Missile Command C42 Cap polarity

tb lilley

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My Missile Command went down last night. Been running problem free for about 3 years.

Voltages all tested good. So, was in process of cleaning and reseating socketed chips when I noticed that the cap at C42 only had one leg connected to the board. Looks like the cap had been soldered in very tight up against the PCB.

Cap is .22uf 25V radial.

I'll order a replacement that'll go in here:

5A4F089C-6D96-4C42-843B-4B7C6B54888F-11366-000009D06D01A2B2.jpg


But I've no idea which way round the cap should go in, as there's no polarity marking on the board.

Diagram doesn't help either:

ae517a5a1c3c1e24afeec998dad88836.jpg


So ... Any ideas which side way around the cap should go?
 
I don't think that cap is polarized but have somebody take a pic of their board to be sure. It doesn't look like an electrolytic cap. I doubt that is the problem with your board also, sorry.

What were the measured voltage on the +5 of the chips?
 
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It's a ceramic disc cap - just been told these are non polar, so you're right.

I've only had a cursory look so far. Voltages read correct on PCB test points. A couple of years ago I replaced big blue, and fitted new diodes on the rectifier board. AR2 also had a full rebuild. New edge connector fitted (harness side) also. So could be power related, but I'm hoping that side of things is still OK.

I'll replace the cap as it makes sense to fix something so obvious - and it's cheap and quick to do. My PCB knowledge and repair skills are limited, so if that doesn't work, it'll be sent of for repair.

I don't doubt what you're saying about the cap probably not being the problem - but can you explain your thinking? The cap must be there to perform a job ..... ?
 
The 4116s like to die. I see yours are socketed. Also if they are not the issue, check the 74LS42(s) and 74LS32(s). They like to fail as well.

Bill
 
The board only had a slight fault to begin with. Small artefacts appearing on screen. Would disappear as each new wave started. Test mode reported ROM, RAM, MAP all ok. Turned the cab on this morning and now nothing. Blank screen. White. Won't go in to test mode. I'll replace the cap over the weekend. Also have, I think, a spare tube of 4116 so will try fresh RAM too.
 
I had the same problem. You can get the same cap from Radio Shack - it's a Mylar I think. Too bad you aren't closer - I still have the second one from the package.

The cap is NOT polarized. When I replaced my cap, my problem went away (yay!)
 
The board only had a slight fault to begin with. Small artefacts appearing on screen. Would disappear as each new wave started. Test mode reported ROM, RAM, MAP all ok. Turned the cab on this morning and now nothing. Blank screen. White. Won't go in to test mode. I'll replace the cap over the weekend. Also have, I think, a spare tube of 4116 so will try fresh RAM too.

Sadly, neither ram test checks all of the 4116's memory locations. I'd still try swapping out all of the 4116s as a starting point.

Bill
 
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