Category Archives: Various

HP 4140B pA Meter/DC Voltage Source: some incorrect assumptions, but finally, a repair

Shortly prior to my departure from Japan I started repair of a HP 4140B. A very desirable instrument for semiconductor characterization. The issue remained that output “A” didn’t provide correct voltages, probably due to some issues in the amplifier.
Two month later, I have returned to Germany, and a spare LF256 J-FET opamp had arrived, so I thought it would be a quick fix – but to no avail.

Fitting the LF256 to the board – I usually leave some part of the old wire in the Teflon isolator, because I want to avoid soldering/melting the solder in the Teflon part – it is all difficult to clean up afterwards, so I prefer to solder the new part to some leftover wire, rather than contaminating the isolator.

But- with the new opamp installed, same symptom, no proper output, current limit function of channel A blinking, but the input of the opamp is good. So it must be something else down the chain. Another look at the schematic…

There is an analog switch, followed by a discrete linear amplifier with a dual J-FET input stage.
After some study of the analog switch (cutting a trace and checking it), the switch appears fine. Next in line, the dual J-FET, and in fact, this is dead – found it by measuring the E-B and C-B transition voltages with a diode tester (instrument powdered down and board removed!!), and the transistor around the FET shows largely different values compared to the working B channel. It is 1855-0049 HP part, available in some single piece quantities but expensive!

Looks still shiny and new, but it isn’t working.

Studying some NSN databases, found at least some data of this part, which had been manufactured in equivalent versions by some other manufacturers as well, probably in the 1970s.

It is a rather not so special depletion n-type J-FET. But it is a dual FET part, and while single J-FETs are no problem to get, dual FETs are rare specimens.

Even in their long past days, these didn’t come cheap… maybe something like 40 EUR a piece in today’s money.

So we need to do further study, and there are essentially two kinds of dual FETs – some that have a specially made dual die, with both FETs on one chip and coupled in various ways to keep them from drifting apart with temperature, etc., and the other kind, which is merely just two reasonably matched separate FETs in one case, for convenience more than anything (and for thermal match).

Screening through my inventory I found these 2N5457 FETs which have pretty similar electrical characteristics, in particular, zero-gate-voltage currents.

The parts I have are all quite uniform so there is no need to select a special pair.

With such replacement with similar parts, rather than identical parts, I think it is a good idea to take no risk, so I took the B-channel dual FET and transplanted it to the A channel. And the B channel, which is anyway only a secondary function of the instrument and doesn’t allow the same fast ramps and functions like the A-Channel, it will be definitely good enough to install the two FETs separately (closely together), rather than the original part.

The dual FET of the B channel replaced by two 2N5457.

The B channel dual FET 1855-0049 transplanted to the A channel.

With these repairs, the instrument powdered on just fine, and the output voltages were spot on without any need for alignment. Even the zero bias setting if the LF256, no need to adjust.

Induction range repair – just a couple of IGBTs, and a 20 Amp fuse

Recently, the induction cook-top of my SMEG range failed, leaving me with potentially expensive repair options quoted at above 1000 EUR, or to do some investigations myself. Surely, the latter option applies in my case. So, after receiving two spare IGBTs by mail, and a high current fuse that matches the “repair option” fuse holder of the cook-top, it took just a bit of soldering to get the thing up and running again.

Mounted the IGBTs, an easy job compared to the tedious mounting of all the coils and cables. It is not quite a service friendly design, and there are many sharp edges that can damage cables and your skin, so better wear gloves and handle everything with care.

After cooking on the range for a while, there is absolutely no problem at all, it’s a 20 dollar fix, if you don’t count your own time – maybe about two or three hours, mostly, to take out the electronics and put them back in.

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!

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 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 8753C Network Analyzer: a new old YTO, and a new old firmware

After another trip to Germany, another HP 8753C to fix. This unit had option 020, 006, a 6 GHz unit, but there is no 6 GHz test set.

First, we need to get a suitable YTO, found a good ASF-8751M, from Israel. Cleaned it up and gave it a proper test.

It is a 4-8 GHz unit, but I easily got good power down to necessary 3.6 GHz. It is a well-behaved unit, with reasonable power consumption running of +15 and -5 Volts. The heater may be better run with 24 Volts, but there is only 15 Volts in the 8753C, and it is good enough it seems.

Some modification of the PLL board, as described before, to approximately double the tuning current, installed a 20 Ohms sense resistor, and installed a BD249C transistor on a good heatsink.

A quick drawing of the heatsink, should you need it. Use 1 mm aluminum sheet. Don’t cut yourself, when cutting the metal!

The YTO, installed in the veritable source assembly. Pretty confident that this will last for a while.

This time, all worked well and the pretune correction functioned immediately, no further adjustments needed. Phase lock seems very stable at all frequencies, scan rates, and band transitions.

Out of curiosity, did a phase noise test of the 8753C in CW mode (fixed frequency mode), getting well below 100 dBc. Pretty good. Maybe better than the original YTO.

For the current unit, I also wanted to update the firmware, and install the 010 option (time domain analysis). The option installation (and EEPROM backup), done like described in an earlier post, but desoldering the EEPROM, and changing three bytes…

The unit is still running pretty old firmware.

Should be easy enough to program some 27010 EPROMs, but the devil is in the detail. After a number of incorrectly programmed EPROM, finally figured out the once of the CD4015 CMOS of the EPROMMER had failed! Fortunately, I had some in stock to fix it.

After these efforts, the 8753C is starting up with the latest (albeit, dated) firmware, and all options.

A few tests with filters and such, a very useful and well working unit. The CRT also very good, no need to install a LCD.

