HP-150 MS-DOS Computer

One quick question. You replaced the real-time-clock battery, didn't you. IIRC, that's physically on the video PCB. Is it possible the video PCB is not correctly seated in the backplane (well 'frontplane' in the HP manual) MAybe. But then I've worked on many HP98x0 machines. All had the same fault -- no display. And yet it wasn't a common fault, or even always on the same PCB. I've had defecty carry FFs, dead shift registers in the M (memroy address) register, dead flip-flops on the I/O PCB, a cracked clock crystal (!), and so on. That's when you need scheamtics// 'Lucky Dip' sericing may work most of the time, and it may save time when it does, but IMHO three is _no_ substitute for actually tracing the fault and curing it. Particularly in your own classic computers (where there isn't the pressure to get the thing runing again by yesterday) One side of the 'S153 drives the half-bright siganl, the other side drives the full-bright signal. They have common inputs and (of course) select lines. Which half is enabled is determined by the intensify line (the 2 eneables to th 'S153 are linked to a 'S04 inverter so that only one half is enabled at a time -- either full bright or half bright). When the machine is working correclty, the half-bright and full-bright signals can therefore never occur simultaneously. I have no idea what the video board shows if they are both asserted at the same time. Yes, but then why do spaces look different from all other characters? [Rb beam frequncy standards] Sure. But it was my impression that if you got a signal from a beam standard, and if the circuitry was 'locked' (rather like a PLL locking), then it was on-frequency. In other words, if it was working (and there were ways to tall if it was locked), then it was accurate. I can beleive there are adjustments to get it to lock. But I am suprised there's one that controls the output frequency. -tony