After some searching I found the problem in #KramerMC. The #DS8286 on the cpu board gets in the way. In addition, it is installed incorrectly because the inputs and outputs are bridged. When I leave the socket empty it runs fine. Wonder if the computer ever worked the way it was intended.
Since I still had two of the plug-in contacts for the bus system in stock, I'm starting to build a RAM expansion to 64k. With address decoding to spare out 0000-0fff (ROM 2732) and 0fc00-ffff (display RAM). Because of the simplicity I decided to use 64k static RAM.
I'm also curious if the buddy might bring a new CPU board today, I'll be surprised.
#RetroComputing #Z80 #U880 #FormerGDR