201's that I had put in the board, they came from a draw and had not been verified as working so I started to check these. But as I started to do this the board crashed and started just displaying a screen full of white X's.
So I used my tester to check the 2114 Work ram at IC's 19-22, it was passing all four of them. I even examined the memory dumps and checked with Mame and all seemed good, even spotting the return address on the stack pointer at the top of memory for the call to the function that displayed the X's. So this then led to hours of wasted time, looking for the problem in the CPU / addressing / Watchdog circuit. There was also the possibility of more broken tracks as there was already a patch wire on a '273 that had been done. The only curious thing I noticed is that the ram was continuously being rewritten with 0x00's which is only done once on startup! This meant the board must be effectively restarting all the time but the watchdog was not active.
So I found a test rom on the net and ran that up. It reported bad 2114 @ $8400-87FF. So I homed in on IC20 & 21. But every time I ran my tester it got triggered while checking the 2 lower rams. After checking in Mame I then found it was clearing the whole 2K twice during the lower 1K ram check. So I hacked the test rom to change this. Then when I ran my test IC20 failed with the first byte but IC21 passed. Unfortunately the test rom stops on the first error but this was enough for me to change IC20.
And the board started booting again. So my conclusion is that this ram was reporting 00's which meant my initial tests passed as Frogger only clears ram on initial startup and the restarting was cased by a corrupt stack meaning it kept jumping back to $0.

Anyway on with my corrupt frogs! Back to testing the 74S201's. It was obvious IC60 was bad so changed out with a 74S301 as I didn't have any more 201's and the board already has pull up resistors so an open collector version would be fine.

It has been a frustrating experience but just goes to show where ram is concerned that it needs to be effectively written to for the tester to find errors. Blanking with 00's will not always do!!

I have just finished the board off with JRok's Frogger high score save mod which involves adding a socket at IC12 for an NV ram and patching the /WR line to the socket.

Edited by GadgetFreak - 06 Oct 2018 at 7:13pm
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bonehead View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bonehead Quote  Post ReplyReply Direct Link To This Post Posted: 06 Oct 2018 at 7:43pm
A very good write up and another pcb fixed. You must be very happy to have fixed such a classic game.
Yet again your tester has proved it’s worth to bringing the board back to life.

Well done sir.!!!!
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smarty View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote smarty Quote  Post ReplyReply Direct Link To This Post Posted: 07 Oct 2018 at 2:37pm
Nice fix and write up Aaron. Thumbs Up

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