Re: 1084 woes

[Linards Ticmanis <ticmanis@coli.uni-sb.de>]

Michael J. Mahon wrote:


> > I guess either the quartz or the PAL chip TCA650 simply doesn't like to 
> > be switched on and off very quickly. I'll try inserting a 220uF 
> > capacitor between the transistor base and ground - combined with the 
> > base current limiting 4.7kOhm resistor that should delay the activation 
> > of the transistor for about 0.1s (if I calculated correctly), quite 
> > enough to get the quartz across those 4 lines of text alive.
>
>
> No crystal oscillator can be switched on quickly.  That is the nature
> of a high Q circuit--it takes a while to settle down.

OK, then my guess was basically right.

> Instead, the output of the constntly-running oscillator can be gated
> (ANDed with a control signal) very quickly, and this is the usual
> means of switching a clock signal on or off.

This is obviously not the solution in this case, because...

> Even if you can stabilize the color reference, it must b e supplied
> on each line--including during vertical blanking--so that the color
> monitor can maintain correct phase lock.  This is another "high Q"
> circuit which must be kept supplied with a stable phase reference
> or color rendition will suffer--usually after just a few lines without
> a burst.

And it did suffer. Quite badly in fact.

> This is why the Apple does not switch the color burst off during
> the text part of a mixed display--to do so would cause unspecified
> behavior of the the "color killer" circuit in the monitor, and poor
> rendition of the color for many lines after the burst was restored.

Basically the circuit looks like this....

Quartz
etc.     Ground
   E     C
    \   /
     \ /
    ===== PNP transistor 2N3906
    B |
     ---
     | |  4.7kOhm resistor
     | |
     ---
      |
      |
     /TEXT from slot.

I wonder what would be the easiest solution to delay the activation of 
the transistor? I thought about adding a someting like a capacitor 
(220uF maybe) in between B and Ground.

The capacitor would be charged through the 4.7k in Graphics mode
and then upon the switch into text mode it would slowly discharge
through the resistor again. So both switching directions would have a delay
introduced. The mean voltage in mixed mode should be high enough to keep 
the transistor inactive at all times then, so the quartz gets its 
working voltage and stays happy.

Unfortunately my knowledge of electronics is limited, so I have no idea
if this would cause my Apple to burn...
--

Linards Ticmanis

 The Master said, "The business of laying on the colors follows the
preparation of the plain ground."