HP 54750 Digitizing Oscilloscope: a CARE package, and a scratched disc

Faster than expected I got the spare power supply from the US, it is in good shape, not dusty or anything. Well packaged.

First, I studied the circuit and the burned/unreadable diodes, because I will be trying to repair the defective supply later, just to have a spare. The diodes are 3.9 V and 11 V Zener diodes, fair enough.

Also dissected some special thermal fuse protected resistors, 22 Ohm, about 5 Watts, and a 130 degC thermal fuse in a ceramic package, an inrush current protector.

Probably going to replace these with discrete 22 Ohm resistors and thermal fuses. Actually, both the thermal fuses and the resistors were shot.

After fitting the power supply, some issues. The instrument starts up, and the screen initializes, showing a gray square, but nothing else. It just doesn’t boot up. Fiddling around a bit, I thought that maybe the battery protected memory got corrupted, or some other issue, so I set the dip switch to force-update and rom-unprotect, and started it twice, without actually loading firmware, but hoping that it would set some bytes or something to make the machine start at least. And it did. Also took out the video and cpu cards, reset all connectors. But finally I believe it was just some memory hickup.

Accidentally, found a stamp – made in 1996 – fits the datecode of the semiconductors.

The machine has been on for quite a while, no wonder the power supply eventually gave in. For precision timebase and jitter measurements, it is recommended to leave these instrument always on, or run it several hours before the critical test (3 picoseconds/div resolution, we are talking about mm distances at the speed of light…).

The firmware is a bit dated, and with the startup issues (that actually completely resolved once it started), I decided to update the firmware. Easy work with a 3.5 inch disc. Trying and trying – always getting read errors on my USB 3.5 floppy drive. EEE??? Some inspection – there is a big scratch in the disc.

I had been harsh to this disc, the only one in my possession in Japan, by storing it in a box with electronic parts and all kinds of things and dirt could easily get into the disc. In the late 80s, I kept these discs in a specially design box, etc.

Now, where to get such a disc it the city of Ube, Yamaguchi, Japan? A quick ride to the recycle store (2nd hand store) – no discs for sale, but I found an old network card, including an unused driver disc! 200 yen!

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The disc is working just fine, and the update proceeded with no problem – version 7.12.

Final critical tests of the 54751A plug in – the samplers are working great, no issues at all!

The new firmware disc, I will keep it in a well sealed ZIP bag, for later use!

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