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 5372A Frequency and Time Interval Analyzer

5372A Frequency and Time Interval Analyzer: out of sensitivity cal. error – easy fix

2015/01/29 Simon 4 Comments

For most precise time interval counting, I use a trusty HP 5370A, which has no less than 20 ps single-shot resolution. The 5370A and its inner workings could be discussed for hours (well, pages) here, but this is not the topic of this entry. This is about the 5372A, a much more modern machine – it still measures time intervals, but has a pretty capable build in numerical analyzer. It has a CRT, excellent quality, magnetic deflection, and this provides a nice and sharp green display. It doesnt’t add much weight, because the 5372A is massively heavy anyway, due to all the shielding, and a huge linear transformer. Why did HP use a linear transformer? No idea! Even the most quiet synthesizers can run with some kind of high quality switchmode supplies. Well, in the end, these machines are not build to be carried around but rest, or work, in an adequately thermostated metrology lab.

The 5372A has some very useful functions, not available for many other counters, unless you spent a lot of time developing your on software, work via GPIB; the 5372A can do virtually all of the most exotic tests, just with the single box.

That’s the main CPU, a MC68020 (the first real 32 bit processor ever!!!), and a floating point co-processor, MC68881, and a lot of traces that let the bits flow around.

This one, managed to get it for very little money, with the build-in precision HP 10811 reference OCXO already worth 100, used. And it even has the very desirable Option 030, 2 GHz input. Ideal for measuring locking processes of PLLs (via a triggered frequency vs. time display – shown directly on the screen, as a diagram/graph!), etc.

With the 5372A there are two important things to consider:

(1) It is extremely heavy, and does not ship well, unless it is packages with utmost care. My unit arrived in a big box, and barely made it – don’t even consider international shipment without a lot of bubble wrap, heavy foam, and double-boxes.

You can see the box already damaged; and there was just a single layer of bubble wrap; but I was lucky, the machine survived.

(2) Second item, never get desperate about the “160 out of sens cal error”. This error is well-known for these units, and the 5373A, and its 99% cause is a dead lithium battery on the CPU board. Just replace the battery, and follow the instructions to calibrate.

These are the main parts: a spare AA Li-SOCl2 cell, the CPU board with the dead battery (Tadiran is a really good quality Li-SOCl2 cell). And the new battery fitted (of presumably lesser, OmniCell, Made in China; but will be fine).

Note that this cell had been changed before, and some solder spilled!!
It seems, it didn’t kill the board, but it is removed now.

The calibration, not very spectacular (using a 8904A Source):

With the calibration done – no error displayed any more.

More detail testing will follow, but according to the self test, all is fine!

HP 8663A Synthesized Signal Generator

8663A Synthesized Signal Generator: a 1 year long repair story

2015/01/28 Simon 2 Comments

Over a year ago, I got hold of a defective 8663A. It seemed beyond repair, but hardly any equipment is, provided, you put in sufficient effort. The 8663A is certainly worth any resonable effort – it is a marvelous piece of test gear, providing full evidence of human ingenuity. It is said that a team of no less then 20 of the best HP engineers took about 5 years, with full support of the mighty HP organization at the time, to develop the 8662A and later the 8663A generators. Ever since then these were the gold standard for any low close-in noise source, for phase noise measurements, and so on. Keysight is offering a replacement now, the E8663D, about USD 50k, nothing compared to the 8662A or 8663A, for its historic value, and comparing the sheer mass of metal, the amount of gold on the assemblies, and the variety of strange little parts.

After some analysis, these were the main defects of the unit discussed here:

(1) A missing attenuator. The 8663A uses a pair of mechanical attenuators; these come in a set, together with a calibration ROM; with one of the attenuators missing, we might need to check flatness and level accuracy once replaced.

(2) Some intermittent failures of the A6A4 output sum loop. Seems to depend on frequency but not limited to any particular range. Supplied some test signals to the A6A4 assy and the fault really seems to reside with this assembly.

(3) A defective output amp. P/N 08663-67002. This is a real disaster. The output amp is a microwave microcircuit, with HP GaAs FET transistors. Needless to say, such assemblies were very expensive already at their time, and spare assemblies, despite long search, are fully unobtainium.

