Monday, 13 October 2014

Tempest. Board repair.

This board failed on me back in March, and I briefly looked at it but didn't really have the enthusiasm to fix it at the time, fast forward six months and here we go again...

So upon re-testing the boards, the X axis output had stopped working, this was caused by a failed TL082 at location D/E12 on the main AVG board, pin 7 wasn't outputting anything, I fitted a new socket and replacement IC and had a stable output again, back to where I was six months ago.

Last week I got stuck into this fault again, (Crazy vectors and Showing Mathbox error in test mode) I have spent a good few hours probing with a scope and logic probe to try and track down where the issue was, I got the schematics printed onto A1 paper at work, I even de-soldered all the proms from another Tempest AUX board to double check my proms had verified correctly, which they had.

Now, convinced that it wasn't a Prom/Bitslice/Socket or Interlink cable, I honed in on the remaining IC's in the Mathbox area. E4 a 7408 (quad 2-Input AND gate) got my attention, now I'm not really proficient enough in fixing boards to understand what was wrong but pin 6 which drives the clock input of B1 (374) didn't look pulse correctly on my logic probe, the frequency of the pulsing was all over the place and the HI/LOW LED's were going 'mental' in an odd way. I tried piggybacking a known good chip onto the 7408 to no avail, not deterred I de-soldered it and put a socket on the board. I then tested the IC in my Programmer which also tests 74 series logic, and sure enough one of the gates reported faulty. I put a new 7408 in and BOOM! The game is working again!

I have ordered new TL082's, MC1495's, trim pots for X/Y size/position adjustments and will fit these when the arrive to hopefully improve the reliability of this board, but for now I think I'm due a few credits. :)