exxos's Atari, Amiga & retro forum

If you have an 8MHz crystal then you can take U79 out of circuit and connect pin 5 to pin 8.

stephen_usher wrote: 28 Jan 2024 21:21 If you have an 8MHz crystal then you can take U79 out of circuit and connect pin 5 to pin 8.

Sadly I don't, though I could as a test use my signal generator.

Need to think about it.

Re-reading your diagnostics. Have you checked that pin 4 of U79 is tied to ground and not floating? That gate only acting as a delay but if pin 4 isn't low then pin 6 will never be active.

P.S. If this is OK, try a pull-up on pin 6 as the chip's line driver may be faulty to it can't power the line but can pull it down so the chip tester will show it's OK.

stephen_usher wrote: 29 Jan 2024 13:30 Re-reading your diagnostics. Have you checked that pin 4 of U79 is tied to ground and not floating? That gate only acting as a delay but if pin 4 isn't low then pin 6 will never be active.

P.S. If this is OK, try a pull-up on pin 6 as the chip's line driver may be faulty to it can't power the line but can pull it down so the chip tester will show it's OK.

I checked that this morning before work and it is.

All I see on pin 6 of U79 is 5v and pin 5 has 4Mhz to it, pin 4 is to ground. I lifted pin 6 to see if it was something on the board, but I still get 5v on pin 6 even with it lifted.

Bother. You could link pin 6 (of the socket) to pin 5 (of the socket), as the gate is only acting as a delay, and see what happens.

stephen_usher wrote: 29 Jan 2024 15:33 Bother. You could link pin 6 (of the socket) to pin 5 (of the socket), as the gate is only acting as a delay, and see what happens.

True, might give that a go later if I have time :)

rubber_jonnie wrote: 29 Jan 2024 16:54

stephen_usher wrote: 29 Jan 2024 15:33 Bother. You could link pin 6 (of the socket) to pin 5 (of the socket), as the gate is only acting as a delay, and see what happens.

True, might give that a go later if I have time :)

That didn't work. I had a reply over on OSIWeb that was very helpful here: OSIWeb to which I replied as below.

"I swapped out U58 this morning, I had a 7400 in there as per the BoM, but dropped in a 74S00 instead and started to get 8Mhz on pin 8 of U79, however the clock is really unstable, flashing up and down in frequency. I have changed C57 and C55, though not the crystal, though scoping that does seem to show it running consistently at 4Mhz. I'm certainly getting consistent values at U58, ad though the voltages are a bit low, what's coming out of pin 8 is good, and what's coming out of U29 pin is good voltage wise, even if the frequency out of pin 8 is only 4Mhz.

But U79 is weird. Sometimes I get 8Mhz at pin 8 as described bit sometime 4Mhz and sometimes its flapping between those values, and that's then reflected up the line.

I have new LS86s coming, but worried about the overall instabilities I'm seeing in the clock circuit.

Sometimes, and I'm assuming it's because the clock gets somewhere near where it should be I get a sort of video signal, but until the clock is stable, which it isn't ATM, all bets are off."

At least I am sure of what clocks should be where now, even if they're not at the moment.

Stupid clock...

I'm guessing that the original design relied upon the timing of a specific manufacturer's TTL logic chip, possibly even production run, for timing and the original clock circuit only barely worked because of this. This is probably why when the design was translated into the UK101 they changed to using an 8MHz crystal and simplified the circuit.

I wonder if when the design was first made 8MHz crystals were hard to come by but by 1979 (UK101) they were cheap enough and plentiful enough.

I would very much suggest changing the crystal to 8MHz and removing U79 entirely, jumpering between it's input and output.

stephen_usher wrote: 30 Jan 2024 12:22 I'm guessing that the original design relied upon the timing of a specific manufacturer's TTL logic chip, possibly even production run, for timing and the original clock circuit only barely worked because of this. This is probably why when the design was translated into the UK101 they changed to using an 8MHz crystal and simplified the circuit.

I wonder if when the design was first made 8MHz crystals were hard to come by but by 1979 (UK101) they were cheap enough and plentiful enough.

I would very much suggest changing the crystal to 8MHz and removing U79 entirely, jumpering between it's input and output.

Yes it's certainly possible that's the case.

I'm going to give it a go and have just ordered the necessary xtal.

I have been trying with my signal generator, but it's all a bit weird.

I could just change it to be like the UK-101 and take pin 3 of U58 straight to pin 1 of IC 29 with an 8Mhz xtal rather than removing and bridging at U79.

Well, the new LS86's arrived and it's still the same, nothing at pin 6 of U79, so only 4Mhz at pin 8. 4Mhz at all the places it should be.

It has 'improved' in that the output from pin 8 of U79 sometimes gets up to 6Mhz, but it's swinging back and forth to 4Mhz and ultimately the clocks are wrong overall.

Fingers crossed the 8Mhz crystal will come today and I can try running a bypass.

<sighs>

EDIT: Posties been, no crystal, just a letter from Barclaycard...