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  CompuPro System Support 1 by Bill Degnan - 03/26/2010 14:11
My CompuPro System Support 1 card is on the fritz, but I am working to fix. The signal is being received from the terminal but I can't see the text on the screen, nothing is returned through the IO card. My first guess is that the 1488 or 1489 chips are bad. Swapping did not fix.

I checked the voltage regulators. The 7912 in position U2 is not getting the expected voltage. I replaced the 7912 to no improvement. I think now maybe the tant cap in position C3 (or maybe C4) is bad. There are four such caps on the board C3, C4, C7, C8.

The manual says that these tant caps are supposed to be 6.8uf 35v tant caps, but when I check the actual caps on the board they're all marked +1.5 25V

Now I need to decide whether to trust the manual or use +1.5 25V instead because that's what's there, and it worked for apparently many years that way?

  How to Test a 7912 (-12v) by Daniel Roganti - 03/27/2010 12:40
The reading of the voltage on the 7912 could be confusing. Since that's the neg.12v regulator, pin 1 is ground(black lead) and pin 2 is the input, the opposite of the pos.voltage regulators. Output is still on pin 3. So maybe you were getting the wrong reading. Pin 1 is on the left when facing the part from the front and standing up.

So there should be about neg.18v of input voltage from the S-100 bus on pins 1,2. Is this what you meant ? Because this is the main power supply we're talking about, which would affect the entire system. If so you would have to take out each card to check if one of them is shorting out the main power on the S-100 bus. I'm hoping the main power supply would still be ok and it's just one of the cards that could be loading down that power. Basically check for a low ohms short (<100ohms) on the pins of each card between the ground pin (pin 50,100) and the -18v pin (pin 52).

If you meant that the 7912 output voltage is bad, which could've be read incorrectly since pin 1 is ground instead on this. So you want to read on pins 1,3 to get the output voltage reading. Then you should see about neg.12v on there. If not, then the problem is somewhere locally on that card.

I checked the tant.caps on mine , all for of them and they are all 1.5uf, 20v. I just noticed that too in the parts list of the manual as the schematic doesn't mention it.

You want to check for any low ohm shorts (<100ohms) around the 7912, after you verified the voltage by following the pins I mentioned above. Between pin 1(black lead) and pin 3 since the problem exists in the output circuit.

  7912 is not receiving voltage by Bill Degnan - 03/27/2010 21:12
The 7912 pins 23 is .643 volts. The input is 0. The other voltage regulators are OK.

  Measuring a 7912 by Daniel Roganti - 03/27/2010 22:45
oh, sorry I was dozing off here :-)

pins 2,3 is not the correct combination

Ground=pin 1
input volt.=pins 1,2
output volt.=pins 1,3

Ground=pin 2
input volt.=pins 2,1
output volt.=pins 2,3

  SS-1 Card Repaired, Replaced -16V fuse by Bill Degnan - 09/12/2010 09:06
Thanks to Dan Rog. the SS-1 board has been repaired and the cut traces fixed. The card was inserted into the system but...nothing...

Worked with Dan via video cam, traced the -16V and learned that there is no -16V on pin 52 of the S-100 bus. Traced the wires and discovered a set of fuses hidden on the chassis for the +16 and -16V lines. Replaced the fuse for the -16V and now the system is back in business, done.



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