by tezza » Thu Jul 21, 2011 10:30 am
Hi Rick,
Some ROM sockets in old machines can sometimes be empty and are used for “optional extras”. The Commodore PET has sockets like this. So just make sure they are really necessary. I guess the manual can tell you that.
If it is a necessary ROM, there are three options. Either seek a junk machine you can extract it from, seek a replacement ROM someone has as a spare part or seek/get/burn a replacement EPROM for the ROM. Given these machines are quite rare, neither of these things are easy. If you haven’t already, I’d engage with the HP85 community through that Google group. Someone might have a spare or they might be able to burn an EPROM for you. Often an EPROM adaptor also needs to be made as easy-to-get EPROMS are not directly compatible. I have an EPROM programmer and might be able to help but you'd need to find a the ROM binary image and information on what type of EPROM is required as a replacement for the ROM (and details of an adaptor if needed)
However, the erratic symptoms you describe don’t seem to be a ROM issue. Even if a required ROM is missing, the buzzing and flaky nature of the display would suggest some other issue also. If it was just a ROM the symptoms would be steady. My approach would be to sort these other issues out first (a steady consistant display, no buzzing and correct voltages). Ensuring the voltages on the board and the VDU are correct are an essential first step in diagnosis. Otherwise it's like someone trying to improve the steering on a car by tinkering with the alignment when the car has four flat tyres.