Heathkit IO-101 Vectorscope/Pattern Generator Insanity .

[Kevin Kuehn]

That was it. The vertical sync pulse was so long it was blanking too far into the beginning of next frame. All it needed was a 1.5k resistor from ground to the vert sync input of the sync mixer gate, so now the vertical blanking width control works as expected. The way it was, the gate input couldn't recognize the change in pulse width(RC differentiator) because it had no DC reference. Apparently that was an inherent design flaw. Now I have to wonder if Heathkit at some point implemented a fix for that small oversight.

[Notimetolooz]

Before you fixed the problem I was going to say that the horizontal sync pulses look a little too wide. Maybe your fix changed that also.

[Kevin Kuehn]

Originally Posted by Notimetolooz

Before you fixed the problem I was going to say that the horizontal sync pulses look a little too wide. Maybe your fix changed that also.

They are fairly close to 5 microseconds. But I didn't have the pulse very well centered on the edge of the vertical graticule, so at first glance it looks wider.

[Zenith26kc20]

I've had one of these for a long time. If I remember correctly, mine does the same thing. I replaced all the IC's and IC sockets. I'll pull it out. Maybe you just got the answer on fixing these critters!

[Zenith26kc20]

I've got mine open here. Digital IC's are not my forte', I admit. Could you tell me from what junction of two parts you put the resistor to ground.

[Kevin Kuehn]

Here's a couple pictures of where I located the 1.5K on the back side of the circuit board. It's between pin 4(ground) and pin 10(input to G1 gate which is IC 9 in the manual). You will need to adjust the Vertical Blanking Width pot(R35) in order to get the best line pattern at the top of the picture. Basically you will be adjusting the width of the vertical blanking pulse, which determines where at the top of the screen the horizontal scan begins. When that pulse is too wide you see the vertical blanking pulse turn off transistion, which is what causes the wiggle in the vertical lines. Hope that makes sense. Let us know how it works out.

[Zenith26kc20]

Mighty big THANKS!!! I fought this unit for days and finally gave up! It sat in my upstairs room for a while now!
I wonder how many other people have given up on one of these because it just didn't make sense!
Again, Thanks!!!!!!!