exxos's Atari, Amiga & retro forum

PhilC wrote: 25 Mar 2024 13:35 Well done. Complete nightmare with those logic chips.

I'll say. Just wondering if I powered on with the wrong voltage at some point, as an LS00 for the video circuit died this morning as well, and that is too many to be a coincidence.

Just need to jump this last flickery video keyboard issue and it should be good.

Been a bit of a marathon for sure.

rubber_jonnie wrote: 25 Mar 2024 15:39

PhilC wrote: 25 Mar 2024 13:35 Well done. Complete nightmare with those logic chips.

I'll say. Just wondering if I powered on with the wrong voltage at some point, as an LS00 for the video circuit died this morning as well, and that is too many to be a coincidence.

Just need to jump this last flickery video keyboard issue and it should be good.

Been a bit of a marathon for sure.

* I'm sure we've all done it " he says whilst thinking "oh yeah I blew up my video Adaptor last week, oops"

PhilC wrote: 25 Mar 2024 15:47

rubber_jonnie wrote: 25 Mar 2024 15:39

I'll say. Just wondering if I powered on with the wrong voltage at some point, as an LS00 for the video circuit died this morning as well, and that is too many to be a coincidence.

Just need to jump this last flickery video keyboard issue and it should be good.

Been a bit of a marathon for sure.

* I'm sure we've all done it " he says whilst thinking "oh yeah I blew up my video Adaptor last week, oops"

At some point I was using the bench PSU for powering on the PS2 Slim, which is 8.5v. Now I don't recall powering the OSI on with that voltage, but I suppose if I forgot to select 5v on memory 1 I could have easily done so if I wasn't paying attention.

Time to add a zener diode and a quick blow fuse to the power input?

stephen_usher wrote: 25 Mar 2024 15:54 Time to add a zener diode and a quick blow fuse to the power input?

Indeed, and potentially put a label on my bench PSU reminding me to check voltages before turning power on. There may already be a Zener in there, need to double check.

OK, so it's fixed.


The solution? I replaced the 74LS04 at U21 with a 74S04.

According to the build doc I have it should be an an LS, but as soon as I swapped it out, the problem went away. I then looked at other schematics and BoMs and all state an LS04.

I tried a bunch of LS04's and the results were always the same, fuzzy screen.

Now I need to test a few other things, like saving basic progs, but for now it's built and running.

Oh, so marginal timings... What fun.

stephen_usher wrote: 28 Mar 2024 14:59 Oh, so marginal timings... What fun.

Indeed...

So here is the list of things that caused me problems during my build.

Everybody will be different, and I'm happy to say I didn't have any shorts at all, but what I did have were some errors of my own making, and some that were baked in to the info I had in front of me.

1. Clock circuit problem 1 - I used a 74LS92 for U29. This is what the BoM said, though the schematic said it was a 74LS93. I followed the BoM. Fixed by using a 74LS93
2. Clock circuit problem 2 - I had inserted a 150nF cap at C65, it should have been 150pF and it was effectively filtering the clock signal.
3. Video circuit - I had a placement diagram that showed 3 holes for the position of C8, with the middle pin indicated, so I connected the right and middle pins. This is incorrect and should have been the left and right pins, ignoring the middle marked pin. This led to 15Khz on the vertical sync instead of 60Hz and a rolling image.
4. Fuzzy video - this was an odd one, but ultimately turned out to be the use of a 74LS04 at U21. I swapped for a 74S04 and the fuzziness went away, so clearly some sort of timing issue. This is a bit of a gotcha as the BoM and schematic agree it should be an LS part.
5. Initial memory location - My misinterpretation, but the 2 RAM chips used for initial testing need to go into sockets U31 & U45, not U38 & U52.
6. Shift lock - There are 3 apparent connections for the shift lock switch. The middle connection is irrelevant and the switch needs to connect the left and right pins only for shift lock to function correctly.

I think that's about it so far, but if I hit any more issues going forwards I'll extend this list.

Hopefully it may help others with issues doing this build.

I think the fuzzy video thing is due to the change in speed (and manufacturing process) between now and the 1970s. Indeed, I'm surprised that the BoM didn't state a specific manufacturer's part as the timings differed so greatly back in the day. There are many systems where the service manual states manufacturer and even down to build date for replacement parts.