Burned in arcade screens just aren't part of the arcade experience I remember, so I try to replace them with burn free tubes when possible.
One particular hardship of owning a Sega Blast City or candy cab in general is that it's highly likely the monitor tube is going to have burn in. While the Nanao MS-2931 monitors do produce an excellent picture, the games that ran on them often had "Insert Coin" or some similar statement that would burn in the screen easily.
Official replacement tubes are really not existent. Only a few cabinets used this specific Nanao monitor. The tubes also have a specific curve that can be a bit more challenging to find as many I have come across with the same curve have bonded yokes which make swapping yokes difficult.
While the original tube on my Blast City had some burn, it still had a decent picture, but the corner convergence really wasn't the best. Unfortunately, without even moving the cabinet where it sat for months, somehow the neck broke underneath the yoke cleanly, almost like it had been cleanly seared off with a light saber. I didn't even realize it when looking at the tube until I moved the yoke itself and it was loose due to the breakage underneath. I'm not sure what would have caused this - yoke weight over time and a previous crack?
While the broken tube was kind of a set back, in a way it freed me up to explore finding a new tube because I could remove the yoke from the broken tube with no risk at this point and experiment with another tube.
Really, the only tube options available are going to come from discarded CRT televisions. But most of the time people just discard them because they figure no one will want to buy them. With few being sold on Craigslist these days, it is getting harder to find them especially without the glass on the screens scratched up from being tipped over on curbs.
While I had swapped many CGA standard resolution arcade monitors with TV tubes before, I wasn't sure about a Nanao MS-2931 as it is a trisync monitor. Would it work? I thought it should as even CGA monitor tubes can run at 640x480 resolution. I figured I would need to find a donor tube with the exact same outward curve. The Blast City was released around 1998, so obviously a tube from that period to about 2005 was probably the target zone. The newer the tube, the less use it likely would have also. Earlier, I had found a Toshiba tube that I thought would work, but ridiculously it had high screen burn from someone watching a news channel.
Fortunately, I came across a person who had been clearing out motel rooms who had a variety of 27" TVs of different brands so I picked up a variety of them in my vehicle.
A key bonus is that none of these TV sets had bonded yokes which can be quite difficult to remove without heating them up with a 12v battery charger to breakdown the adhesive. Otherwise you'd essentially break the tube neck attempting to take off the original TV yoke. Because I needed to put the Nanao MS-2931 tube's yoke on the replacement tube this made it much easier.
However, properly positioning and converging the Nanao MS-2931 yoke onto the TV tube was quite time consuming. The first donor TV tube I tried had wonky geometry. Getting the yoke in just the right spot so that you don't have magnetic color distortion issues was also a good test of patience. After using many chevron magnets as well as convergence strips, both home made and ones that I had taken off of discarded TVs, the result speaks for itself.
The TV brand and model of the donor tube I ended up using was a Symphonic model ST427F manufactured June, 2005. The TV tube itself was a Funai A68QBT892X.
Here are a couple of images showing a test grid using a CPS-2 JAMMA PCB, and a game play screen in CGA 15Khz mode.
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