exxos's Atari, Amiga & retro forum

sandord wrote: 03 Oct 2025 14:14 I don't know how to measure things related to termination because I don't really understand how termination works.

Normally just 75R on the "monitor side" There is 75R in series on the DAC as well. So if you had say 5V output on the DAC with 2x75R thats 2.5V output. If the monitor side termination is more like 50R, then it would drag the voltage down to more like 2.00V.

There is no functionality to control this output on the DAC IC itself. Its inherently supposed to output the correct voltages. Like I say, my monitor brightness is turned down a fair bit and the image is still too bright indicating the voltage is actually slightly higher than its supposed to be if anything.

When I was designing the ST2VGA adapters, I ended up where the voltage was too high and the image was extremely dark to appointing almost like a black screen. Basically oversaturation..

But I don't really understand what's going on because the image you posted originally looked fine...

If HDMI is added to the STe (using RGBtoHDMI), will this DAC upgrade board improve on the HDMI output as well, or does it only affect the original Atari DIN13 video output connector when attaching an Atari colour or mono monitor (or a VGA monitor through a suitable adapter such as your ST2VGA)?

Pacman wrote: 03 Oct 2025 20:14 will this DAC upgrade board improve on the HDMI output as well,

Possible yes.. No way anyone can really know unless someone tries it though.

I let this project sit for a while and decided it was time to finally pick up where I left it :oops:

I measured the R-signal on the monitor connector from both my ST and STE (the latter with the DAC upgrade) with an all-white screen.

This is the ST:
And the STE:
I'm going to look if I can measure the signal somewhere right after the DAC board to see if it perhaps gets attenuated somewhere in the signal path.

There seems only to be a 75 Ohm resistor (R407) in the path before it reaches the video connector so that shouldn't be significant to the output level.

For new readers: the transistor is not in the STE anymore, that's where the DAC board has taken over.
By the way, I measured without a monitor connected.

sandord wrote: 10 Jan 2026 16:52 There seems only to be a 75 Ohm resistor (R407) in the path before it reaches the video connector so that shouldn't be significant to the output level.

Actually it would.. There should be nothing on the RGB lines at all..

A lot of resistors should have been removed..

https://www.exxosforum.co.uk/atari/last ... /index.htm

I did remove all of the resistors that also are removed in the pictures.

The one I'm talking about is between the DAC board and the video connector, like the diagram shows.

Bridge over the 3 75R resistors on the DAC or STE..

exxos wrote: 10 Jan 2026 17:37 Bridge over the 3 75R resistors on the DAC or STE..

Thanks, the voltage jumps to 3.24V when I bridge the resistors on the DAC.

I'll have to calculate the proper value for replacement resistors so that I'll end up with about 2.88V, like my ST does.

Well, I do have a fancy scope but I also lack basic understanding of electronics.

But let me try to make more sense of this anyway. I'm going to make a lot of assumptions so please do correct me where I am wrong.

So, when I look at the schematics, I think R407, R409 and R410 form some kind of voltage divider.

Since those resistors are still on the board (the instructions didn't tell me to remove them), they do influence the voltage level after the 75 Ohm resistor on the DAC PCB, even though they aren't directly in the path from the DAC to the video connector.

When I bridge R409, the voltage drops from 2.30V to 1.76V so that seems like a voltage divider at work to me.

I don't really understand why the voltage divider is in the design.

Also, I don't think I should change the resistors on the DAC board but rather fix something on the main board instead.