clb wrote on 2024-11-14, 19:03:Gotcha, so the cold vs hot components have been ruled out. […]
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Gotcha, so the cold vs hot components have been ruled out.
Also, not sure if a coincidence, though in an older video I did about CRT Terminator, I posted about its compatibility quirks with different VGA adapters: https://youtu.be/i14WJheyG14?t=875
There in the compatibility matrix
I commented about an observation that some VGA adapters took an unconventional approach to require video signal to be sampled at the falling edge of the clock signal, as opposed to the phase being on the rising edge, which is the norm.
ET4000AX was one of those unconventional adapters. So I wonder if that could relate to the root cause. Because if I the sampling phase is off (in this case with respect to hsync and pixel data), then exactly that type of one pixel horizontal jitter would result (sometimes the hsync signal might get interpreted one pixel early, vs one pixel later). Then if the LCD display VGA pixel lock struggles with trying both sampling phases, it might not synchronize to get the right lock. -> trying another LCD display might give interesting info.
That, or the adapter has a roaming hsync signal would be my guesses here. Oscilloscoping hsync vs a busy pixel data signal line could be interesting. (will require referencing between the two to see how stable they run with respect to each other)
Got more information.
First, I have a correction to make. The ET4000 *was* always somewhat noisy in the 286, but not nearly as bad as in the 386. I think my brain drive space is running low 😀
It's still noisy on a different monitor, this one a much newer LCD.
I tried the ET4000 and the same super I/O card in both motherboards, connected to the same keyboard, power supply, and monitor, and they both have the issue, but the 286 is much better. It's almost good enough to ignore which is probably why I forgot about it.
Here's a scope capture of the ET4000 output on the 286. Channel 2 is V sync, 3 is H sync, and 4 is red. The probe grounds are each connected to pins 6,7 and 8 which are grounds. I'm not seeing noticeable jitter, but they sure seem noisy.
https://youtu.be/ZcARGZo_Du4
Next I connected the scope to a Prolinea I have, same exact method. Much less noise. That has to be the issue.
https://youtu.be/Li4Vk0kyWho
I did try powering the 286 with my bench supply, +5V only, but it wouldn't get through POST so it likely doesn't like the missing voltages. The VGA output was still noisy though.
Something odd I noticed on my card though: pins 84 and 85 are bridged and it doesn't appear intentional. It does appear factory though. As the chip is the 144 pin version this means these two are bridged:
HS pin
(datasheet explanation: Horizontal retrace synchronization signal, supplied to the CRT monitor)
and
TKN1
(datasheet explanation: Token Status output:
TKN (1) = ET4000's MCU is processing font cycle.
TKN(O) = ET4000's MCU is processing pixel cycle.
TKN ( 1:0) are redefined if CRTC index
35 bit 5 = 1:
TKN ( 1) = interlace mode active.
TKN(O) = even field. )
This is very suspicious and I feel like removing the bridge and retesting it. These two pins do not appear to belong together. EDIT just removed the bridge and no change.