HP 9825 Update (Patreon)

Ken and I gave some attention to our neglected HP 9825, which had been displaced by Apollo stuff. The next board to repair is the keyboard. We know the reset is not working, neither the direct signal from the key, nor can the processor read the correct status byte through the custom KDP chip, which is a bit concerning. 

I decided to power only the 5V section to check the TTL (the tape drive, the printer and the KDP chip take a whole bunch of other weird voltages). And right off, it tripped the supply.

I still don't have a thermal camera, I really should get one. In the meantime we used our highly trained fingers. That S00 was really hot.

No wonder, it had a melted pin that even left a burn mark on the PCB. Which might be a good thing, that short might have protected the other circuits.

So we replaced our 74S00. And surprise! The board still trips the current limit! But not by as much anymore, we are about 220 mA over our "good" reference keyboard.

More use of the magic fingers revealed that two LS04 inverter chips were suspiciously warm. Not toasty, but warm, which is not normal for a low power LS series TTL.

But surprise again! Both tested good on the Tauntek:

And the Tauntek told us that they were only drawing 4.9 mA each. So definitely they were not the cause of the problem, but the victims of it.

The 74LS04 outputs were connected to the proprietary KDP chip and a fairly unusual 7425 TTL chip. So unusual, that neither the Tauntek nor the TL866 had it in their databases. However just taking the 7425 off the board was enough to bring the 5V rail back within normal power consumption, with our 74LS04s now returned to their cool and composed selves. So the 7425 was the root cause, even though it was not hot itself - too good of a short to dissipate anything probably.  

With the 74S00 replaced and the 7425 gone, the board did not trip the supply anymore. So finally, we test what we set out to do, the reset signal. After more complicated adventures that I'll have to relate in the video, we got our reset signal back, the yellow pulse here:

We did not have a SN7425 in any of our collections, nor did they have them at Anchor, nor at Excess Solutions - it is really that unusual! I'll have to order a new one before we can tell if the board is repaired. The board has plenty of other stuff connected to it: the printer, the display itself, the tape, so I am not quite convinced yet that our adventures with that board are over.

In the meantime, as you'll see, we'll have plenty to keep us busy.

Marc