UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Gemini 80-Bus System

jncoleman · 12th Jan 2020, 3:11 pm

John,
I would start by looking at all the signals around the Z80 chip. Do the strobes (MREQ, IOREQ, RD, WR) look sensible? WAIT, INT, NMI, BUSREQ should all be be high. Are the power terminals OK? If this is all correct, then are the memory strobes getting to the ROM? What are its OE and CE doing? I've only just glanced at the 813 circuit diagram but I see that the 2732 CE is coming from an involved looking arrangement controlled by the high-order addresses and an RS bistable, itself controlled by reset. If CE isn't going active, then I'd trace through that next. I assume you've got all the data sheets so you can see what the waveforms should look like. My own 813 / SVC set doesn't work either, although in my case I'm getting correct outputs at the 813 edge connector, so I'm suspecting the SVC. I'll be interested to hear how you get on.
Nick.

12th Jan 2020, 3:11 pm	#305
jncoleman Triode Join Date: Oct 2019 Location: Carlisle, Cumberland, UK. Posts: 11	Re: Gemini 80-Bus System John, I would start by looking at all the signals around the Z80 chip. Do the strobes (MREQ, IOREQ, RD, WR) look sensible? WAIT, INT, NMI, BUSREQ should all be be high. Are the power terminals OK? If this is all correct, then are the memory strobes getting to the ROM? What are its OE and CE doing? I've only just glanced at the 813 circuit diagram but I see that the 2732 CE is coming from an involved looking arrangement controlled by the high-order addresses and an RS bistable, itself controlled by reset. If CE isn't going active, then I'd trace through that next. I assume you've got all the data sheets so you can see what the waveforms should look like. My own 813 / SVC set doesn't work either, although in my case I'm getting correct outputs at the 813 edge connector, so I'm suspecting the SVC. I'll be interested to hear how you get on. Nick.