There is only a single trace connected to Row1 on the PowerBoard but I can't figure out how it would cause a short to GND.
It was used on older pre-fliptronic WPC games for lane change.
Is there a short between U8 pin 5 (r1) and Ground ?
picture can be found here:
https://pinside.com/pinball/forum/topic/t2-flipper-works-but-not-flipper-switch#post-2001669

A ground short has nothing to do with the power/driver bird at all. Ground shorts are detected solely on the MPU with the driver board having no impact at all.

The switch matrix is entirely local to the MPU.

Make sense?
--
Chris Hibler - CARGPB #31
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The new place for pinball repair info

ehm, now you made me doubt so I had to convince myself on the test rig.

test_setup1_(resized).jpg

Oh. I see. That's a -1 driver board, that senses the flipper switches for the purpose of lane change and HSTD initial entry. Yes, I can see how that might happen. Good catch. It is an odd failure.
--
Chris Hibler - CARGPB #31
http://ChrisHiblerPinball.com/contact/
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The Place to go for Pinball Repair Info

If.... J113-5 is bent to J113-6, then there is still no connection between Row1 and GND on the PowerDriverBoard.
But when the ribboncable is connected on J211 - J113, it will create a path to GND via J211-6
It doesn't necessary need to be a '-1 board', this can happen on all types.

J113-5_indirect_cnct2GND_(resized).jpg

If you are sure that the PowerDriverBoard is the cause of this 'gnd-short-row1' , can you go to TEST T1 (switch edges) but with the ribbon-cable between J211 and J113 disconnected. Disconnect only from the CPU-board will do.

Is there still a GND-short-Row1 in the switch matrix ? If not, does the short come back when the ribboncable is back in place ?

I helped a guy with this problem on TAF and it took forever to find the issue was on the PDB (the opto was shorted if I remember correctly).

It sounds like your U20 chip is blown. I believe the U20 is socketed on TOM, but could be wrong. If so it'll be easy to switch out.

The OP has already verified the CPU is good.

OP, I know you suspect the problem is on the PDB, but since this started with someone crow-barring the coin door I would remove J212 and see if that helps. The fact you did not find a short with zaza's test would also indicate that possibly the PDB is not the issue.

A ground-short can also be caused by an opto board. When a comparator is defective in such a way, that an output always conducts, ground potential is permanently given on the row-line. Before searching for a mechanical ground-short, you should disconnect all connection cables from the opto board, that are leading to the CPU board. This way, it can be determined in a simple way, if the cause is to be sought on an Opto Board and also on what opto board.

Yes, it is easier to create a gnd-short with a crowbar to the coindoor-slamtilt or ball-trough-jam than the suggested PowerDrive short on J113-5, but that would be a 'too easy' fix

any news here???