Missile Command - step by step

[yorkshire_spam]

Post edited to summarise faults and fixes (original 1st post shown below)

Fault #1

Symptom: no V-SYNC

Check: 32H 4H (outputs on pin 14 of E5 and 13 of D5, inputs on A6 pins 11 and 12) - both ok

Check: Outputs of counter @A4 - outputs stuck low.

Resolution: Replace LS191 @A4 - V-sync working

Fault #2

Symptom: no sound (self test)

Check: Pokey pin 37 - ok

Check: LM324 pin 5 @ N10 - stuck high

Check: Continuity between Pokey pin 37 and LM324 pin 5 - no connection

Resolution: Reflow pin 37 on pokey sockect (dry joint) - audio output working

Fault #3

Symptom: RAM Fault. Self test indicates ram @ M4 is faulty.

Action: Replace RAM IC (socket)

Symptom: RAM Fault. Self test still indicates ram @ M4 is faulty.

Action: Swap RAM from location that tests as good to M4

Symptom: RAM Fault. Self test still indicates ram @ M4 is faulty.

Check: Use Fluke to test RAM - RAM DCD ERR @ 0009 BIT 4

Check: Inspect PCB - damaged track between M4 and N4 corresponding to DEAD4 (RAM Address line)

Resolution: Flow solder over break in track - RAM now verifies as good with fluke AND self test.

Fault #4

Symptom: Static blue screen

Check: Pin 40 (_RESET) on CPU - stuck low

Check: 74-93 @ D4 output - no activity

Resolution: Replace 74-93 @ D4 - blue screen now "bounces" as watchdog is "barking"

Fault #5:

Symptom: Jumping blue screen, no self test etc.

Check: MADSEL - no activity

Check: 74ls74 flip flop @ F7 - no ouputs

Resolution: Replace 74ls74 @ F7 - game "works" but with screen corruption

Fault #6:

Symptom: Game "works" but with screen corruption

Check: Full diagnostic with fluke (BUS/ROM/RAM) - all verify fine, ROM checksums tally.

Check: Go back to the start and check the sync circuit outputs with a scope instead of a logic probe - "corrupt" output on pin 3 of LS191@A4

Check: Verify downstream ICs function as expected - apparent short between pins 5 and 6 of LS153@K3

Resolution: Replace LS153@K3

Result: Game runs!

---------------------------- Original First Post ----------------------------

I'm working on a couple of MC PCBs at the moment, so I'll write them up here as I go. (So I don't forget!)

In general the MC self test is pretty good.... but quite often boards have faults that prevent them getting that far.

So the first PCB I'm working on won't self test. At this point I always start with the SYNC circuits (confusingly nothing to do with SYNC line on pin 7 of the CPU)

The SYNC circuits are responsible for all the timing signals on the board, anything wrong here and you won't get far.

So... I have a good 10MHz, 5MHz and H signals including H-Sync.

BUT I have no V-SYNC.

V-SYNC is based on the 32H and 4H (and _FLIP, but that's not a consideration right now)

Both of those (outputs on pin 14 of E5 and 13 of D5, inputs on A6 pins 11 and 12) are fine.

Following the circuit through checking with a logic probe I can see that a couple of outputs on counter at A4 are stuck low.

Replacing the counter at A4 means I now have V-SYNC and all the "V" signals seem ok (1V .... 128V)

Still no sound on self test.

I check that my test rig is working right - yep the "TEST" is low on the MC PCB.

Checking pin 37 on the Pokey shows the pattern I'd expect for self-test audio (it pulses in the sequence of the self-test "beeps"!), so there's something wrong with the audio output as well. That's my next step for investigation, probably starting around the LM324 on the audio output.

yorkshire_spam2018-11-20 16:32:24

 

[DanP]

Nice fix Spammy. Interesting to know that the sync circuit is actually for something else on MC...

 

[yorkshire_spam]

.... the saga continues.
So I have a nice clean signal (clearly the 8 beeps and the buzz from the self test, but I can't tell which are high or low beeps with just a logic probe!) on pin 37 of the pokey.

BUT! Pin 5 on the LM324 at N10 is just sitting high!

Continuity check between pin 37 pokey and 1 leg of R38 indicates no connection.

Turning the board over reveals a dry joint on pokey 37.

Re-flowing that joint gives me sound.

Now I have a buzz and 8 clear "beeps" - the ram test is working.

The 3rd beep is high pitch, but the other 7 are all low.

That indicates bad ram at M4.... so that's the next job!

 

[yorkshire_spam]

one step forward three steps back.
So I removed M4 4116. Replaced it with a socket and added a 4116 chip.

In test mode the beeps still indicate an error on M4 (but that's not a massive surprise, MC sometimes gets the RAM check wrong.)

Out of interest I swapped M4 for one of the other socketed RAMs (previously replaced by me)

No change, beeps still indicate a fault at M4.

So I swapped the chips back.... clearly at this point I bggrd something up. As now the PCB won't self test.

SYNC signals all look ok.

Fluke indicated a RAM error, "RAM DCD ERR @ 0009 BIT 4" so it backs up the previous self test issue - something in the RAM address/data side of things is screwed.

BUT all the ROMs checksum fine in circuit.

So I'm left wondering why it's not self testing anymore. I'll go back over all the timing circuits and then start looking at the addressing/write status on the watchdog circuits.

Grrrr.... I like a challenge, but sometimes these boards go too far!

 

[backflipper]

lol damn boards taking things to far, I like that.

Good to read another fix log.

 

[Equites]

Check your ROM sockets on these.

 

[yorkshire_spam]

All ROM sockets on the board have been changed for turned pin sockets and the ROMs verify fine with the fluke plugged in.

