HP-150 MS-DOS Computer

I am not sure quite what you mean here. Do yuo mean that all characters (other than space) appear as solid blocks? Boy do you need the schmatics! I've looked at the 150 'Video Alpha Display Sybsystem' and the overall design is what you'd expect, but the details are odd. And I susepct it's one of those odd bits that's causing the problems OK, the basic dsegin is a CRT controller -- here an SMC9007 (U315) which addreeses the video RAM. The output of the video RAM is 16 bits wide (character and attrboutes), the character part -- 10 bits of it -- goes to the address lines of a character generator ROM U512. The output of that goes to a shift register (U511 and U612, 'S195) which does the obvious dot serialisation. The attribute logic is compiclated, but based round a 16L6 HAL (mask programmed PAL) U314. I do _not_ have the PAL equatiuons. The outputs of that are latced (U614, '174) and feed the 'Dot Stream Mixer', a 'S153 mux which combines the alpha dots and the graphics system dots, and which then produses the Full Brightness and Half Brightness signals to the 'Sweep' (monitor) PCB. The complicated bit is round those shift registers I mentioned. There's some logic to, I think, make the line-drawing characters touch on-screen. This is controlled bu U46b ('S74) and is based rounf U78b ('S112) and U713b and a ('S09),, U712d ('LS00) and U610a ('S32) There's even a couple of gates between the 2 shift register ICs, but I think if that was the problem you'd get half of each character displayed correctly. I think I'd start by looking at pin 3 of U713. That's the alpha dot stream going into the mixing logic. What you're looking for is fine enough pulses to be character dots, rather htan complete character cells. If You've got that, suspect a problem round U715. If not (and I think you won't have it), go back to U78b, etc. Oh, I quited like Nicolet and LeCroy :-). Not that I own any of their instrumnets, alas.. Exactly. Well, it could also have been a 5243L, which is the 10MHz version (the 5245 being 50MHz). I have one of each. Actually, a fair number of the PCBs are common to both instruemtns. I also have some, but certianly not all, of the plug-ins Alas I have to 'make do' with the internal crystal. I am told that HP sold a rubidium beam (sub)standard that was in a 19" rack module, and which you simply cabled up to the external oscillator input on the 5245L. I don't hjave it, thouhg. That's the problem. Unless you have a Rb or CS beam reference, it's almost impossible to calibrate the crystal, and the crystal can and will drift slightly over time. Not enough to bother me (I am darn sure the instruemnt is more accurate than any modern thing that I could afford), but still. -tony