No video output, could use some advice
Posted: Fri Feb 19, 2021 2:01 pm
Hey team!
So I've been playing around a bunch with my zeddy in order to get it back to health and then beyond. My goal with this thing is really to make it useful to me and as a learning device.
So far I've built a power supply for it, done an RF->composite mod and built a new keyboard. This was all working great up until I did *something* during the removal and reinsertion of the ROM that now makes it not display anything. I don't actually have a display that natively does composite, so I've been leaning on a cheap AV to HDMI adapter that I have which is a little finnicky about signal, so it's entirely possible I'd be getting a white display on a CRT.
I've already looked at a few other posts for similar problems so I've got quite the body of diagnostic information already, so I'm going to present all of that here and now. I have a 2 channel 200MHz scope, a LogicPirate logic analyzer, cheap digital multimeter and a TL866ii ROM programmer available for any further debugging. I'm not super experienced with electronics so feel free to explain basic things to me - I won't take it personally
Essentially I would like to know what should I test next and why? This is all part of the learning exercise for me I'm happy to post scope traces, more pictures, whatever!
Photos
Top Bottom Keyboard (just for kicks) Power
Getting the right ~10V/~5V split on the 7805
ROM
Having discovered the problem *after* I removed and reinserted the ROM, I originally thought this was the problem so I built an adhoc 2364 -> 2764 adapter and dumped the ROM. It's a perfectly intact copy of the revision 1 550 ROM which I have checked with a few different methods (checksum, diffing the disasm, booting in an emulator) so I'm actually pretty confident the ROM removal is a red herring.
RF out
Tested at the end of the RCA cable, I'm getting a steady "square" wave which has a frequency of ~15.6KHz and a period of ~64us with a ~5us floor, which I believe is consistent with a correct composite signal for an all-white screen.
16 on the ULA is very similar - perhaps suspiciously so given that there's the RF -> composite adapter in between the video pin on the ULA and the actual cable.
My AV adapter is defaulting to a blue screen however, and not displaying anything. It's not the adapter as I successfully tested it with my BBC.
ULA
I've only tested the clock input and output on this and they look just fine - 6ish MHz in on 35 and 3.24MHz out on 14.
RAM
Doesn't boot on internal 1K or external 16K rampack. Both were working in the before-time, but I absolutley wouldn't exclude the possibility that I've done the same stupid thing twice and fried both.
Z80
This is where I've got the best indications that somethings not right
16/INT: Pulsing
17/NMI: High (~5V), should be pulsing
18/HALT: Low (~0V), should be pulsing
19/MREQ: Pulsing (~1.7MHz)
21/RD: Pulsing (~800KHz)
22/WR: Pulsing (mostly high)
24/WAIT: High, should be pulsing
25/BUSRQ: High
26/RESET: High
27/MI: Pulsing
28/RFSH: Pulsing
I additionally tested A14 and A15 which I believe are supposed to have some action on them (sorry, I'm basically just transcribing my notes) and they're low (~38mV which I'm going to go ahead and call ~0V).
So I've been playing around a bunch with my zeddy in order to get it back to health and then beyond. My goal with this thing is really to make it useful to me and as a learning device.
So far I've built a power supply for it, done an RF->composite mod and built a new keyboard. This was all working great up until I did *something* during the removal and reinsertion of the ROM that now makes it not display anything. I don't actually have a display that natively does composite, so I've been leaning on a cheap AV to HDMI adapter that I have which is a little finnicky about signal, so it's entirely possible I'd be getting a white display on a CRT.
I've already looked at a few other posts for similar problems so I've got quite the body of diagnostic information already, so I'm going to present all of that here and now. I have a 2 channel 200MHz scope, a LogicPirate logic analyzer, cheap digital multimeter and a TL866ii ROM programmer available for any further debugging. I'm not super experienced with electronics so feel free to explain basic things to me - I won't take it personally
Essentially I would like to know what should I test next and why? This is all part of the learning exercise for me I'm happy to post scope traces, more pictures, whatever!
Photos
Top Bottom Keyboard (just for kicks) Power
Getting the right ~10V/~5V split on the 7805
ROM
Having discovered the problem *after* I removed and reinserted the ROM, I originally thought this was the problem so I built an adhoc 2364 -> 2764 adapter and dumped the ROM. It's a perfectly intact copy of the revision 1 550 ROM which I have checked with a few different methods (checksum, diffing the disasm, booting in an emulator) so I'm actually pretty confident the ROM removal is a red herring.
RF out
Tested at the end of the RCA cable, I'm getting a steady "square" wave which has a frequency of ~15.6KHz and a period of ~64us with a ~5us floor, which I believe is consistent with a correct composite signal for an all-white screen.
16 on the ULA is very similar - perhaps suspiciously so given that there's the RF -> composite adapter in between the video pin on the ULA and the actual cable.
My AV adapter is defaulting to a blue screen however, and not displaying anything. It's not the adapter as I successfully tested it with my BBC.
ULA
I've only tested the clock input and output on this and they look just fine - 6ish MHz in on 35 and 3.24MHz out on 14.
RAM
Doesn't boot on internal 1K or external 16K rampack. Both were working in the before-time, but I absolutley wouldn't exclude the possibility that I've done the same stupid thing twice and fried both.
Z80
This is where I've got the best indications that somethings not right
16/INT: Pulsing
17/NMI: High (~5V), should be pulsing
18/HALT: Low (~0V), should be pulsing
19/MREQ: Pulsing (~1.7MHz)
21/RD: Pulsing (~800KHz)
22/WR: Pulsing (mostly high)
24/WAIT: High, should be pulsing
25/BUSRQ: High
26/RESET: High
27/MI: Pulsing
28/RFSH: Pulsing
I additionally tested A14 and A15 which I believe are supposed to have some action on them (sorry, I'm basically just transcribing my notes) and they're low (~38mV which I'm going to go ahead and call ~0V).