Category Archives: Various

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 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.

HP 8561B Spectrum Analyzer: a smoking power supply

Recently, busy days with all kinds of business trips and vacation in between, but finally time to go back to the workshop and enjoy some repairs in free time.
I got this analyzer for cheap, but it is not working unfortunately. When I plugged it in, smoke came out. Not a good sign, but let’s first find the source of the smoke. Easier said than done, because this “compact” unit has many fragile boards, and many screws, but I managed to get down to the innermost part, the power supply. Still wondering how HP designed this unit, it must have been a mere engineering nightmare, but these units are surprisingly reliable, 30 years old, or older.

Well, it didn’t take long to find the culprits, some old RIFA caps!

One blown, the others not looking much better. So I decided to replace them all, including the 2n2 Y-rated caps. The 100 nF X2 caps have a 20 mm raster, not a common size nowadays. We may as well replace them with original RIFA parts. Not cheap, at about 3 EUR per piece, but the unit is definitely worth it.

The power supply compartment is specially shielded, and I used the opportunity to clean out the dust.

A few days later, the new caps arrived (Y caps were still in my stock, WIMA brand).

The old board is looking marvelous with the new caps mounted.

A moment of truth – furtunately, the caps were the only issue, all working fine!

HP 4192A LF Impedance Analyzer: a leaking backup

Finally, to complete my collection of HP Impedance Analyers, I found a 4192A really cheap. As always with cheap things, there is a catch – this unit has some scratches, and doesn’t power up.

Well, usually no big deal, so I placed a bid and some time later the big box arrived. Similar to other HPY (Japanese-made) impedance analyzers, this unit has a lot of empty space inside, and is big and bulky, but at least, this simplifies repair.

Opening up the covers, the main issue is quickly found – the NiCd memory backup batteries have leaked some alkaline substance to the board and case, reading to some damaged components.
Fortunately, the corrosion is not looking too bad, at least the PCB traces are present, and the solder joints seem to conduct electricity.

The front view, you can see the scratches and dirt, but an overall complete unit. No boards missing. Despite their age, these units are normally still traded at 1-2 kUSD, and list price used to be close to 15 kUSD in the late 80s. New units of similar accuracy and range will easily cost you the same, in 2019 dollars.

The board affected, the A7 power supply assy. A switchmode supply. According to the manual, HP used a switchmode supply to reduce the weight and make the unit more portable (???? – what is portable about this box).

The bay holding the power supply, you can clearly see some traces of corrosion, but it is only superficial. The NiCd electrolyte has a tendency to leak out and then slowly creep with moisture all over the place.

These are the General Electric troublemakers!

Best cure for such leakage – wash with plenty of hot water.

Then scrub with a toothbrush, and scrub with vinegar (don’t use any concentrated acid). Vinegar will neutralize any traces of alkali electrolyte.

This is some of the worst placed, but fortunately, the traces were not affected much, and even the leads have a lot of good metal left.

Many good and well known parts in this unit – the CPU

… many Eproms holding very few kbytes each…

Pricy DACs.

And, the first fix – replaced the NiCd batteries with a commercial NiMH pack. There is a 1 kOhm resistor on the board, charging from less than 5 Volts – so this will be fine even for NiMH (less than 0.03 C trickly charge won’t cause any significant deterioration of NiMH cells).

Also – replaced 3 cracked RIFA 15 nF Y-rated caps.

Further repairs will have to wait until I come back from Germany in a few weeks, because some parts on the power supply board show damages, a ceramic capacitor (10 n, 100 V) that didn’t like the electrolyte and a diode (similar to 1N4148).
The electrolytic caps still look OK, but we will see in a while.