Cromemco D+7A weirdness
Cromemco D+7A weirdnessAll --
I’ve been on-and-off trying to get my D+7A board working satisfactorily but I’m having some challenges with the D/A side of the board. The A/D and digital input/output ports seem to work fine, but the D/A side of it won’t calibrate. The D/A isn’t too critical but it is needed for the sound output from Dazzler games.
Basically, you use a little program to set the D/A to output 2.54v and you adjust the gain pot so your DMM shows 2.54v. Then, you set the output to 0 and use the zero pot to cause the DMM to show 0. What I’m getting is a slowly upward ramping voltage which stops at about 2v. When I program it for a 0v output, it essentially stays at the 2v level.
I checked all of the reference voltages on the board and they’re within tolerance. I’ve swapped all of the chips in the signal path (D/A, two op-amps and analog switch) with no difference in output, and I also made sure that I was getting the right bit pattern through the buffers to the D/A (same signal path as the digital output port, which works).
Very odd indeed. I don’t see any broken caps, resistors or the like in the signal path either, although I may put it under a magnifier next. If these symptoms ring a bell to anyone, let me know.
Collector of Classic Computers
Build Master and lead engineer, Altair32 Emulator