Tempest fix

G'day folks,

Just to let people know how my Tempest problems worked out, Scott Swazey
spent an hour or two on the phone with me helping me diagnose my
deflection problem. He was quite patient when I confused the four
SG1495's while taking reading, so my hat is off to him! <SWISH>

I've describe my symptoms before, so I'll just summarize by saying that
I seem to loose the bottom half of the screen (and it wasn't the monitor
because I checked the boards in another Tempest). I'd suspected the
opamps (since I've found them been bad before), but after replacing all
four I still had the same symptom of half a screen?

At this point I thought maybe XREF might be high, and if the voltage was
stable and used as a reference for deflection would explain the
scrunched line. But Joe (and others on the vectorlist) quickly showed
me that this signal was used by the DAC and not the output stage of the
deflection circuit. So I moved on to the analog switch figuring that
maybe I was seeing a similar failure as Joe did on his Lunar Lander (but
on just one axis).

Scott quickly eliminated that as the problem, and after having me check
the YOUT signal at various points along the circuit (at opamp outputs
for the most part) narrowed the search to the four SG1495's, since the
first pair of opamps were producing reasonable results and the second
pair of opamps produced a goofy YOUT.

Further investigation by grounding XCOR (this is where I started
confusing A/B 12, A/B 13, C12 and C13) showed predictable results. And
XCOR and YCOR both gave reasonable readings, so the SG1495 at C13 was
the prime suspect! All that socketing of opamps paid off at this point
by allowing me to quickly bypass C13 by feeding the output of the first
pair of opamps into the second....and presto....a full (albeit smaller)
picture! And swapping another SG1495 into C13 completed the repair!

Again, many thanks to Scott for taking time out of his busy schedule to
help me deduce this problem with my limitted test equipment (a pocket
DMM)!

                Steven S Ozdemir
                sso@dsc.com

ps - For those of you who cringe at where I'm getting my SG1495 and
other parts, I've scavenging off of a bad Tempest board to save this
Tempest board and two other Tempest boards.
Received on Thu Dec 4 15:31:03 1997