ZX81 High Res timing issue

[msknight]

Here's a good one.

ZX81. Fitted with internal 16k RAM chip and tried various capacitors to "slow down" the chip select, but it won't work with proper WRX high res games. Characters meant to be proper high res (not pseudo high res) appear utterly garbled on screen. (pic attached of a "3d maze")

It doesn't appear to be the RAM because I've tested with an external RAM pack which has been converted to WRX High res and has been proven working on another zeddy. I've checked that 5v is being fed to the internal RAM (ie. disabled) when the external RAM pack is attached. The internal clock fed into the processor appears to be fine and as expected. (pic attached)

I've tried another ULA, but that hasn't changed anything. However, I am waiting for another couple of uLA81 which are back ordered from New Zealand. I don't have any ETA on their arrival, so nothing reliable to swap out with.

Total work that was done to the board... 5v regulator swapped. Video converted for lumacode RGB-HDMI. Two main caps replaced. ULA was socketed. Power and tape jacks replaced. 16k ram installed internally. Pic attached.

My experience is limited, so when it comes to something like this, I'm well out of my depth. I've got no clue where to start sticking my probe.

[msknight]

I've taken some scope readings from the RAM pack when attached to the good zeddy and the bad zeddy.

On the good zeddy the /rd and /rfsh signals are coming in nice and square at 5v. (bottom two pics) On the bad zeddy, the /rfsh has a wobbly top to it, and they are both down at 4v. (top two pics)

[msknight]

The source of the fault was found.

Even though I had two Ferranti ULAs, it appears that they were both faulty in the same way... or potentially not tolerant enough to perform well enough to be able to use WRX high res graphics.

I found this out by pulling a uLA81 from another zeddy and trying that.