exxos's Atari, Amiga & retro forum

rubber_jonnie wrote: 05 Apr 2022 16:06

stephen_usher wrote: 05 Apr 2022 15:54 It's a pity that the termination didn't help.

Given the frequencies of the interference the power is highly unlikely to be the source.

I doubt it's the power too, but since removing the socket is trivial I may as well try.

Removing the socket made no difference. Grrrr!!

Having a look at the picture that you took of the board, what is the ribbon cable for? And where do the wires at the rear of the board go?

What happens if you move the video socket away from the board at the rear and pass them to the front of the unit?

stephen_usher wrote: 05 Apr 2022 16:55 Having a look at the picture that you took of the board, what is the ribbon cable for? And where do the wires at the rear of the board go?

What happens if you move the video socket away from the board at the rear and pass them to the front of the unit?

The board with the Ribbon cable is the Ultimate 1MB. I have tried moving it away as there is a reasonable amount of give on the ribbon cables, plus now I've removed the video out socket the cables are well away and leading to the front.

I may try and drop the OS ROM and MMU back in see if having no U1MB makes a difference, but I don't think it will.

No difference.

Where is this board getting its clock from? Could there be a clock mismatch problem?

stephen_usher wrote: 05 Apr 2022 17:58 Where is this board getting its clock from? Could there be a clock mismatch problem?

The clock is derived from A 74LS74 (U21 in the 800XL, U24 in the 600XL. There is only a single LS74, so there is only one spot to tap from.

The circuits is a bit of a faff though. There is 4.433618 Mhz crystal connected to the LS74, but it also seems connected to another circuit that contains a 3.546894 Mhz crystal and that pin is the one that is tapped for what the installation instructions say is '3.5 Mhz'. That seems to be the clock the 74LS74 uses.

I'll check all the frequencies of the crystals, see what they say. I could potentially disconnect the clock and feed it from my signal generator, see if it works any differently.

There will have to be ceramic caps next to the crystals to cause the resonance. Might be an idea to check their values and what they should be according to the schematic.

rubber_jonnie wrote: 05 Apr 2022 23:13 The circuits is a bit of a faff though. There is 4.433618 Mhz crystal connected to the LS74, but it also seems connected to another circuit that contains a 3.546894 Mhz crystal and that pin is the one that is tapped for what the installation instructions say is '3.5 Mhz'. That seems to be the clock the 74LS74 uses.

The second crystal at 4.43 MHz is present in PAL computers only. It provides the PAL video color carrier frequency. Both clocks are connected like a PLL, not much different than ST systems with modulators that also have two oscillators.

ijor wrote: 06 Apr 2022 04:20

rubber_jonnie wrote: 05 Apr 2022 23:13 The circuits is a bit of a faff though. There is 4.433618 Mhz crystal connected to the LS74, but it also seems connected to another circuit that contains a 3.546894 Mhz crystal and that pin is the one that is tapped for what the installation instructions say is '3.5 Mhz'. That seems to be the clock the 74LS74 uses.

The second crystal at 4.43 MHz is present in PAL computers only. It provides the PAL video color carrier frequency. Both clocks are connected like a PLL, not much different than ST systems with modulators that also have two oscillators.

Cool, thanks.

stephen_usher wrote: 05 Apr 2022 23:49 There will have to be ceramic caps next to the crystals to cause the resonance. Might be an idea to check their values and what they should be according to the schematic.

Yep, will get them checked out.

In the meantime, the heatsinks for the VIC-20 just turned up, so once the new VIC chip turns up I can get them in and fingers crossed no overheating.

The VIC was posted from the Netherlands on the 31st March, so a week in transit now, but sadly no tracking. Hopefully it'll get here soon.