My conversion of the SD Systems VDB - 8024 for VGA monitor is working fine.
If anyone else needs to try a conversion to move up to a larger monitor
size, or if your old mono CRT is failing, let me know and I will post the
instructions for the conversion. The cost is about $7.00, using two faster
ALS TTL ICs and a 28.63636 MHz crystal plus a converter cable (made of an
old VGA signal extender cable). You get white characters on black screen. An
old 80486 cooler heatsink may be needed for the SMC CRT controller chip
also, but the clock is just at the top rated speed for the SMC part.

Bruce

The xtal freq was doubled, from 14.43 MHz to the 28.63636. It is necessary
to increase the minimum horizontal freq. of a typical 80X24 mono CRT driver
from about 15.5 KHz to 31 KHz. The raw clock is used for the dot clock in
the video shift register. It is further divided to produce the character
clock into the CRT5037 chip.

I'll cover the mods I made on the SDS 8024 board as an example of things to
consider.


Mods Required for SD Systems VDB - 8024

NOTE, while I changed some TTL  and LS chips to ALS or F series, it may not
be necessary, I just did it to ensure good waveforms since the top end of
the original chips was surpassed (max rated freq. 25 Mhz, but many will
likely run at 28.6 Mhz)

Y1 - change from 14.43 Mhz to 28.63636 Mhz - basic timing element
U2 - change from 74LS04 to 74ALS04 or 74F04 for OSC circuit
C3 - remove (may not be necessary)
U1 - change from 74163 to 74ALS163 for character clock and Z80 clock generation
U24 - change from 74LS165 to 74ALS165 for video shift register
=== U24-11 drives the onboard Z80A. With 14.43 Mhz xtal the Z80 clock is 2.5
Mhz.
        With the 28.68686 Mhz clock it will be 5 Mhz, which should work in
most cases.
        If not, lift pin 13 on U1, and short trace pads U1-12 to U1-13,
giving a 2.5 Mhz clock again.

The CRT5037 now generates a horizontal signal of about 31Khz, and a vertical
sync pulse of 120 Hz. This seems not to bother the VGA monitors. I have run
it successfully on a low end 15" VGA and mid quality 17" VGA monitor with
perfect results.

CRT Cable to VGA Monitor Wiring

+Horizontal drive to pin 13 on VGA connecter
-Vertical drive to pin 14 on VGA connecter
+video signal out to pins 1,2 & 3 on VGA connecter (RGB)
Ground to pins 5,10 typical on VGA connecter (and 6,7 & 8 if needed for RGB
return)

While H and V polarity are shown as +H and -V, most VGA monitors sync on any
polarity.

******** HOT - HOT - HOT *********

Fasten an old '486 heat sink to the CRT5037 at the center (die cover).

Regards,

Bruce

FURTHER TESTS

I tried running the VDB -8024 with the original TTL and LS parts again. It
is not reliable, and the horizontal scan time too large.
In addition, the Z80A did not work well at 5 Mhz, so the pin lift and trace
jump for U1 was needed. A quick test with an 8 Mhz Z80 did not help the
situation, and for now I have no need to speed up the CPU.

Bruce

Hint... the die is actually attached to the *bottom* of these old IC
cases. The heatsink will work a lot better if you can get a piece of
metal under the IC, and use it to conduct the heat to the top. (Heatsink
manufacturers make heatsinks to do this).
-- 
Lee A. Hart