26 Dec
2025
26 Dec
'25
7:13 a.m.
The VT101 uses a 68 ohm 1/4W resistor. I don't
have one of those "in
stock",
but I do have 100 Ohm 1/4W. I have a 33 ohm but not
sure if this might be
a
big high wattage one, I will have to check tomorrow. I
guess I can try
both to
see what happens.
I tried both a 33 ohm and 100 ohm resistor. I wasn't able to put it on the
DC011 output, instead I had to put the resistor on the 74S299 input as it is
socketed and the chip is expendable.
The video signal was still fine, although I can only use the External Video
Output Connector to verify. However, the 74S299 still seems to get just as
hot.
Interestingly though the waveform did change. Using a 33 ohm resistor there
was still a spike on the falling edge but it didn't go negative. Using a 100
ohm resistor the spike almost disappeared
https://rjarratt.wordpress.com/wp-content/uploads/2025/12/vt100-74s299-clk-s
ignal-with-100-ohm-resistor.png
So now with the 100 ohm resistor it does look practically identical to the
VT102 signal, but it still seems to get too hot. I think I will just put it
back to what it was and live with it. I will try the 74F though when I get
it.
Thanks
Rob