HP 71b repair advice - won't power on with batteries or on AC adaptor

[KimH]

(07-14-2021 12:41 AM)teenix Wrote:  I fiddled with it again this morning and checked the waveforms with my oscilloscope. They matched all the service manual pictures including the 32KHz clock signal so I thought the display or drivers were damaged.

To my surprise when I looked at the display it was working. Why this is so - not sure and nothing obvious, but it seems to be ok now and still going after reassembly.

One thing I did notice when looking at the 2.5Mhz clock signal into the CPU it was working when the ON switch was down, then stopped when the ON button was released.

If I remove the power completely and discharge the supply rails, then turn it back on the clock signal stays on all the time with "Memory Lost" on display.

All the keys appear to work including [ON] and [f] [OFF].

I'm not familiar with the 71 operation, so hopefully it will be ok. They must have had big shirt pockets back then for this beastie :-)

cheers

Tony

That's great news - congratulations on your first repair of the TITAN

The behavior of the clock running when you press a button (ON in this case) is normal. You may know this, the CPU has 3 states of power - DeepSleep (OFF), Idle (waiting for input, display on) and Running (well, when it actually does something).

Might be too much information, but heck - a LEX-Program called KEYWAIT manages to have the CPU go into Idle when a/your program is pausing and waiting for input "Enter Temp:" for example. Without the KEYWAIT function the CPU would gobble power while waiting, nifty little detail which ought to have been part of the original design.

The fact that Memory Lost has the CPU running is a bit odd, it maybe be doing something in the background for a few seconds then go idle. I will try this next time I have some time.

BTW - didn't Darren have 2 machines