23 Oct
2013
23 Oct
'13
12:03 p.m.
All,?
I have a Compaq Portable ("luggable"), revived from damp storage, and I'm
trying to work through some of the troubles it has.
First one I'm trying to sort involves the video card. It has the original CGA card in,
with the internal output hooked to the internal screen. It has a DB-9 connector for an
external C/EGA screen and also NTSC composite output on an RCA plug.
I've not been able to test the external 9-pin because I don't have a screen, but
I'm going to guess it's working because the internal screen display is good.
The composite out is terrible. On several television sets it jumps, losing horizontal
sync. The image supposed to be on screen is legible, just.
Anyone have experience with this issue (a particular cap, a chip particularly prone to
failure) or the schematics for the board? It's tough to trace, being a 3 layer
sandwich.
If anyone has the schematic for the vdu driver board also, I need to work on that too,
it's suffering from dry capacitors.
Thanks
Phil Andrews
Sent from my Samsung Galaxy S?4
24 Oct
24 Oct
12:37 a.m.
On 23/10/2013 20:03, Philip Andrews wrote:
All,
I have a Compaq Portable ("luggable"), revived from damp storage, and I'm
trying to work through some of the troubles it has.
First one I'm trying to sort involves the video card. It has the original CGA card
in, with the internal output hooked to the internal screen. It has a DB-9 connector for an
external C/EGA screen and also NTSC composite output on an RCA plug.
I've not been able to test the external 9-pin because I don't have a screen, but
I'm going to guess it's working because the internal screen display is good.
The composite out is terrible. On several television sets it jumps, losing horizontal
sync. The image supposed to be on screen is legible, just.
Anyone have experience with this issue (a particular cap, a chip particularly prone to
failure) or the schematics for the board? It's tough to trace, being a 3 layer
sandwich.
If the internal screen is fine then the syncs are at or close to the
proper frequency, so any problems are going to be around the NTSC
modulator, which would be fun to debug without a scope...
On the other hand whilst I have never tried the Composite on CGA, but I
have tried comparable modes on several other computers, including my
rather old Atari STE, and a very modern Raspberry PI and it sounds like
its working (almost) just fine. The composite out on CGA cards always
was pretty useless, and "legible, just" pretty much describes any
80-colum output on composite. Try 40 a column mode (2 or 3)...
(Oh and my experience is with PAL, NTSC would probably be worse.....)
If you have a TV with a SCART then the circuit below would work..
http://www.electroschematics.com/377/
but I guess you are in the US and and your TV will have CYMG inputs.
Actually that circuit would probably work with CYMG but the colours
would be wrong.
If have a modern LCD TV with a VGA input you could try something
similar, but leaving the syncs separate and feed it into the VGA. This
might sound daft but often LCD TVs will sync to normal TV on the VGA. IT
doesn't work with monitors, but it does with some TVs. As a quick and
dirty test you could just use 470 ohms on all leads and omit the "I"
line....
Dave
If anyone has the schematic for the vdu driver board also, I need to work on that too,
it's suffering from dry capacitors.
Thanks
Phil Andrews
Sent from my Samsung Galaxy S?4
--
Dave Wade G4UGM
Illegitimi Non Carborundum
4077
days inactive
4078
days old
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dave.g4ugm＠gmail.com -
phil＠viscom.net