Great, my VT52 is shot.

[Mattis Lind]

Den sön 19 apr. 2020 kl 17:22 skrev Jon Elson <elson at pico-systems.com>:

> After looking at the video, and Mattis' description of the
> sweep method, I'm guessing a counter
> in the chain has gone bad.  Assuming this uses a 5 x 7
> character matrix, then there would be probably
> a divide by 6 for the scan across each character plus space
> between, then a counter for the number
> of characters per line, a counter for the 8 or 9 raster scan
> lines for each row, and then a counter
> for the number of rows.  At least one of those counters
> might be counting too fast.  Does the vertical just give one
> reset pulse, or does it give a pulse to bump the trace down
> the screen one scan line at a
> time?  Also, a cap in the vertical sweep circuit may have
> gone bad, causing the vertical to move too
> fast, as evidenced by sloping lines on the screen.
>

As I wrote earlier in the thread I think it is a good idea to check the V
sync and H sync signals to check that they are right in pulse length and
shape.

There are indeed electrolytic capacitors in the vertical deflection circuit
but I am not sure if that would make the beam move much faster since for a
faster move of the beam also requires a higher voltage over the deflection
coil to create a faster ramp up of the current trough it. But it is
definitely worth checking. Especially since the scan lines are sloping a
bit and not straight as one would expect.

The interesting thing is that the counter chain that develops the V and H
sync signals are different from the on the ones that generate addresses to
the picture memory. There are separate X and Y counters that addresses the
memory and then for each scan line the lower bits of the accumulator are
used for addressing the char ROM together with the information from screen
memory.

The VT52 is a quite complex juggle where counters interact with the program
which runs in sync with all operations.

So,  yes,  a counter might have gone bad. Myself I have never seen a
counter that has counted too fast unless the input was too fast. But I
probably still have peculiarities to experience.



>
> Just guessing from a distance.
>

So do I. And now we need Chris to do some measurements on it to get further.

/Mattis


>
> Jon
>