The Losmandy 492 Digital Drive dates from, wait for it, April 1992. Yup 4/92. Ah, where were those creative marketers in 1992? In the late '80s Celestron commissioned Tangent, which provided the Advanced Astro Master digital setting circles controller to come up with an accurate battery operated tracking system for the upcoming Ultima 8. The new unit was hoped to replace the rather expensive and sort of make shift adaptation of the Vixen DD2 used in the PowerStar mounts. In 1990 the Ultima 8 belatedly got its Tangent system.
By this time plans were afoot for the C11 to be sold as a package with a new German mount being developed by Losmandy (a/k/a Hollywood General Machining). This package was introduced in 1992 as the G11. It was equipped with the 492 system which was an adaptation of the Ultima drive. While the Ultima was built around a closed loop servomotor system an open loop step motor system was selected for the G11. Given the higher torque requirements this is not surprising. The 492 was later adapted for use in the Losmandy GM-8 mount. Until recently still available as the standard drive system for the G11 and GM-8. It is currently not available from Losmandy.
So what to do when your GM-8 492 gives up? Well G11 owners who upgrade their drives to a goto system can be a source. But there is a small problem if you are looking for a GM-8 system and there are only G11s available. I was recently faced with is and here's how it was easily overcome with the help of Mike Herman, who hangs out here and at the Google Losmandy group.
The major stumbling block between the GM-8 and G11 492s is the that GM-8 has a 180:1 worm and the G11 360:1. Therefore, the step pulse timing has to be different. After talking with Mike the solution seemed inexpensive and fairly easy. All that was required was a 6MHz crystal for the microcontroller's clock and a EPROM flashed with the GM-8's control parameters, which Mike was able to provide. The EPROM is a 27C256 and dare I say this is a Classic memory design? It was a development of the first EPROM invented in 1972. With a capacity of 256K bits it can store 32K bytes. EPROMs are programmed at high voltages (usually 28V) and are erased by shining high energy UV light at them through a window. so it's fairly easy to pick out the 27C256 in the photo of the 492 controller, just look for the window.
Fortunately, the EPROM is not soldered to the circuit so removal and replacement is easy. The the G11's 12MHz has to be removed. and it is soldered to the circuit board. I removed it by heating the points under the board and pulled it out. Rather than attempting to reuse the through holes that involves some risk to damaging them and the printed traces they are connected to, I used a surface mounted replacement crystal. This was soldered directly to the legs of its capacitors. And there you have is a GM-8 controller.