I found a damaged track DEAD4 on the dram addressing circuit (common to all pin 11s on the 4116s) I meant that M4 onwards didn't have DEAD4 connected.

I've bodged a bit of solder onto the track to reconnect.



Back to no sounds on the self test, but it looks like the watchdog is barking, so I tied WDDISABLE to ground and got this...



I'm thinking there's something screwed in the ram addressing post A->BA in the addressing (as the ROMs all address and read fine), probably somewhere around the 157s where they go from BA->MAD in addressing, but I'll start doing some checks later.

EDIT.... having checked the schematics, my money is currently on the 74-42 at E8 as it affects _WDOG and _COLRAM... but we'll see!

EDIT #2: Here's a video of the board running with WDOGDIS tied low:

https://youtu.be/lKgoYQElnS0

yorkshire_spam2018-11-15 15:07:50

 

[yorkshire_spam]

A quick update on this although it's not actually fixed yet....

With the fluke in the processor socket the board now verifies in terms of bus, rom and ram. (Long ram test passes) but clearly there's still a fault with the WD barking

I'd previously tested all the timing circuits but only with a logic probe. At this point it's time to break out the 'scope.

I started working through all the timing lines and there's something not quite right on the 64H line (E5 pin 13), so I swapped the 74ls160 at E5. BUT the new known good IC gives the same "quirky" output. So I suspect I have a track/pin short or a seriously faulty IC downstream. As 64H (and other H signals) feed the logic in the DRAM address decoder I think it's a reasonable candidate for the fault I can see on-screen.

I'll post a proper update with some scope traces etc. when I finally fix the fault.

 

[yorkshire_spam]

I am such a muppet. I forgot that E5 is an LS160 and D5 is a LS163....

So the output pattern on 14,13,12,11 on E5 will not be the same as D5.... it should look like this:


Which it does.

Back to the drawing board on this one!

 

[yorkshire_spam]

A couple of steps backwards on this board (but that's just the nature of these things!)

I came to do some more work on the board the other night and it had reverted to the typical Missile Command blue screen of death.

So I pulled the CPU and ran some basic checks with the fluke - rom, ram and bus all seemed ok.

I checked the _RESET (pin 40) on the CPU and it was stuck low.

A bit of tracing back through the schematic revealed a dead counter at D4 (74-93)

Replacing that got the screen flicking again, but still blue. Watchdog is now barking, but looking at the DRAM circuits I can see that the MADSEL is stuck.

I checked pretty much all the logic that feeds the MADSEL and it all seemed fine, but a check of the 74ls74 flip-flop at F7 using the HP comparator reveals it's outputs are goosed.

Piggy-backing a working IC at F7 get's me back to where I was 2 weeks ago - the board is running but with the glitches you can see in the video above.

I'll stick the faulty IC and the replacement on the scope and try and get a screenshot to post here for reference.

Then it's back to trying to figure out the screen issue. I'm going to dig out the Data I/O 29B and get it running so I can dump the prom at L6 (I socketed it earlier in this PCBs life, so it's an easy check)

 

[yorkshire_spam]

So I went back to the start with the scope and went through all the timing signals one at a time....



That's the signal from pin 3 of the LS191 at A4, previously replaced!

The second trace is a second LS191 in the HP Comparator clipped to A4.

Pin 3 is the 1V signal, used in addressing the DRAM during the picture output, so I'm pretty sure I'm on the right track with this. BUT replacing the IC makes no difference.

So.... I'm thinking I have a short/partial short or a serious issue with one of the 2 ICs downstream on the 1V signal?

 

[yorkshire_spam]

It lives!
I couldn't find any shorted tracks on the PCB, but a continuity test on pins 5 and 6 of the LS153 @ K3 indicated a short.

Removing the IC from the PCB and there's still a short, BUT no longer a short between the corresponding tracks on the PCB - so it's definitely the IC not a PCB issue.



The final thing to do is to find out if the missing bit at the top of the score is down to the PCB or the monitor.

Also, I'll re-write post #1 in this thread as a summary of the faults found and corresponding fixes.

Cheers, Sam

 

[dave11674]

Lovely post
I have a MC at work
Doing the same corrupt screen

I think with your help yorkshire_spam ill get it back up and running

Ordered a few logic chips off ebay so will keep you posted

 

[yorkshire_spam]

Hi Dave, good luck fixing your board, if you need any help give me a shout.
Cheers, Sam

 

[dave11674]

Hi mate

Is there a way to get test beeps from the boards on the bench ?
I have a speaker ready to hook up but not sure if its
Gnd and aud1 to speaker
Or aud1 and 2 to speaker ?

Playing with a board thats just seems to be resetting all the time.
Flickering red screen

Ive done the ram mod with 4164
Swapped eproms in from a working board
Put a known good working cpu in too.



Thanksdave116742019-07-06 13:42:38

 

[yorkshire_spam]

If you hook audio 1 & 2 up to the speaker it'll be VERY quiet, but just about audible.
I have a small mono amp on the bench built from a simple kit and powered from my test rig for when I work on Atari boards.

 

[dave11674]

Just ordered a mini amp off ebay mate £11
Should do the job

 

[yorkshire_spam]

Have you checked all the timing signals with a scope? that's where I'd start.

 

[itruk]

Bit of a bump.. Been offered a Missile Command but it has a fault. The seller just says it comes up with a red screen.
Is this fixable or hard to tell by that description?

A US seller so I'm unable to get hold of the board myself.

Thanks

 

[yorkshire_spam]

To be honest blue/red screen of death isn't really much to go on. It really just means that the board isn't running for some reason and whatever values the ram is set to give red (or blue) on screen.
Most likely repairable, but what's wrong with it is another matter! (Ive only come across one MC pcb that I haven't managed to fix yet)