Category Archives: Test Equipment Repairs

For some, it is like solving crossword puzzles: fixing defective test equipment. Preferably, mid-70 to early 90s vintage.

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!

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!

HP 54750A Digitizing Scope: it ain’t work with no power

At the famous Yahoo auction, in Japan, I found this great instrument, a 54750A with a 54751A plugin. This is no less than a 20 GHz bandwidth(!!) sampling scope. I used to own one before, but have sold it some years back. At the time of introduction, these were the top instruments, in the 1995 to 2000 years. Still today, a 20 GHz scope doesn’t come cheap!

Normal issues with such unknown, non-working units are broken plug-ins, shot samplers, or other various difficult to fix issues. The samplers tolerate no ESD, and no more than 2 Volts! So please don’t let just any kid play with it. Not sure what happened at some companies in the past, when the engineer shot the 10 kUSD plug in… the whole unit traded for about 50 kUSD at the time, not too long ago.

This unit, it just won’t power up at all. No sign of any activity. So I took out the plugin, and started troubleshooting. The power supply.

These supplies were purchased from YOKOGAWA, a really high end supplier, and there are protection circuits for all voltages, including interlocks for the fans! But this unit has no need for any interlocks, because there is absolutely no power.

A little bit of examination shows discoloration and defective diodes.

The diodes overheated, because they are short. These are primary side diodes.

Also one of the transistors around this area of the supply failed. A 2SC3866 high voltage transistor, nothing too special, but none at hand at the moment!

Some analysis of the circuit – it is clear that this is the startup (auxiliary) power supply that will start up the main power supply. Also the input current limiting resistors (2 units that have 22 Ohm and a 130 degC thermal fuse each) are blown, no wonder that there is no action at all.

So I tried to connect an external supply, but there seem to be some other functions and details, so I can’t get the unit to start up. At least the red LED that indicates the auxiliary power, it is lit. But nothing else. Better not to proceed without schematics – which aren’t available.

So I have two options – wait to be able to go to Germany again, where I have a 54720A mainframe that has the same supply, and do some tests and investigations on that supply, or, see if there is a spare supply available somewhere. Kindly enough, a US enterprise offered a spare for USD 25, a great price!!! Plus 70 USD shipping to Japan…. Well, I purchased it and now waiting.

The 54751A module – even if the mainframe won’t work again, it is great find. These usually go for USD 1.5k, working condition. And I do have spare samplers back in Germany, in case it is a damaged unit (but as the mainframe has a power supply failure, I suspect that the module is fine, and that the unit was taken out of service because of the non-working power supply).

Some current offers…

HP 3335A Synthesizer/Level Generator: Unlock issue, soldering issue?

The recently fixed 3335A, it still has some issue with the reference input. From time to time, or when touching the BNC cable connecting the 10 MHz OCXO output with the reference input, the *UNLOCK* light comes on.

First I thought it has to do with the low cost and cheaply made BNC cables, but even with very good BNC connectors, the issue persists. So, let’s open the thing up another time. I don’t like partially fixed instruments – even though this error could probably be solved by just not touching the cable….

Fortunately, the BNC connector has a long cable internally, and can be disassembled without destroying the crimping.
Checking it, the center wire has no connection. Looking at it in magnification, it seems that only ever the very tip has been in touch with solder, but inside of the golden BNC center connector, there is no solder. So it is clearly a HP manufacturing issue, someone didn’t solder it quite right.

After soldering it again, with plenty of solder, and removing the excess, all is working well! Finally!

HP 3335A Synthesizer/Level Generator: a good fake transistor? at least, a working unit!

Recently, I got a very nice but non-working HP 3335A, with a defective power supply.
After a little wait, the spare PNP power darlington transistors arrived. 2N6052, pretty general purpose power transistors.

The look very shiny and heavy chromium plated, I got 4 pcs, and two had some marks of screws, and 1 had irregular connection wires. Doesn’t look like a genuine Motorola, but well, it is no rocket science anymore to make PNP darlington transistors.

After installing the transistors (I checked the contacts on the board, they seem to be good), some quick test with the load transistor showed perfect regulation of the power supply both on the -15 V and -5 V rails.

With the module assembly connected, unfortunately, the -15 V rail goes up to about -3 V, by current limit regulation.

No other way to find out than by checking module by module. The 3rd last checked turned out to be the faulty one – A7 assembly, mixer.

A little shorted 2.2 uF cap…

At the far lower left.

Replaced it with a 10 uF ceramic multi-layer capacitor.

To check the quality of the transistors, I sacrificed one piece, and cut it open – seems very solid inside, with copper heat spreader, well-mounted die, good bonding wires. So even if it has not been made by Motorola, it seems like decent quality.

