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The RGB2HDMI cable documentation specifies that the sync should be picked up from IC14 (74LS86) pin 3. That works nicely for Issue 2 boards. I'd be grateful if someone would tell me the equivalent pin for Mk II boards. There are rather substantial changes in the schematics of those two boards.

Hard to make out the details on my copy of the ISS2 schematic but it looks like that pin is the 'padded for PAL' /hsync, in which case the MKII equivalent is IC14 pin 6.

Thanks for that Steve. That's what I'd assumed but, as the waveforms were significantly different, I thought it wise to ask this expert forum. The attachment shows an Issue 2 IC14 pin 3 and a MkII IC14 pin 6.
Pin 6 wasn't working as the sync input to the RGBtoHDMI (no amount of adjusting RV1, after lifting R65 and the 4.43MHz crystal, would get the DC voltage levels correctly aligned). This method had worked with the Issue Twos (IC14 pin 3 and RV2). All my Dragons (two Issue2s, one MkII) have DragonPlus power boards). After some investigation I swapped the MK II PSU (early version) with an Issue 2 one (a more recent version). At that point the MkII started working correctly with RGBtoHDMI whereas the Issue 2 machine displayed the fault symptoms. Time to examine that PSU board (doesn't look to be an inline resistor thing and the voltage outputs are fine). Very likely a difference in the Video DIN area rather than the power side. Investigations continue.

Alan

The MKII trace looks like what I would expect. The ISS2 trace looks like something weird is going on like the scope connections are reversed, or is not connected to the LS86 output. Ignoring the inversion and distortion the two waveforms are the same.

The Iss2 trace, on re-measuring, now looks the same as the Mk II one. Most likely an error on my part.
The fault on the PSU turned out to be a lack of -5V owing to a blown fuse. That fuse had already been uprated to 125mA (from 100mA) as per a recent recommendation. It hadn't died a violent death and a replacement of the same rating is holding fine for the moment.

The only schematic I have for the Mk II is poor (very blurry) so I can't easily see what effect the lack of the -5V line would have. It'd certainly cause disruption at the op amp in the video circuitry and the areas around IC 11 & IC9.

Anyway, success.
Thanks again Stew, and apologies for getting your name wrong in earlier posts. My Dragons are probably now in better condition than my eyesight.

Alan

No problem Alan,

Great to hear you got it working and you certainly have my sympathy regarding eyesight. Mine has deteriorated to the point that I can no longer do fine work without a big illuminated magnifier.