Well to be blunt the CM-5 is a junk monitor, If I were to try to buy I would not want the CM-5. Therefor a small price reduction would be good. Besides I have an RGBtoHGMI convertor. And there are a few TX's on eBay right now over priced as usual.
Boot in Model III mode by doing the reset break key combo. Signal should be stable there. If it is than boot up in The Model 4 mode using LS-DOS or TRS-DOS 6.x. In the Model 4 NGA there should be a trim capacitor to stabilize the signal. Should be upper left corner if I remember correctly. Yes...
I once got a Jr that was basically new in box. When turned on nothing happened. I was going crazy. (Usual for me!) anyway long story short the ROMS somehow
went bad. So someone sent me the ROM images, put them in the Jr. and all went well booted up and worked fine after words.
When you try to put the top back on look into the top disk drive cutout and make sure the CRT neck isn't hanging on any wires. Also make sure any power supply wires aren't preventing the top from going on. Just a couple of ideas. Even my Model 4 hangs slighly in the back right corner sometimes.