The ETX 90 and ETX 125 were optically superb scopes, but the mounts left a lot to be desired, especially the early models. I deforked my 125 a while back and it works very well on both my EQPro and ZEQ 25 mounts (although I am much more comfortable with it on the ZEQ!). My old 90 model (a 1995-vintage non-goto scope--I think Meade referred to this model as the ETX 90 RA) worked pretty well on 3 AA batteries. Tracking was decent enough for visual work and for imaging bright, solar system objects. The other day, I dusted it off for some solar imaging and found that (most likely) the circuit board had died. I decided to defork the OTA so I could use it with the iOptron mounts.
As I've already deforked it, I can't provide pix of the process, but I can answer the question both asked: where are the screws you need to remove to release the OTA? Here's the pic you need:
In the above image, the OTA would be vertical and the screws you need to remove fit in the four holes you can clearly see in this image. You'll need an Allen wrench (not sure what size, I just have one that fits!). After you have removed them all, simply pull the OTA forwards to remove it; it should slide out pretty easily.
I mounted my OTA on a dovetail. As the dovetail was not designed for the ETX 90 OTA, I drilled a couple of extra holes into it. The holes should align with the brass, threaded screw "sleeves" in the oblong block at the back of the OTA.
Here's a pic:
All-in-all, it was a 10-minute job and the 90 is now usable again (see my last blog entry with the solar image).