Power Ripple in DEC BA11-K

It is a very useful tool, sure (I use one all the time), provided that (as with all test gear) you realise what its limitations are. So was I, didn't stop me getting caught though... Let me expalin. The machine in question was an HP9820 (as mentioned in my otehr message). Now, with thse machines, I start by checkign the power supplies (all rails fine), then the clock board (both the bit clock and the microcode clock were running), and then I put enough of the machine togethr for it to run (that is all 4 CPU board,s the memory unit and the display board) and see what it's doing. Normally I strt with quick tests to look for 'sillies', if that doesn't find the fualt I usea full-blown logic analyser to trace the CPU microcode and see jsut what it is doing. Anyway, as I mentioned in my previous message, it's bit-serial. There's a 16 bit shift register claled the 'M register' which does a conversion are shifted into the MSB end of this register (as you'd expect), and after 16 clocks, there's a new address o nthe address bus. The outputs of the M regiser are brouight out to pins on a test conenctor on top of the memory box backplane, and I ran my logic probe over them. This showed that the top 4 bits were all chaning, the bottl 12 bits were all stuck low. Now, the M register is made up of 4 cascaded 4 bit shift register chips. I decided based on taht that the chip handling bits 11..8 had failed so that its ouptus were all stuck low . Therefroe the lower-order chips were being laded with 0's all the time. So I desoldered the appropriate chip on the memroy address PCB and replaced it. No change. Not just 'still nothing on the display', but the M register outputs on the test connector were still doing the same things -- top 4 changing, bottom 12 all 0's Then I useda logic analyser. I found that yes, the top 4 bits were changing, but the timing was craxzy, they were not just chaning on the correct clock edge. It appeared they were not being latched at all. So in fact the next shift register in the chain -- the one I'd replaced, always had a 0 at its serial input when it was shifting. Hence its ouptus were always 0 Out with the highest shift register chip in the M rwegister, and I replaced it with the one I'd taken out from the next position and which I guessed was good. It was. This time I got the correct display. If I'd used a logic anaylser at the start I'd have found the fault first time. -tony