Missile Command cocktail A/R II -5 reading -15!

mhkohne

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Donor 13 years: 2013-2025
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I had a failure (R30 blew and I lost a ram on the main PCB) due to a bad connector. Since I was ordering parts anyway, I ordered an A/R II cap kit from Bob Roberts and put it on. I also did the sense mod, because I know that I'm not going to be able to replace the bad connector right away, and the sense mod will keep my A/R II board from blowing up for the moment.

Now, I just got time to put it back in and have a go. I put the A/R II back into the system, and I did NOT connect the main PCB (it's still on the shelf). I powered up (no smoke!) and started probing the test points. 22 reads a little high, +5 is at 4.97, +12 looks OK, but the -5 is at -15 volts!

Obviously this isn't right - does anyone have any idea where I may have screwed something up? I'm going to take it back out tomorrow and re-check all the capacitor values and the transistor part numbers to make sure that I didn't put one in the wrong place. If that doesn't pan out, I'll have to start back-tracking down the -5 volt supply.

Any hints of things to look for would be greatly appreciated.
 
Hey I PM'ed you with a couple of suggestions. One other thing to try is to insert a 200 to 500 Ohm resistor between -5V and GND on the board, and then measure the voltage again. The slight current draw might bring the -5V regulator back into regulation.

If it still reads -15V let us know! This is a problem that pops up once in a while I don't think anyone has been able to nail down the cause yet.
 
OK, thanks guys. It was just that the PS was unloaded. I clipped on a 330 ohm resistor and it settled down to -5 volts very nicely, and I've now got the PCB back in and the system works!

Now all I need to do is put a cap kit on the monitor and adjust that...
 
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