Atari Jaguar CD Repair: No Image

I'm trying to repair a Jaguar CD and I'm pretty much out of ideas at this point.
Maybe someone has seen a similar problem before.

I recently got a Jaguar CD addon that unfortunately doesn't work. When connected to a (working) Jaguar, the power LED is on but there's zero image output from the Jaguar.
Same thing if a game cart is inserted to the unit.
Both the case and the mainboard are in really good condition. It doesn't look like it's been opened before.

Things I've tested/done so far:
- Both cart slots are clean, no missing or bent pins. I'm certain the connection is good.
- Can't find any broken or suspicious traces or vias
- There's clean +5V at Orbit/Butch (U9) and the BIOS EEPROM
- Power Reg U5 outputs +7V, U13 +5V (with PSU connected)
- The BIOS EEPROM is socketed on this unit. So I was able to easily check the contents with an EEPROM reader. It matches the BIOS found online.
- Checked for continuity between the BIOS and the cart slot to the Jaguar.

And something I noticed as well which could be related to the problem:
Output Enable (OE/, Pin 24) of the BIOS is always at +5V, with and without a game cart inserted.

Most testing was done with the CD drive disconnected and no Jaguar CD power supply.
I've never owned a Jag CD before, but as far as I know that should get the unit to boot and show the 'plug in power supply' screen.

Any help is greatly appreciated!

 

I would start by cleaning everything, these things are are like the Sega 32x in many ways. Any add on that daisy chains cart slots is going to have this type of problem

 

Yep, that's the usual problem and the first thing I did. But not in this case, I checked the connection for each pin with a multimeter, it's all good.

 

Replace capacitors if still using originals. Most electronics have odd issues if manufactured before 2004. Sega Game Gears and tube displays are good examples of this.

 

https://console5.com/wiki/Atari_Jaguar_CD Schematics. I had forgotten about that riser board, I would inspect both sides of that. Problem is that test for continuity only confirms a connection between the probes. I have seen a lot of connectors that look clean but have surface contamination that causes parasitic capacitance and resistance enough to cause data signals to fail.