Re: Display issues

>On the Lisa 1: the display looks bright and crisp, with little
>evidence of waviness whatsoever. There is some very minor
>distortion: scanlines at the top of the screen are more widely
>spaced than those at the bottom:

There is an adjustment for this on the video board, tweak the pot called "V. LIN".

>The main problem is that the screen brightness occasionally goes
>haywire. It doesn't flicker-instead, at random intervals, the
>brightness will ramp down smoothly, as if someone triggered the
>"privacy screen dimming" feature. Sometimes it won't ramp down
>completely-it'll just go partway. Other times, though, it smoothly
>fades to black, and then after just a second or two, it will POP
>back to full brightness.

I don't recall seeing this one, but I think this is one of the failure modes of a flyback transformer.

I have a video board that shuts off then comes back after a while due to the 7824 voltage regulator (U1) on the board overheating, but I don't recall it going dim.

Other possibilities may be the digital contrast control circuit on the I/O board and the Contrast trimpot on the video board.

When the display shuts off, do the contrast/brightness controls on the power supply have any effect?

>On the Lisa 2: first of all, the screen is pretty dim and takes a
>while to "warm up".

Depending on what you mean by "a while", adjusting the contrast & brightness knobs on the back of the power supply may help. If they don't I imagine it could be a few different things on the video board, or perhaps the CRT itself.

>When it does, the edges of the display and
>vertical lines have a very slight waviness to them. This waviness
>varies horizontally no more than a quarter millimeter, let's say,
>and vertically the "period" is maybe 3mm or 4mm. The waviness will
>"roll" up the screen and will gradually go away the longer the Lisa
>is running.

Also see:

   http://lisafaq.sunder.net/lisafaq-hw-vid_wavy.html

I have seen this symptom caused by ripple on the 33V output of the power supply as described in A4 of the link above. In this case you can minimize this by moving the display to left as far as possible using the H PHASE control on the video board. If it is bad enough to warrant it, you can move H PHASE all the way (which will fold the left edge of the image), then move the image back to the right by adding a capacitor to the video board.

>I've also observed occasional positional flickering, where the
>picture will displace itself horizontally very briefly-barely a
>flash-before snapping back into place. (May be a video board issue
>here?)

Also see:

   http://lisafaq.sunder.net/lisafaq-hw-vid_jumpy.html

Along with other kinds of video twitching and rolling, this is a common symptom of aging of the trimpots on the video board. Although tweaking them can make the display stable for a while, replacing (all or some of) them is often required to make the display reliably stable again.

These are the values:
5K Cont R5
1K Hor Phase R34
100K Height R22
100K V Hold R19
200 Width R2
100K V Lin R31

These are current replacement parts from DigiKey (match the value to the trimpot to be replaced, or get 3 of the 3352T-104 and one of each of the others to replace them all):

3352T-201
3352T-102
3352T-502
3352T-104

James

-- 
-- 
-----
You received this message because you are a member of the LisaList group.
The group FAQ is at http://lowendmac.com/lists/lisa.html
To post to this group, send email to lisalist_at_email.domain.hidden
To leave this group, send email to lisalist+unsubscribe_at_email.domain.hidden
For more options, visit this group at http://groups.google.com/group/lisalist
--- 
You received this message because you are subscribed to the Google Groups "LisaList" group.
To unsubscribe from this group and stop receiving emails from it, send an email to lisalist+unsubscribe_at_googlegroups.com.
For more options, visit https://groups.google.com/d/optout.