My customer won this working unit on Ebay, and agreed to pick it up from the seller. Between the end of the sale, and it being picked up the seller decided to get his tech to replace the batteries as they were leaking. Following this it would partly boot but hang, not good! Thankfully a heavy discount was negotiated to cover repairs, and in it came to my shop.
I have not worked on one before, and I was amazed to find it is based on the first IBM PC, ISA bus and all. This one had a lot of Forat mods, including the SRAM256 card which I have read is a favorite for folks to blow up by inserting backwards. I checked the batteries that had been fitted, and was only getting 2.4v, and one read shorted. I purchased new lithium batteries and a charger which confirmed that one was indeed shorted. New batteries were fitted. Powering on found the screen blank, and this was traced to the CPU being stuck in reset. The transistor between the batteries and this line was bad, and replacing it released the reset. Checking the CPU clock found it running less than 1/10 correct speed. The osc chip SN74LS04 and its socket were badly corroded by the battery acid; parts replaced and clock working correctly.
We were back to it hanging, but things were now solid as the battery/acid damage was out of the way. Address line A6 was found to be low amplitude, and the SRAM256 card was dragging it low. Removing the card put the 9000 into a mode where it was looking for the SRAM 256 card (Forat firmware and card), so it would not come up. From the schematic, -12v would go to A6 on the SRAM card if it were reversed (surprise!). I traced the shorted A6 line to a PAL on the SRAM256 card, and socketed the chip. Lifting the PALs leg enabled the card to be plugged back in. Bruce Forat was contacted for parts, but by the time he came back with a price I was well into reverse engineering the card fitted detection in the PAL using a logic analyzer and 8080 disassembler. Clearly the address decoding part of the PAL was still good as after zapping the code that looks for the card everything came up. The customer wanted to go this route as he wanted to see what else was bad on the 9000 (i.e. was it a write-off) before spending more on it. As the PAL is socketed he can eventually order the PAL from Bruce, and fit it along with the original firmware eprom.
Testing showed the floppy controller was bad, and a chip change fixed that (did the cable get reversed at some point?). Last thing was to replace the screen with a more modern high contrast led back-lit one. Customer sent me a YouTube video of it in action that night, he is delighted!