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jimbob · 28 July 2012, 20:27

Thanks for the tips Khyron. After a closer look at the schematic I see what you mean about the DACS. U31 and U32 just buffer the outputs of Denise into binary weighted resistors then a summing amplifier. I had thought the chips were the DACs after reading in another thread about troublesome DACs.

This prompted me to suspect U31 or U32 even more strongly since if the buffer had failed somehow, this could cause excessive current draw from Denise explaining the heat problem. Because of the heat I was reluctant to power it up long enough to probe about so I just used my trusty multimeters continuity/diode tester and found what I'm pretty sure is a failed input on pin 2 of U32. Straight away, this will mess up the MSB of the red signal and the excessive current draw and heat probably messes everything else up too.

These buffers are 74HCT244. I think I can just replace them with any standard CMOS equivalent. I'm not quite sure if they are inverting or non-inverting buffers though, Anyone?

@altcomputing, thanks for the offer but I'm pretty hopeful denise itself is OK now, see above. I also thought the video encoder has nothing to do with RGB output but it's not true. U12 does provide analogue RGB to the 23pin port. The digital RGB signals taken from the buffer outputs are not used by standard monitors, I think they're compatible only with some old style 8 colour type monitors since only the MSB from each colour is used.

28 July 2012, 20:27	#4
jimbob Registered User Join Date: May 2006 Location: Kilmacolm Age: 46 Posts: 632	Thanks for the tips Khyron. After a closer look at the schematic I see what you mean about the DACS. U31 and U32 just buffer the outputs of Denise into binary weighted resistors then a summing amplifier. I had thought the chips were the DACs after reading in another thread about troublesome DACs. This prompted me to suspect U31 or U32 even more strongly since if the buffer had failed somehow, this could cause excessive current draw from Denise explaining the heat problem. Because of the heat I was reluctant to power it up long enough to probe about so I just used my trusty multimeters continuity/diode tester and found what I'm pretty sure is a failed input on pin 2 of U32. Straight away, this will mess up the MSB of the red signal and the excessive current draw and heat probably messes everything else up too. These buffers are 74HCT244. I think I can just replace them with any standard CMOS equivalent. I'm not quite sure if they are inverting or non-inverting buffers though, Anyone? @altcomputing, thanks for the offer but I'm pretty hopeful denise itself is OK now, see above. I also thought the video encoder has nothing to do with RGB output but it's not true. U12 does provide analogue RGB to the 23pin port. The digital RGB signals taken from the buffer outputs are not used by standard monitors, I think they're compatible only with some old style 8 colour type monitors since only the MSB from each colour is used.