From: RDC@oz.org
Hi All,
"Knut Roll-Lund" wrote in message
news:3JKdnUUqCLDCYsPbRVnzvQA@telenor.com...
> solarfox@DON'TMESSWITHtexas.net wrote:
>> On 31 May 2007 05:21:39 -0700, Kallikak didst
>> expound thusly:
> To me it seems you see part of the correct characters, it is just that the
> lines in the character are messed up.
>
> My guess would be bad contact on the character generator ROM (it sees one
> line as open which in TTL means high).
>
> Z12 74LS93 outputs the line "addresses" L1 L2 L4 but it is in the divider
> chain and must be counting correctly as you have the normal number of
> lines (of characters) on the screen. The character generator ROM might be
> broken but it is more likely it is only bad contact. L4? If L4 was stuck
> out of Z12 there would be no video at all. Contact cleaner spray in the
> ROM socket might do wonders.
Looking at the repetitive nature of the characters on the display:
(Bearing in mind that the first line of each character with the 6670 is
blank.)
Top half of each char is different to the bottom half: L4 is OK.
Each second line is not always the same: L1 is OK
Each lot of two lines repeats itself:
For example: ( the "I" char)
---
---
-
-
-
-
The L2 input on the char generator seems to be jammed low.
As Knut says, either Z12 has a stuck L2 output or the 6670 has bad contact
with the socket,
or (hope not!) the 6670 is faulty.
It's worth bearing in mind that Japanese manufactured Model 1's were sold in
Australia, which
have a completely different cicuit to that detailed in the Tech Manual.
It may be worth confirming with the seller that this is a USA manufactured
unit. (Sticker underneath).
Leslie
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