HP 4140B pA Meter / DC Voltage Source: Special low currents, special connectors, and various FETs

It is another great auction score, a HP 4140B meter, used widely in the semiconductor industry and automatic test stations. Also handy in the lab to test all kinds of diodes, Zeners etc.

It has two +-100 VDC voltage sources, and a ultra-sensitive pA meter built in.

The pA meter seems to work, but one of the voltage source current limit LEDs flashes, although nothing is connected. This will need some repair. The other voltage source is working just fine, so there is no issue with the control board or DAC at least (one DAC is sourcing the voltage for both voltage outputs).

The current input is using some very unusual and high value range resistors… megaohms, gigaohms! Rarely seen before…

The range resistors are switched by reed switches, but not very common design. The coils are actually at the underside of the board, and no physical contact to the reed case, which could lead to leakage currents in the picoamp range.

There are some (plated) iron rods going through the board. These will get the magnetic field to the reed contacts.

These precision resistors, they don’t seem to come cheap. Maybe HP got a discount at the time… at least it doesn’t appear recommendable to start building such pA meters from scratch yourself… rather get some old used units.

The input assembly uses a dual FET to sense the null current, and the FET is a U401, rather common device. Maybe some nice experimentation or null detector can be done in the future with such designs.

The FET is mounted in the board, within a ground plane, and shielding between and around.

From the top, although there is not much heat generated, generous utilization of space, it could probably made fit to 1/4 of the volume?

The defect of the voltage source, it could be easily traced to the A5 board. This has a track and hold circuit, with a FET input opamp. The 4140B is one of the few instruments that I only touch with gloves inside! Better don’t leave residues and fingerprints on these gigaohm resistors and teflon standoffs.

Turns out the input to the amplifier is good, but the output is defective. A simple LF256H opamp, quite a common part.
Waiting for the spare… but pretty sure that replacing the opamp will fix the A5 board.

Another difficulty, the main connector. Originally, the 4140B came with a set of cables and a connector assembly, but this is mostly lost in some drawers of the previous owners.
So I did a test with a rather temporary assembly, but it is showing the correct currents, so all is good in general.

Finally, I found a cheap triax cable assembly.

The connector, it is gold plated inside, and better don’t touch!

Delicious Apple Muffins

This a well time-proven recipe, and will yield 20 mid-size muffins.

500 g wheat flour
3/4 package baking powder (about 10 g will be sufficient)
165 g sugar
a little bit of salt
1 small and flat teaspoon of ground cinnamon

All the dry material, mix them well, and best pass all through a flour screener.

Cut two large apples into small pieces, or use 3 small apples. Don’t store them for a long time, as they will turn brown. Best use some firm apples, not overly ripe or soggy.

Heat up the oven to 180 degC.

To the dry mixture, add 250 mL milk, 90 g of neutral taste oil (also works with 90 g of soft margarine). 2 Eggs.

Mix the dough well. Then add the cut apples, and mix well again.

Fill the dough into muffin papers (in some holders of metal form), fill to almost full, but don’t spill the dough (use 2 spoons for best results).

Immediately bake in the pre-heated over, for about 30 minutes.

Let cool down. You may apply some heated/liquified jam or glazing, but I just eat these plain, and also kids like it that way.

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!

=”490″ class=”alignnone size-large wp-image-4682″ />

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!