Ok so here’s the next to last of my Super Nova stories. Everything was working great until the Space Lab decided to stop spinning, again. This time the points weren’t scoring so it was back to the switch catcher board after verifying no broken wires etc. Also none of the other switches that connect through the switch catcher worked.
By this time I was pissed off and had enough and just threw the two chips I had at it. One is an LM339 which isn’t an uncommon chip to fail. The second one is a 74LS32, the last is a 74279 which is a chip I don’t keep in stock.
In other posts I’ve shared a technique I use for locating faulty 74LSxxx chips. It’s not sexy, fancy or even fool proof, but it has helped me quickly isolate faulty chips.
What I do is put my multimeter in diode test, place the RED lead on ground, which is usually opposite pin 1 on the same row, and touch the BLACK lead on all the other pins. With the exception of the +5 pin, each pin will usually read between .450-.750. The power pin will usually read about .270 or so.
If I find one pin reading considerably lower than the others I suspect that chip and test it in my tester or just replace it and verify the repair. One word of caution is if you’re doing this test in circuit and find one unexpected pin dead shorted to ground, you need to make sure it isn’t grounded intentionally before condemning it.
I found one pin reading much lower than the others on the ‘279. Since I didn’t have a replacement on hand. I pulled it and tested it out of circuit. All pins read about the same.
Back to the schematics, I see the pin was connected to one of those green ceramic disc capacitors. I measured the cap it showed a much lower reading than the others. It’s a .1uf x 50v, I have tons on hand to replace on older Bally switches. Hint: you should keep some on hand too. I get them from Jameco, their part number is 2146302.
I reinstalled the ‘279 and re-tested. Space Lab is spinning once again. Since the game is going far away, I decided to just replace all of the ceramic disc capacitors to prevent further issues with the switch catcher board.
There are a lot of times those ceramic caps are just used for filtering and the circuit will usually function ok for a little while without the cap. Remember, when caps fail they usually go short causing all kinds of problems. If the circuit starts working without it, then it’s bad. Put a new one in and you’ll be good to go.
For my fellow gear heads out there, that was how I used to quickly diagnose faulty engine control components on late ‘90s Fords, especially mass airflow sensors. I’d just unplug it, start the engine again and if it ran better I’d put a new one in, problem solved, every time!
My next and last article on this Super Nova will be repairing the background sound issue. Till next time, Frank.