HP 3325B Synthesizer/Function Generator: a quick fix, and a hot transistor

Recently, I got a defective HP 3325B, it is a very useful generator even for today’s standard. It features some highly linear ramps, has great frequency resolution and a powerful output (10 Volts p-p into 50 Ohm). This unit reportedly had major issues, no output, and failures with startup. So even before switching it on, I removed the panels to check. Nothing obvious at first glance.

After a quick power on, some smell from the output section, and clearly, there are some burned resistors, and one of the power stage transistor is terribly hot, so hot that the solder melts… don’t burn you fingers!

Removed the board altogether (take care not to damage the connection flat cables!), and even the solder had some spray by heat effect, so I cleaned the area well.

To get access to the resistors, and to also do a proper test, all the transistors in the area were removed, and the transistors desoldered. All cleaned up pretty well, the board seems to be of good quality.

The 3-440 transistor aka 1853-0440, cut open. It has a tiny chip, difficult to see the damage with my means, but it is shorted to base.

The resistors, the only issue is a slightly discolored 47 Ohms carbon composition resistor, part EB4701, a 0.5, 10% tolerance resistor. Quite expensive to get, and the part, despite some signs of heat, tested good and within tolerance. So I decided not to replace this transistor, because it has an effect on the high frequency performance of the circuit.

The power amp, it is a marvelous push-pull design. It relies on complimentary NPN-PNP transistors that have high frequency power.

Nowadays, the PNP RF transistors of this sort are rare, probably they even were rare and expensive during their time.

The damaged resistors, fortunately, after a good amount of searching, I found the bags here in by temporary Japanese workshop.

The transistors, these 3-440 are equivalent to the 2N5160, and I happened to have 3 of these back in Germany, new old stock. Purchased them some years back, because they are generally not easy to get.

After these replacements, I run the adjustments and performance checks as per service manual, with no trouble at all. Also the self test passes flawlessly. We can call the generator fixed.

Out of curiosity, I checked with ebay, and there are very reasonable offers of what appear to be Chinese copies of 2N5160 transistors. They have the Motorola label, but to my knowledge, the date code is much past the obsolescence of these parts at Motorola. So I am waiting to receive these parts, and will give them a good test and study, to see if these are good replacements, or just fake.

HP 6205C Dual DC Power Supply: a generous binding posts fix

The repair itself, it is not particularly noteworthy, because this supply has served me well in the last years, in fact, it had been switched “ON” all the time to power an experimental setup.
The initial repair of this supply has been documented before, and on the pictures there it is quite visible that this supply had damaged binding posts. Seems that the prior user dropped it on the front panel.

Now the noteworthy facts, a kind reader of this blog, an American fellow, had a few of these posts at hand, from a HP plotter. He kindly sent them to me, free of charge!

So, as a result of the kindness of the reader, and the standardization of the parts HP used in their equipment, the power supply is now in better shape than ever before.

Did a few tests, like, checking ripple current at full load, and electrical safety – ground resistance, but all looking good.

HP 8753C Network Analyzer: Serial numbers, options, EEPROMs

The HP 8753C comes with some software options 010, time domain (essentially, a built-in FFT function), and the even more useful harmonic analysis, option 002. These work without any further calibration, and used to be available as a code to enter to the instrument , with service function 56, to update the option status.

Thanks to a kind gentleman, such codes are available now, and normally you can add them to the 8753C without any expert knowledge and risk.

Unfortunately, for this instrument, the method to add options by code entry didn’t work. How come? As much as we know, the option code depends on the serial number, let’s check if the serial of the CPU board is the same as that of the instrument (ending in 00860). A first hurdle, how to read the serial – it is not showing upon startup for the 8753C, but you can get it by first executing service function 55, which will fail, and then go to Display-Title.

To my big surprise, the serial shown is incorrect, only 4 digits, missing the “8”.

Accordingly, we need to dig deeper, and the serial number and other information is stored on the U23 EEPROM, a 2kByte chip, Xicor.

It is a very long lasting device, no reason to believe that it will fail anytime soon, but there are always risks. First, I read all the coefficients via GPIB, and then carefully desoldered the chip.

Actually, desoldering went very well, even just with plain tools, a soldering iron and a manual solder sucker.

The programmer, put together from a few jumper cables, and an ATMEGA128A board. When reading, I hardwired the WE- write enable input to VCC, to make sure that no data are lost. There are also 6k8 pull ups directly on the ZIF socket, to make sure the input stays “High” even if the jumper wire is not connected well.

In the EEPROM, clearly there is the incorrect serial, it is not actually missing a digit, but has an incorrect character. Maybe it got modified when the CPU clock failed (remember that this board had a bad osciallator?

Now, we need to put in a single character, an “8”.

I don’t normally need to program 2816 EEPROMs, so rather than taking chances with some incompatible programmers, I made a small program, to just set a single byte, at a given address. In this case, writing an “8”.

With the serial number corrected, put the EEPROM back onto the CPU board – using a precision socket.

Using the secret code that only works with the matching serial – and with the write protection of the CPU board disabled – the option install worked perfectly fine.

Now, the 8753C shows the options upon startup, and the time domain and harmonic analysis functions show up in the menu as softkeys.

Afterwards, I checked the EEPROM contents again, there are only 3 bytes changed, in-line with what can be found in online forums. Also tried to activate the 006 6 Ghz option, not much use for me, but the option code is same as seen for the 8753D, etc. There are 3 bytes, right in front of the serial, with the upper half-byte bits all set (0xFx), and the lower half-byte encoding the options in a bit-wise fashion. With no options, the three option bytes are all zeros 0x00.

If you need any of these EEPROMs or related advise with the 8753x units, just drop me a line.