TL;DR: I got a brand new Rottendog DIS200 display board and both upper displays have all the commas lit, all the time. Waiting to hear back from Rottendog, but in the meantime I'm wondering if anyone else has run into this issue or has suggestions. See photo below and history/details below that.

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History:
-- I recently acquired a Big Guns. Of the old displays, the upper left & lower left were completely out, and the upper right & lower right were functioning but not 100%. (Commas were not lit on upper display.)
-- I got my hands on another old display board for troubleshooting. This board had both upper displays and a lower right display (missing lower left). When I hooked it up, both upper displays came on (no commas) and the lower right one did not. This seemed to imply that at least the game was trying to send the signals to the displays.
-- I tested the output of the high voltage board, it was reading around 108V. I understand it's supposed to be around 100V, and it WAS powering the displays, but just to knock out two birds with one stone, I decided to purchase a new Rottendog DIS200 display.
-- It was explained to me by a friend that the new DIS200 display board doesn't use the high voltage supply, so I pulled the fuses from the HV circuit before installing the new board, to prevent any possible damage.
-- Hooked up the new board, and all four displays came on and they function perfectly -- except, on the upper two displays, all the [lower half of the] commas are lit up.
-- I found another thread about Cyclone that mentioned replacing ribbon cables to address this. Just to be sure, and to eliminate possible causes, I replaced the 26-pin ribbon cable going to the display board. I also replaced the other 20-pin ribbon cable for good measure (not sure if that is part of the display system). No change, commas are still lit.
-- I sent an email to Rottendog on Sunday night. I haven't heard back yet (I'm sure they're busy) so in the meantime I thought I'd start a thread here to see if there was anyone else who'd had this experience and/or had any suggestions.

NOTE: I will not attempt any modification / surgery on the display board itself until I formally work through it with Rottendog, since it's brand new and I don't want to void any possible warranty/exchange possibility.

The first time I hooked it up, it was loose outside the machine. (Didn't want to install only to have to remove if it didn't work for some reason.)

Thanks, I guess I'll have to see what Rottendog says. Still no reply to my email, so perhaps I'll call during business hours tomorrow.