Now… doing a proper test…. several hours on, to see if there are other weak links. The attenuator at least is working, and the output is good!

Mettler AE 163 Dual Range Analytical Balance: Swiss Made equipment, in Japan

Regularly screening through Japanese auction sites on the lookout for some gems, I found a great AE 163 Dual Range analytical balance, completely non-working condition. No display at all. From the picture it looked like a rarely used clean unit (be careful when buying some old lab equipment, some might have quite some damage by chemical vapors etc.). I scored it for 7 EUR, great!! Plus another 20 EUR in shipment charges, but at least it was packaged very well and arrived with no damage in transit.

The specifications are better than most modern analytical balances ranging in the 3-4 kEURs, with 0.1~0.2 mg linearity, built-in calibration weight (accurate to 0.2 mg – very hand to recalibrate the balance after taking it to another place, or just to confirm that it is working fine), and these were the high end balances of the 80s, still in use today in various labs. I remember to use such balance during my time as a researcher at the University of Eugene, Oregon, a while back…

The balance has about 4 circuit boards, a display/keypad (an ingenious single bar keypad, easy to handle with gloves on, etc, without disturbing the balance), a control board that also has the main power supply, a sensor board for the force compensator, and a current driver board for the coil. These balances work by force compensation, i.e., there is a magnet coil that will compensate any weight you but on the balance by electromagentic force. And there is a pretty sensitive position detector (a light gate) to keep the regulation control loop going.

After some probing (there are no schematics unfortunately, but anyway, difficult to fix because there are mask-programmed controllers and custom ICs), found that one of the supply rails is down, shorted by some tantalum. 10 uF blue paint-dip type.

Decided to replace them all, including two 1 uF tantalums. Tantalums can last a long time, but some series tend to fail one after the other.

With quite little effort (also because of the nice serviceable design of the unit), all working again.

Here is a closeup of the force coil, it should have a coil and a strong magnet inside.

The light gate of the position detector.

These will also need to be replaced, 3n3 Y-rated capacitors, getting brittle after 28 years…

The balance also had an add-on, a serial interface. The circuit is quite complicated for its function, using mask-programmed CPU, but that used the be the most reliable technology at the time (and still working today).

Also with that interface add-on, replace the tantalum caps, and the Y-rated caps (mains is fed-through to the balance from this add-on module. Not sure why they added another set of Y-caps, as there is no mains related circuitry inside (2n2 value caps).

HP 3335A Synthesizer/Level Generator: a marvelous piece of engineering and precision

For several years I have been looking for a HP 3335A, which is probably the most precision level generator that is available. Some of the more recent devices struggled to keep up with the performance, so the 3335A is still used in some calibration labs, and has thus been quite expensive even until now. This time, in Japan, I found a non working unit for 10 EUR (!!!) including a precision OCXO reference. It is a very clean and late unit (about 1991), but not showing any signs of operation.

The front panel, it is pristine, with no scratch or anything. Maybe a rarely used instrument from some remote Japanese cal lab or university.

The key part, the attenuator, it is a marvel of engineering, don’t touch it without reading first the repair instructions.

The 1990 HP catalog entry, 0.05 dB absolute level accuracy, 0.07 dB flatness. Such performance did not come cheap, 13000 US dollars in 1990….

To achieve the frequency resolution, a fractional N loop was used, it is one of the first instruments that used such PLL technique.

The RF boards are in two metal enclosures, pretty heavy extruded aluminum.

After some tests, pretty obvious faults – the -5 and -15 V rails are dead. There is unregulated voltage present, so it must be something with the regulators, a classic design with darlington pass transistors, and opamps to regulate the voltage and current.

The strange thing, with the modules disconnected, the rails come back to the precise voltages.

Made a plug with a 27 Ohm resistor to load the rails a bit, and, they immediately drop to near-zero. Seems there is not enough capability to drive current.

Took out the pass transistors, and tested these by driving 1 mA of current – but no amplification or anything. Open circuit.

Opened up both transistors, and both have the same defect – the die is not attached to the case any more, somehow, time, heat or something destroyed them (overload is unlikely, because the current regulation loop and foldback is working).

The original parts 1853-0415, power darlington, seem to be equivalent to PMD13K60 of a mysterious Lambda semiconductor company.

There are NPN and PNP complementary sets available. It is a fairly standard darlington power transistor.

From the web, we get another proof, there HP installed the parts with the original part number, rather than re-labeled to HP number.

The replace, the 2N6052 seems to be a good replacement. So I ordered a few – waiting.

As a backup, also ordered some KD366B which seem very strong and well fabricated.

