Well, I know what I'd do.

I would take a multimeter on Volts and hook up the negative lead to signal ground and positive lead would start poking around:

On same board as contrast variable resistor (aka "DM" board) ...

1) DM board's IC827 pin 1.  Move contrast adjustment and see if voltage goes up and down.

If that's ok ...

2) Move positive lead to CN824 pin 13 (turn contrast, look for voltage going up and down).  If not ok, inspect series resistor.

On JKAN-LC board (which is cabled to DM board's CN824) ...

3) Move positive lead to JKAN-LC board's CN202 pin 13 (same test moving contrast).  If not ok, suspect cable connection on either DM or JKAN-LC side.

4) Move positive lead to CN203 pin 8.  Same test.

... And follow the bouncing ball all the way to the LCD.

If the first step fails (measuring at the opamp output) then I'd disconnect the cable from CN824 and remeasure.  If that's still not OK then I would start inspecting the components on the DM board for operation.  Turn off and unplug the keyboard and then use resistance to check ohms across the variable resistor to make sure it's changing when you move the contrast.  And if that checks out walk through the various components.  Make sure the op amp power checks out.  Etc.

This is fairly straightforward to someone who's done tech work or any electronic design.  Shouldn't be difficult to find someone in your area to take a look.  There's certified techs Yamaha has listed or you can go elsewhere.  This one shouldn't require the highest proficiency with Yamaha gear.

Btw: I had something similar happen to me.  Not GC but a different retail store.  They paid for the repairs and I took the equipment to my own repair center.  Wonder if that's something GC would entertain.  

Omg Jason! That information is gold! I apologize for not responding sooner, but the email notification that I had a reply to my post ended up in my Junk Bin, and I rarely look in there for anything. 
  When I have the time and the house is quiet I WILL get this diagnosis underway. I do understand the lingo and look forward to the challenge.

 Thank you again Jason, the detail of the reply is really incredible, you spent a good amount of time replying. It's greatly appreciated. 
-Don

No problem.  Happy hunting.

Also, depending on the external enclosure damage it's possible there's a board flex/stress issue that impacted traces.  It would be strange, to me, that the boards would continue to function for other logic and the problem to be isolated to this one thing.  Still, to be complete, you could ohm (continuity) everything out to make sure the traces/pads/solder to lead (as in pronounced leed) of component are physically in tact.  And if not you could replace traces with top or bottom side wire.  The stuff used for wire wrapping (solid core, 30 AWG - often blue insulation so referred to as "blue wire" by repair techs).  And you could jumper from pad to pad.  Check the solder joints first before suspecting a broken trace if continuity doesn't check out.

I don't know how production is these days but the older boards would have solder joint problems all the time requiring reflow.   Most everything these days is done lead free (you may see a silkscreen with "Pb" and a slash through it or "RoHS" stamp).  What I'm getting at is make sure if you have a flux pen and solder that the chemistry and composition is compatible with lead free (if the boards are lead free - or make sure compatible with leaded solder if that's what these use) or you'll be introducing problems from your rework.  Temperatures are also higher for lead free solder so adjust accordingly.   All of the reflow I would advise a no clean flux pen.

Outstanding reply Jason! Thank you so much for the heads up and crucial info. I had no idea about the solder/flux info 👍    I haven’t had the quiet time I need to focus uninterrupted on the instrument. 
  I have screen shot everything so I have it on hand… just in case. 
 Thanks again Jason!

No problem.  I don't know if the XS is Pb free (RoHS) or not.  The year of release is sort of in the fold when moving to lead free was a popular and regulatory required thing. In the industry of electronic design here in the US, the company I worked for operated under various exceptions until having gotten all the ducks in a row to pump out PCBAs that are RoHS compliant.  I'm not sure if Japan got ahead of the curve or dragged their feet like the company I worked for did.

The second page of the service manual mentions the solder contains lead so it's fairly explicitly denoting these PCBs are not RoHS.  So your flux and solder should use what at one time was called standard composition.  These days lead free is the standard but then it was the other way around.

Hi Jason, Well, I finally found an opportunity to troubleshoot this contrast problem last night. But I hit a wall right out of the gate. I'm continuing the post here instead of a message for anybody else who may run into this problem.

  To access the JKAN-LC board, there's a metal cover over it that had to be removed. There a connection to unplug and a ribbon cable to CN824. The ribbon cable clamp is not cooperating. (I can't seem to find an option to attach pictures in this forum?) I'm familiar with these ribbon clamps on laptops. I tried to opening it with just my fingernails and the edges of the plastic clamp are crumbling away. 
  These ribbon clamps are usually a no brainer. They pivot open or slide out of the way. While trying to remove the ribbon cable, it did come out of the clamp..undamaged. The problem is, I have to plug the ribbon cable back in to continue my test. I can't find a way to open the clamp. I know it's elementary, but I don't want to damage anything. I can "kinda" push it back in, but it doesn't want to go all the way in and I DO NOT want to fold the ribbon. 
  I also thought that maybe the part I think is a clamp, actually lifts up straight up off the board, and I took it apart incorrectly but I wanted to check before I start prying on anything. 
  One other question, on CN824, how do I know which pin is #13? The pins aren't marked. I know these snafus are exposing my rookie-ness. But I understand your well written step by step flow test and I love a good challenge.

Ahhh it's the little things in life. Thanks for your time. -Dez

Opps! On the second to last paragraph of my most recent post on 3/20/26' I noticed I type something incorrectly instead of "and I took it apart incorrectly".. it's meant to say "I don't want to take it apart incorrectly".

you can post zip files. So zip up the pics/other and attach the zip file.

I upload to imgbb then post the link here of the image file itself and that ends up placing an inline image.  When you upload an image to imgbb there's a "copy" button you can press to get a link on your clipboard.  This isn't the link to paste here.  What you do is put that link into your browser which will show your image on imgbb then right click on this image and pick the "save image location" (or similar) option which gives you the link of the raw image.  And paste this on to message.

To find pin 1 there's usually a dot or triangle on the PCB.  The connector may also have a marking.  The service manual should show this too.  I'm just getting back from a gig so I'll take a look later.

Most Yamaha ribbon connectors have a "collar" with a lip you pull up lightly which frees up the cable.  Sometimes they have ones you just pull.  I don't know exactly what's in there and a picture isn't going to necessarily make it obvious.  I guess I could cross reference part numbers with a keyboard I have taken apart already and will find a match if lucky.

There's not necessarily a need to take out the cable at first and if all continuity checks out then you're good.