I am reverse-engineering a board which provides a VGA output signal for connecting an oscilloscope to an external monitor. I have quite a bit of experience with PCB design but there is a feature on this particular board which I have never seen before. It looks like a few SMD resistor footprints which are not populated with components, though the space between the pads appears to be a dark gray color, almost like carbon. What confuses me is the fact that there is soldermask over the pads themselves:

enter image description here

These footprints are on the R, G, and B video signal pins, as well as Hsync and Vsync. I am reverse-engineering from scans of the PCB. I have the ability to take the actual device apart and probe the card, but it is a bit of a pain in the neck and I would prefer that to be a last-resort. I am hoping I can get an answer without having to crack the thing open.

Any idea what these features might be? My suspicion is that they are some sort of carbon printed resistor, but I don't believe I've seen anything quite like this before.

UPDATE: I bit the bullet and I ripped the thing apart (as much as I didn't want to). The "devices" on the R, G, and B lines measure about 75 ohms, the ones on the Hsync and Vsync lines measure about 72k ohms. 75 ohm values are consistent with the characteristic impedance of the R, G, and B lines, so my guess is they're some sort of termination resistors. Not sure about the Vsync and Hsync though. Pull-ups/pull-downs?

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    \$\begingroup\$ Looks like place for ESD protection diodes, but not mounted. While most of the pads are covered, it looks like there is no soldemask between the pads, and small amount of the pads are exposed, which leads me to think they might serve as spark gaps. \$\endgroup\$ – Justme Jun 2 '19 at 21:11
  • \$\begingroup\$ To your update, unless you did not isolate the VGA pins by removing components or cutting traces, you are most likely measuring the impedances of DAC output circuitry for the RGB output which will surely have 75 ohms right at the DAC. Perhaps the 72k ohms is result of leakage current for the sync pins as the multimeter is forcing voltage to unpowered output pins. When measuring these components, they have to be isolated from the rest of the circuitry. \$\endgroup\$ – Justme Jun 3 '19 at 5:36
  • \$\begingroup\$ @Justme That is true, I wasn't thinking very straight last night. I guess I was assuming the outputs of the drivers would have been high-impedance when not operating. That being said, I did find a schematic or two that show 75 ohm termination resistors on these signals, so that was really what made me think that's what they were. Unfortunately I'm not sure how to isolate the devices without damaging the board, which I am definitely not willing to do under any circumstances. \$\endgroup\$ – DerStrom8 Jun 3 '19 at 22:26

I'm pretty sure they are carbon printed resistors. I've seen similar ones on display adapters. They do it so save parts to make it cheaper if they produce it in a high enough volume, because they can produce it using a single screen print process instead of 5 pick and place parts. But if you want to get a precise value to reverse engineer the card i think you have no other choice but to measure the resistance.

Here are some more infos about the topic (eevblog video).

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  • \$\begingroup\$ I am pretty sure they aren't carbon resistors. There is just a gap in the solder mask. Besides, what sense it would make for analog RGB video and digital HV sync pins to have such horrible tolerance resistances, and placed near the connector anyway? They just might be test points for probing signals and grounds in a production test. \$\endgroup\$ – Justme Jun 2 '19 at 21:38
  • \$\begingroup\$ @Justme See my edit. \$\endgroup\$ – DerStrom8 Jun 2 '19 at 23:14
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    \$\begingroup\$ Things like this may need to be tuned in the final configuration anyway; don't worry so much about what they were but design the replacement able to accommodate what might make sense. \$\endgroup\$ – Chris Stratton Jun 3 '19 at 0:39

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