[cctalk] Re: Hot Video Shift Register on VT100

(cctalk(a)classiccmp.org) also I have gone back again to look at this even more carefully. I am now fairly sure that there *is* a difference between the VT100 and the VT102. On the VT100 I get this https://rjarratt.wordpress.com/wp-content/uploads/2025/12/vt100-74s299-clk-s ignal-with-short-ground-lead.png. This is using a ground spring, with the 74S299 removed and the probe and ground spring directly connected to the socket. It shows a slight -20mV negative spike, but it varies between -20mV and -80m. On the VT102 I get this https://rjarratt.wordpress.com/wp-content/uploads/2025/12/vt102-74s299-clk-w ith-shorter-ground-lead.png. This used a longer ground lead and with the 74S299 in circuit, but even though the ground lead is longer the signal looks cleaner, there is no negative spike on the negative edge. In fact on the VT100 you can see that the signal then rises to about 0.4V, which is the minimum it goes to on the VT102. cause necessary or I think the difference must be that on my VT100 the DC011 output that goes to the 74S299 is directly connected. On later revisions of the VT100 there is an inductor and on the VT102 there is a resistor. These would presumably reduce the slew rate as you would suggest and indeed the slew rate seems lower on the VT102 trace. If this is the case then I guess the heat is by design, but the design was changed to stop this. I also wanted to respond to Dwight's suggestions about a couple of tests. I have done quite a few tests already actually: 1. With the load removed 2. Replaced the part. 3. Checked the apparent input spikes (see above) Dwight, I think this covers your first test with the 250 ohm resistor to ground, but I don't understand the last test you mention? Thanks Rob