First things first – the attenuator. Found a spare attenuator that should be resonably close to the orgininal one. Did a quick check – it has very flat frequency characteristics anyway, so the ROM calibration coefficients might not be too significant (the 8663A has +-1 dB level spec, but typical accuracy appears to be more like +-0.2 dB; relative levels about +-0.1 dB). Someone tool the attenuator from the ‘dead’ unit, including the bracket – well, I didn’t spent too much time (as you can see) to fabricate a replacement. Good enough.

The intermittent fault of the A6A4 assembly – these assembly seem to be the weak point of the 8662/8663 series – I have already fixed a few of these. With schematics around, no big issue to fix – still it took a few hours this time. Turns out, the pre-tune DAC, which is a discrete circuit using 4 FET switches (1 per bit), showed erratic behavior. This was traced to one of the FETs, of the common 1855-0020 type – I took one from an early 80s HP parts units (8569A).

This is the bad guy:

The most critical item, the 08662-67002 amp. This has a low frequency (<120 MHz), and a high frequency input (>120 MHz), which are routed to a common output, providing about 20 dBm of power, at low distortion (about 35 dBc), over the full band from DC to 2.5 GHz.

After opening up the microcircuit, it is pretty clear that the last of the FET transistors is blown, and shorted to ground. This is all sapphire substrate, wire bonding, high frequency art. Beyond my capabilities (do you have a wire bonder at home, and a steady hand, and the skill and knowledge? Please teach me!). However, this world is not all bad, and rescue came along, back in good old Europe – in its South-West corner.

Turns out that a HMF-1200 is a suitable replacement for the proprietary-unknown original HP part.

Here the work of the kind friend, who certainly has tremendous skill and is a master in his field:

This is the associated board, with a PIN switch, and some bias regulators.

After all these repairs, and some adjustments (which took another few hours; including amp bias, lock detectors, ALC, FM VCO, …), the machine is working again – uptime so far, 48 hours at full power – consuming 500 Watts, and 100 mW at the output. Like powering a 100 W light bulb, from a 500 kW supply…. not quite efficient but a good heat source for the house, during these cold winter days.

To come: some flatness and level checks of the attenuator, but don’t expect any bad surprises.

An interesting document, found during the search for spare assemblies – the US Air Force also seems to be looking for repair, for 4 pcs of the amplifier assemblies, for their F15 Tactical Electronic Warfare System Test Set, P/N 001-006730-003.

May be a good business opportunity, but not for me!

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Microwave Receiver: AM detector/AGC circuit fix, A3B5 assy

2015/01/17 Simon Leave a comment

After fully refurbishing and fixing the MSR-904A receiver, and some months of service, another look at the AM detector. It is working fine, but at times, is seems to have some microphonic resonance and random shifts of level. Nothing that prevents use of the machine, but a small flaw worth correcting.

Locating the faulty assembly – pretty obvious when knocking at the A3B5 assembly. This takes in a sample of the 250 MHz IF signal, and performs the AM demodulation, and also generates the AGC signal.

These are the inner workings, the signal is first attenuated, about 20 dB, then amplified, about 25 dB, using an Avantek GPD-202.
GPDGPM gpd-202
The signal is then futher amplified via a tuned transistor amplifier.

Demodulation is performed by a zero-bias HP Schottky diode, a HSCH-3486. This was state of the art at the time, and still, there aren’t many diodes around that are any better.

hsch-3486 hsch-3206 hsch-3207

Turns out that this diode must have suffered damage earlier, and it was fixed with a drop of epoxy!

The HSCH-3486 are not so common any more (with the SMD package HSMS-2850 being widely used), here a comparison:

hsms-2850 hsms-2860

The HSMS-2850 and HSCH-3486 use the same technology – the HSMS-2850 is more or less a SMD version of the earlier HSCH-2486 (glass package, similar to DO-35).

Now, should I replace the flaky diode with a HSMS-2850? Well, after some thought, rather keep the parts as they are, for such a rare instrument. With some effort, this is what I found:

Glad I didn’t have to buy at USD 20 each, that’s what some vendors are asking for.

