[Coco] [Color Computer] COCO II A/V Mod

[Computer Doc]

Double check to make sure both transistors are functioning correctly.  I
need some HP oscilloscope probes, so I can't verify your 20mV measurement,
but if you have the correct DC voltage on each transistor pin minus the
voltage drop across each transistor junction then your circuit should work.
It seems like I remember output video rms voltage is very small like 1V
peek-to-peek.  Video Circuits are not my forte but a close friend of mine
has taught me quite a bit over the decades.  It would seem to me that 20mVs
is just too small to effectively produce a good video image on a composite
video monitor if any at all.  Also make sure that you soldered the color
monitor driver circuit to the correct pins of the MC1372 IC.  The following
is another link (the original I can't find) of the same simple color monitor
driver circuit that produces a composite video output for the color computer
that I used.  When I built it, mine would not work either.  I got to
checking DC voltages to make sure the transistors were turned on and it
turned out that the final NPN was not turned on!?!.  I suspected Radio Shack
must have sold me a bag full of NPNs that must have had some PNPs mixed in
with them.  It turned out that MOST (12 out of 15!) transistors in the bag
was PNPs!  It was labeled NPNs!  I got the transistors switched out, but I
have not gotten back to replacing the PNP with an NPN yet.  It's so simple I
should have done it already, but I have been busy building a Motorola
6802/6846 microprocessor & ROM, I/O, Timer based computer system that I had
started way back in college. I never finished it because the project was too
overwhelming and I did not have enough time to complete it. I've been
reinspired to tinker with 8-bit microprocessors again and I just have not
gotten back to repairing my composite video driver circuit.  I have my color
monitor driver circuit  connected to my Coco 2 also.  I pray you find the
problem so you can enjoy your Coco 2 also.  Happy Computing!

Kip Koon

Here's the link:
http://coco_docs.tripod.com/id53.htm


-----Original Message-----
From: coco-bounces at maltedmedia.com [mailto:coco-bounces at maltedmedia.com] On
Behalf Of Ale Smukler
Sent: Friday, May 25, 2012 9:40 AM
To: ColorComputer at yahoogroups.com
Subject: [Coco] [Color Computer] COCO II A/V Mod

Hi! I'm an argentine Home Computers collector and am interested in adding a
audio / video composite outputs to a COCO II (my model is a "melted
keyboard" one). I saw that there exists a circuit, but I can't find it!.
Could you help me, please?

Muchas gracias!


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