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Missile Command - PCB #2

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yorkshire_spam View Drop Down
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    Posted: 22 Nov 2018 at 1:28pm
So I've made a start on fixing my spare Missile Command PCB....

Symptoms: Monitor screams and shuts down (no image visible)
Checks: Verify H-SYNC and V-SYNC 
Result: Signals look ok.
For reference here are some healthy traces for each.
HYSNC:
VSYNC:
To my surprise both look fine!
Check: Composite SYNC (Pin 3 LS32@J7 OR position 7 on connector J19):
Result:
Here's the trace for Composite sync from the PCB (green trace = vsync, yellow trace = csync)
This looks wrong!
Compare with a healthy c-sync trace:

Check: Verify 74LS32 @ J7
Result: VSYNC input (pin 2) looks fine, HSYNC* input (pin 1) isn't switching cleanly and if floating/high
Check: Verify 74LS74 @ A9
Result: Inputs (pin 2 & 3) are clean, but outputs (pins 5 & 9) are not.
Action: Replace 74LS74 @ A9

The board will now sync to the screen - will not self test but gives a static yellow screen.
Watchdog is barking. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote yorkshire_spam Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 2018 at 11:45am
Fault #2:
Symptoms: Board dead, blue/yellow static screen
Check: Clock@CPU 
Result: No clock signal
Check: Trace clock signal back from CPU
Result: first IC with valid inputs but not outputs is 74LS74@A7
Action: Replace 74LS74@A7

WD still barking and board will not self test.

Fault #3:
Symptoms: Board dead, blue/yellow static screen
Check: ROM/RAM/BUS using Fluke
Result: 
ROMS dead/faulty:
035820.02
035823.02
035824.02
Action: Replace faulty ROMs (borrowed from other PCB as my EPROM programmer isn't working!)
 
PCB now boots into self test, but clearly still has "issues":
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Post Options Post Options   Thanks (0) Thanks(0)   Quote yorkshire_spam Quote  Post ReplyReply Direct Link To This Post Posted: 27 Nov 2018 at 5:20pm
Fault #4:
Symptoms: Graphics in the wrong place, upper half of image AND lower half BOTH on the lower portion of the screen.
Check: Pretty much bloody everything associated with the MADSEL and ram addressing.
I've been round and round in circles on this until I noticed a discrepancy in the schematics. The memory map shows that for all addresses A15 is low. But I noticed that the BA15 feeding the MAD13 line via 74-157@L2 and 74-08@M3
Pin 9 on 74-08@M3 was sitting high, but A15 on the processor was flipping high/low.
So I hooked up the 'scope on pins 2 and 18 of 74-244@F2 (buffer that converts raw A15 to BA15) and the fscking thing started working.
The only possible thing I can think of is that something UNDER F2 wash shorting the output high and hooking the probes up moved it? I ave actually no idea how it started working, I can't actually make it fault now.
Anyway.... COLRAM is still faulty so colours are wrong and blue text is "missing".... onto the next fault!

Edit: I should have checked the MAME driver! It has a different map to the one on the schematics!
http://tech.quarterarcade.com/tech/MAME/src/missile.c.html.aspx?g=1775


Edited by yorkshire_spam - 27 Nov 2018 at 5:23pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote yorkshire_spam Quote  Post ReplyReply Direct Link To This Post Posted: 28 Nov 2018 at 9:49pm
Double DOH! Missing blue turned out to be a loose connection on the monitor cable!
I'm gonna hook up a trackball and wire up the buttons on the jamma<->missile command adapter to give both PCBs a full test now.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote yorkshire_spam Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 2018 at 3:31pm
Update on fault #3:
The board seems unhappy with NEC 2716 eproms to replace the Atari mask ROMs, but is fine with Hitachi ones!

Update on fault #4:
After storing for a while the fault returned, in the end I found a dry joint on pin 2 of the 74--244 @ F2
Re-flowing that seems to have fixed it.
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