Category Archives: Test Equipment Repairs

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

Oscilloscopes

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

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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…

Generators - Low Frequency

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

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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!

Generators - Low Frequency

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

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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!

Various

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

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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).

Generators - Low Frequency

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

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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!

Various

HP Attenuators: another great method to fix them

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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.


Various

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

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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

HP 8566B Spectrum Analyzer: lock and roll issues

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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.

HP 8566B Spectrum Analyzer

HP 8566B Spectrum Analyzer: Defective and fake transistors, and finally, a HP 4-404 equivalent

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Recently, I got a HP 8566B at very low cost, unknown working condition of course. First, it would not power on, at least not fully. Some activity on the 8566B, but no display at the 85662A display unit.
Clearly, next step, to open it all up and do a good survey. Quickly found the issue in the 85662A power supply board. While all the other boards where good and clean, the power supply boards are directly in the airstream and showed some leg corrosion of transistors. Especially, 2n2369A transistors, and the famous 4-404 HP transistors (not that the 4-404 is anything special, but there are no replacements mentioned by HP.

Recently I found in some late 8662A boards that HP subsitute the then obsolete 4-404 with a MPS6521, a high gain NPN transistor. So I did the same and put in BC337-25 (-40 may be a closer match but not at hand here).

With these fixes, the 8566V turns on just fine, but it doesn’t turn off well. The 5.2 V rail goes high a little, and then nothing happens, no power down, when you switch the analyzer off. So measures all currents and voltages in the 5.2 V supply, and finally traced it to a defect of the main power transistor, a 2n5886 equivalent power NPN transistor in TO-3 case.

Checked it out internally, the defect is actually within the area of the bonding wire connection. So the transistor became better, non conducting, once I pulled of the bonding wires and measured directly on the die.

Quickly ordered some 2n5886 from China, a bag of 5 for less than a dollar a piece including shipping. Well, you get what you pay for. Look at the small die, the missing plate (die is directly bonded to the steel case, the bonding wires much thinner).

Doesn’t look like a genuine ON brand device, even the marking is not in accordance to the datasheet.

For comparison, the HP-Motorola part:

Tried to fix it with a 2n3055 temporarily, but it turns out there is not enough gain with that transistor. So the rail would only go up to 4 Volts.

The 2n5886 is a quite remarkable device, high current and substantial gain.

Finally, I got hold of this “Motorola” device, it may be genuine, at least it looks like solid quality. So I unstalled it for a test.

You can see it installed, a new and shiny transistor. Soldered it in generously applying solder.

Voltage is spot on without any adjustments.

Now all seems to work, except the various IF filters need a bit of alignment.

Although they are brittle, for these adjustments ceramic screwdrivers are definitely handy. Make sure to isolate other screwdrivers, easily there can be shorts when adjusting, resulting in complicated repairs of the IF signal chain.

Finally all adjusted within the toleraces, there are all too many adjustable capacitors inside this unit!!

At least, a well worthwhile repair, because the CRT is like brand new. It seems that someone had replaced it, but then put very little further use on the unit. I plan to use it together with my microwave phase noise measurement installation, for rough characterization of microwave sources prior to engaging the complicated phase noise test gear.

HP 8662A Synthesized Signal Generator

HP Attenuator 33321SB AKA 33321-60026 ATTENUATOR: fixing a defective spare part

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After some consideration, I decided on the fix of the the 33321SB attenuator. The defect – the plastic of the grey holders broke off, probably, a matter of age and a matter of stress in the special construction of the 33321SB with the side connector.

It is not the first attenuator that has such defect, so it seems to be a certain weakness of the HP design, albeit, a weakness showing up much after the design life.

Originally, I planned to fix it back home in Germany, with some precision machine tools – drill two holes, fit/glue two plasic rods, and the mount it in the original fashion. But currently, it is uncertain when I can go to Germany again, so I decided on an alternative fix. There are several unused HP attenuators around here, so I checked their internals for a contact of the same design (length). Take care, there are many different configurations of these attenuators that all differ in the length of the contacts, the distances between contact, and so on.

Found a good donor – now, cutting the piece with a razor blade.

The part is obviously fragile, better don’t touch the contact.

After assembly, it seems to work well and fits well. To avoid further risk of breakage, added some 2K epoxy glue underneath the metal spring – it is not visible on the picture. So I hope it will be a permanent repair. Otherwise, we will fix it again using better tools and methods.

With the spare part repaired, and installed in the 8662A, the generator is working well again, at all power levels.