OSI 600D Superboard Build

[rubber_jonnie]

Oh well. Shows that it wasn't the voltage level then.

Definitely worth a try.

I also pulled the sockets for the LS00 and LS86 to see if there were any shorts below, but nothing there. It's just that LS86 that gives zero output on pin 6.

Pin 5 has input no problem, and I have double checked that pin 4 is grounded.

So no 8Mhz.

I've also replaced C57 & C65 just in case, but no difference.

I did have the pull up on the LS00, and tried moving it to the LS86, but I still only get 4Mhz out of the LS86.

However, the 8MHz xtal just arrived, so I will try that, but it probably won't be until this evening due to work.

[stephen_usher]

I do hope that the 8MHz crystal and the simpler circuit fixes things.

From the look of the output on your telly it seems that there are things trying to be displayed.

[rubber_jonnie]

I do hope that the 8MHz crystal and the simpler circuit fixes things.

From the look of the output on your telly it seems that there are things trying to be displayed.

Me too. Surely by dispensing with the clock doubling trickery we can get 8MHz straight to the LS93.

It certainly does try to display stuff from time to time, so fingers crossed I can get the clocks working and it'll start to do a bit more.

[rubber_jonnie]

Well, this is the gift that keeps on giving...

I installed the 8MHz xtal and get a good 8Mhz signal at U79, pin 5. So I bridge the socket to allow U29, the LS93, to pickup that signal on pin 1, which it does just fine.

OK, so over to pin 11 where I'm expecting to see 1MHz, but I don't, and pin 12 where I'm expecting 500KHz there's nothing either.

The UK-101 clock is this:

UK101-clock.jpg (41.87 KiB) Viewed 1781 times

And the only difference I can see with the OSI circuit, apart from the LS86 for clock doubling, is the cap at C65 between pins 1 and 3 of the 7400 (S or LS can also be used AFAIK)

So I will try different versions of the 7400 and I will try lifting the cap.

But, I did swap the xtal back to 4MHz and get the same behaviour out of the LS93, so good input and no outputs, so also wondering if I should try and find some 7493's rather than the LS version.

[rubber_jonnie]

Oh.Boy> ARRRRRGGGGGGHHHHHH.

So I lifted one leg on C65 and guess what? The clock only started working as it should, and it turns out that it should have been a 150pF not a 150nF capacitor!!!

I'm not sure if it was my blunder, but I don't care because now I have good clocks I can trace the clock round the board and start figuring the rest out. I have some 150pF's coming but that was super annoying.

This is pin 8 of the LS86 after removing C65:

PXL_20240202_150858907.jpg (197.48 KiB) Viewed 1776 times

A good solid 8MHz rather than the 4MHz I was getting.

Here you can see the 8MHz at U29, the LS93 (Blue trace) and 1MHz at the CPU:

PXL_20240202_151959580.jpg (233.77 KiB) Viewed 1776 times

And in this one, the blue trace is the reset line coming high again after hitting break.

PXL_20240202_150853850.jpg (202 KiB) Viewed 1776 times

[stephen_usher]

So, effectively you had a low pass filter on your clock!

Glad you now have a clock anyway. Do you get any output now?

[rubber_jonnie]

So, effectively you had a low pass filter on your clock!

Glad you now have a clock anyway. Do you get any output now?

Bloody annoying but at least I found it

No output yet but not had a chance to look further tonight. At least the clocks are good I can start to troubleshoot the rest.

[rubber_jonnie]

So not much time to sit down with the 600 today, as I have a few other things to do, but I went back and rechecked the CPU clocks which are still all good, though I will fit the correct value 150pF cap when it comes.

It seems that the video signal does now mix with the sync, but I'm not convinced I'm getting the correct video output.

Here is the 15KHz signal on it's own (Blue trace):

PXL_20240203_114822159.jpg (238.69 KiB) Viewed 1729 times

And here it is after adjusting the pot:

PXL_20240203_115842604.jpg (223.77 KiB) Viewed 1729 times

Yellow is just the clock at the CPU.

The closest machine I have to this is the TRS-80, another black and white machine, so I powered it on and scoped the video signal:

PXL_20240203_131556524.jpg (199.71 KiB) Viewed 1729 times

It looks similar, but I just don't think I have a fully formed video signal here.

I also double checked the CPU to see if the address and data lines were active at all, and as it goes, when I use break, I briefly see activity when the machine restarts, so it seems like it's trying to run.

I also got another blip on screen that looked positive, but by the time I got my phone, it had gone, so no photo.

So next I'll work through the video circuit, see what's going on. Ho hum...

[stephen_usher]

That looks like a signal to me, however, just as with the ZX81 there's no backporch so a colour TV (and modern LCDs) will not work with it.

Analogue colour TVs will have a very dark picture and modern LCDs will just see nothing at all.

You'll need a mono-CRT for it to work unless you can find something which will inject a backporch.

[rubber_jonnie]

That looks like a signal to me, however, just as with the ZX81 there's no backporch so a colour TV (and modern LCDs) will not work with it.

Analogue colour TVs will have a very dark picture and modern LCDs will just see nothing at all.

You'll need a mono-CRT for it to work unless you can find something which will inject a backporch.

Yes, I did note the TRS had a backporch, but was wondering if the lack of one on the 600 might be a problem, but I don't know enough to say yay or nay, so thanks for that, it's helpful.

I did also look at the signal on my MSX (It was handily next to me) and I could see the backporch and colourburst, hence why I went for the less complicated TRS.

So are you saying that with a signal like that, which is composite, it may not display on my Dell LCD that has composite in?

I mean that would make sense if the signal is good, or at least looks good from the perspective of the 'scope. Do I need to find myself a composite CRT? The only CRTs I have are the OPD display and an old TV, but the TV doesn't have a composite input.

IIRC the 600 can use a modulator, but I don't have the parts.