Tempest pincushion correction

Hello 'listers

I posted a while back asking about a problem I was having with a Tempest pcb that the pix was pulled in towards the center of the tube on the top and bottom(mon. in cab). Didn't get any good suggestions and searching through past postings helped little so I had some time the other night and fooled with it.

Jed Margolin mentions the pincushion correction circuit on his site (www.jmargolin.com) but does not go into the theory of how the circuit operates. Since I'm no engineer, I removed all four of the analog multipliers(mc1495) from the pcb and also from a spare pcb and subbed them one at a time.

The bad ic turned out to be @a/b12 which is generating the x correction signal fed into the 1495 @c13 (if I'm following the circuit properly anyway).

I have seen quite a few questions concerning this and would like to better understand the theory of this section of the pcb. Any takers?

On another note....the board does not appear to be fussy with what type of op amp is used. I stuck in garden variety 1458 types and they worked just fine. I don't know why super high impedance ic's were spec'd for it, but....as I said, I'm no engineer.

Hope this has helped all of those with bendy tempest pix probs....gives you a place to start anyway.

Have a good holiday everyone

The Boardjunkie

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