The NPN transistors seem still Ok, but who knows, they might have the same manufacturing defect, so I ordered some BDX87B (which are ST devices universally compatible with various NPN power darlington transistors up to about 100 V; nowadays, all these power darlingtons seem to have the same die inside).

Now, let’s wait for the transistors, stay posted!

HP Attenuators: another great method to fix them

Thanks to a kind contributor there is a new way of fixing the HP step attenuators that are ubiquitous in the various HP and Agilent generators, analyzers.
These attenuators exhibit some common failures modes-

(1) blown pads – fix by replacing with pads from good donor units. Keep in mind that even if the pad value is the same, there are pads of different geometry/length!
(2) mechanical issues with aged O-rings, easy to fix, best use some FKM O-rings
(3) the broken-off contact fingers, difficult to fix unless you have some precision equipment like a good milling machine or fine drill.

Here is an alternative way to fix it – use some two component epoxy glue after removing the remaining plastic parts from the contact finger by heat (heat gun or hot plate, about 200 degC).

To ensure a permanent repair, a piece of FR4 is affixed with epoxy glue. Sure enough this is not quite ideal for high GHz frequencies, but no problem up to 2 or 3 GHz.


HP 856x and 859x Series Spectrum Analyzer: Rubber keypad issues with 8561e 8562e 8563e 8593e 8593e 8596e, etc.

The famous and still very common HP 856x and 859x analyzers come in two versions of keypads, the earlier A, B models have hard-plastic buttons that go to individual switches, while the later models feature rubber keypads. Sure such rubber pads are good to touch and easy to use, however, very commonly they develop issues over time for these HP instruments, the buttons will eventually only react to the strongest push, making the analyzer bothersome to use.

Having fixed many of these, here the instructions how to fix the issue, and the repair seems to hold up well (some instruments already fixed 8 years back still good today).

To remove the keypad, you have to take off all the front panel, carefully disconnect the SMA connectors, and make sure not to damage the power cable. Best do it on an ESD surface, or other non static surface like an old moist carpet, a piece of cardboard, or wood. Make sure all is clean (this instrument doesn’t tolerate cut-off wires and solder droplets inside, floating around on your workbench).

Disassembly proceeds with some good screwdrivers.

The keypad has some extensions, these must be pushed out, don’t pull off the keypad from the front!!

Soak the whole rubber in 70% Isopropyl alcohol (I take 99.9% and mix with distilled water), good enough to soak for 5 minutes at room temperature, then just take it out, dry overnight on a paper towel, maybe cover it up with some paper if you are in a dusty workshop.

The board with the gold contacts, I first wash it with 99.9% isopropanol, then use an abrasive sponge (ultrafine), to give it a light polish, just one stroke, at an 45 degree angle over the contact area, and another stroke prependicular to it. Don’t scratch off the gold! Afterwards clean and polish a bit with a paper towel and pure isopropanol. Let it dry overnight.

Then, after assembly (don’t overtighten the SMA connectors, don’t squeeze or damage any of the cables, don’t use force on the boards), all will be good.

Working like new, how pleasant to use!

HP 8566B Spectrum Analyzer: lock and roll issues

With the 8566B main issues fixed, I carried out extensive tests, also switching it on and off many times, running it for a while. So far so good. With all the tests, discovered only one issue, unstable display for spans above 5 MHz.

The instability is difficult to see on the picture, it is a bit random, and at slow sweep, the trace becomes very wobbly and noisy. Jumps around. The common suspects are the A19, A20, and maybe A21 assemblies, these control the YTO. There are various capacitors on these boards that may fail or leak. Or some dead bits on the DAC, or similar. A bit strange that it works so well below 5 MHz span – isn’t it? Not really, because the 8566B has a different mode of operation, depending on the sweep width. Below 5 MHz, the LO stays locked all the time, above, it is only locked at the start of the sweep, and then the YTO is swept just be increasing the tuning coil current in a linear (and linearized) fashion.

After considerable study, probing, checking capacitors, including desoldering some – no success. Some more checks, solder joints fixed, finally, so occasional improvement. Touching the assemblies A19, A20, some response.
From that, suspected a contact issue with the board edge connectors, and indeed, these were not very clean. So I gave them a thorough treatment with polishing cloth, rubber erasor, alcohol. Reseated the boards, A19 and A20.

Finally, the sweep is very stable. Seems a lot of unnecessary concerns about capacitors and such, but well, finally, fixed. Screen shows a reference (stored) trace with slightly below 5 MHz, and a longer time max-hold display, slighly above 5 MHz span. Clearly, not much noise and instability. All looking good.