Had a doozie today! - Missile Command repair log

bit_slicer

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This took all of yesterday and most of today:

Missile Command

* Jailbars, screen still
* Replaced all sockets
* Address space scans PASS THE PROGRAM ROMS, fails the DRAM at D(0), D(2)
* Replaced and socketed all DRAM. No change
* Found a bad inverter @ D3. Replaced. Screen flips, watchdogs now
* Checked watchdog circuit. OK
* DRAM fails intermittently across addr(2) boundaries. Suspect H3. Replaced. No change
* DRAM fails intermittently across addr(0) boundaries. Suspect F3. Replaced. No change.
* DRAM passes in best-case mode. Walked the one across all decoder boundaries. OK
* Some noise on RAS CAS lines. This bothers me. Suspect C8,B8. Replaced C8,B8. No change. Noise is better (an aside, I noticed noise in more places. I may summarily replace electrolytics on the voltage inputs.)
* Checked INT circuit. OK
* Checked Address Decoder circuit. OK (To the extent that it affects RAM/ROM!)
* Checked R/W circuit. OK
* Checked the SYNC circuit. OK
* Swapped in known-good 6502 and Pokey. No change.
* Checked the Microprocessor Data Input Interface. No MUSHROOM or MADSEL. Hmmm.
* Checked the DRAM Address Selector. No MUSHROOM or MADSEL, but logic is ok. OK
* Checked the DRAM Write Enable circuit. OK.
* Re-checked all the '244's. OK
* WTF?

At this point I'm nearly at a loss. I decided to fix another MC board to have something to compare to.

* Compared DRAM R/W cycles between two boards. Both similar in timing. OK
* Re-scanned all memory. Program ROMs and DRAMs all pass.
* Re-checked all signals at the 6502. OK
* Re-checked the Address Decoders. This time looked at ALL the outputs.
* RAM, Pokey, NIO, PROGSEL0,1,2, OUT0, COLRAM, WDOG, INTACK all OK.
* IN0, IN2 ok, IN1 is BAD! No response from N1.

IN1 reads in TEST, SLAM, fire buttons, and HDIR and VDIR. But it also reads in the status of VBLANK, which must be important for the CPU for some kind of timing info.

* Replaced LS139 @ M5. GAME COMES UP TO ATTRACT!!!

Damn obscure! So lesson learned, next time look at ALL of the decodes. :)
 
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Nice work! That board should work nicely for long time now!! Now to get my Centi boards out to ya...
 
+1 on the nice work!

There's a certain satisfaction that you get when fixing a hard to repair board. :D
 
+1 on the nice work!

There's a certain satisfaction that you get when fixing a hard to repair board. :D

Indeed! :) Especially when it's one of those impossible problems that keeps you up at night, follows you into the shower, and at the dinner table, among other intrusive venues. The whole time you're trying to 'think like a CPU' and the problem wraps around your head like some evil Mobius strip because it makes no sense whatsoever. But yes! In the end when the inevitable conclusion is reached, in hindsight it all makes perfect sense like some twisted sci-fi plot where the guy realizes that he was really dead the whole time.

I haven't had a Missile Command board beat me yet. *knocks on wood* Although there are times when I thought I might have to send that dreaded email!
 
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