Like many before me, I have a poorly issue 4 board that I hope to resurrect.
In a previous life this was heavily modified with piggybacked RAM, selectable ROMs and what I believe was a screen noise cancelling circuit. I have stripped off the homebrew changes and reverted to a minimal 2K configuration with ICs that are known to be good. This is an approach that served me well when fixing an issue 5 board.
The first problem appears to be that neither the video or main clock are running at the correct frequencies. The main clock seems to be running at ~1.5KHz. I've done the obvious things, checked power, connections, tried a couple of different LS04s in IC45, removed IC44, changed the crystal. All no dice.
I'm concerned that the noise cancelling might have had some changes to the clock circuit, although I cannot see any track cuts, and it is not obvious to me why it would have.
I'm using the schematic here for reference:
I assume it will be applicable to both the issue 4 and issue 5 boards as far as clock generation is concerned (it seems to be).
Has anyone else seen similar behaviour? Any pointers would be gratefully received.
discussion of games, software, hardware & emulators relating to the Acorn Atom
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1024MAK wrote:How (as in what test gear) are you measuring the clock frequency?
And just as important, at which connection / pin are you measuring it?
Thanks for the tip, looks like I had a combination dead IC44 & an attack of analogue numptiness. I'm testing with a cheap USB logic analyser, and it did not occur to me that the clock circuits would need appropriate loads to resonate at the correct frequency (at least I assume that is what is occurring). So with a working LS393 in IC44 and examining the clock on pin 10 of that, rather than anywhere associated with IC45 I can see 1MHz. I'm guessing this is also why I'm seeing wonky frequencies on IC9, my observations are upsetting the experiment. As the video clock feeds directly into the 6847, I don't believe I can observe it directly. However, looking at *HS and *FS on pins 38 and 37 they look to be clocking at 15.7KHz and 59.9Hz respectively so I think all is now well on the clock front.
Right, onward. What is not working next...
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