A bit of soldering, without taking the whole assembly apart:

Finally, a quick sensitivity and microphonic test – no issues at all.

Quite interestingly, someone must have fixed this assembly before – note the ECG opamp, 1991 datecode (it is a LM1458 replacement; for the AGC circuit).

HP 8642B Synthesized Signal Generator

HPAK 8642B Synthesized Signal Generator: backlight replacement

2015/01/06 Simon 2 Comments

The 8642B is an excellent generator, very clean, at least at offsets >1 kHz, hard to beat. It is also very heavy, thanks to a special modular concept that HP was pushing at the time. Their intention was to make the unit more serviceable, with the result that the generator is super heavy, and so expensive that it never was a real commercial success for HP. Frequency range is from below 100 kHz, to 2115 MHz. Pretty useful, with amplitudes from -140 dBm to 20 dBm.

The generator has a rather large (for the time) LCD display – fully story can be found in the HP Journal, December 1985.

The backlight has a very thoroughly designed light diffusor, which directs light from two 5V axial bulbs evenly to the LCD. Sure enough, these bulbs can burn out.

Some webpages claim that such bulbs would only last a few 100, maybe 1000s of hours, but such statements are incorrect. The bulbs used, 5 V, 115 mA, will typically last about 40000 hours, much longer than common household light bulbs.

These little bulbs have 1.9 Lumen each, not bad. To replace with a LED, 14000 mcd, at 25 deg angle, are about 2.2 Lumen. Close enough.

Found some 3 mm superbright white LEDs:

0.04 USD each!! Amazing!

Also these won’t least forever, white LEDs do lose intensity over time, like, 50% remaining intensity after 20000 hours.
I decided to run them below the rated current, at about 16 mA (120 Ohm series resistor with 5.2 V supply). Maybe this will make them last a bit longer.

Note that the backlight is software-controllable (special functions 134 and 234). This is how it is implemented (let me know if you need to full schematics of the 8642A or 8642B):

The resistors (1.8 Ohm) make sure that the lamps operate at 5.0 Volts, not 5.2 Volts, and don’t interfere with the operation of the LEDs. Quite amazingly, running at 5.2 V vs. 5.0 V would reduce the life span from 40000 hours, to 25000 hours!

Agilent sold these bulbs for about USD 18 each!!

No mechanical modification of the 8642B at all, the LED and the resistor fit well into the cavity (don’t worry about the lens of the LED – the light will find its way.

The result:

– not to my full satisfaction (non-uniform brightness, looks a bit dark on the left hand side, albeit, very easy to read).

After a bit of head scratching – turns out I soldered in the left LED with incorrect polarity – so it can’t work.

With this little mistake corrected, all is good:

Now, let’s hope that these LEDs will last. Never mind, I have a bag of spares!

Note: the display assembly is said to be rather ESD sensitive. Make sure not to damage it!

Micro-Tel MSR-904A Microwave Receiver

Anodizing 7075 Alloy: Micro-Tel handles

2014/12/02 Simon Leave a comment

Quite a few good tutorials exist for anodizing of aluminum, and pretty decent results can be achieved in any home shop equipped with a a sink and a few chemicals. For good results, with all the basic items mastered, the most critical item is the aluminum alloy. Generally speaking, any type of pure aluminum, and Al-Mg alloys are very much suitable for the anodization process. Zn, Si, Mn (and to some extend, Cu containing) alloys don’t work well.

The handles for the Micro-Tel MSR-904A receiver were machined from 7075 alloy, because of its strenght, and availability. 7075 has about 6% Zn, and 1.5% Cu, both of these elements are known to cause trouble when anodized. However, one can still try.

A quick, step-wise description of the process:

Step (1) – throughly clean/degrease the workpiece: first, I use methylated spirits, then, hot water and detergent, then rinse with water. Wear clean gloves.

Step (2) – etch with about 10% caustic soda. Room temperature.

As you can see, the part will turn black. This is typical for certain alloys.

Step (3) – use about 5% nitric acid to remove the black layer. Dip for a few minutes only. There will be some faint grey residue which needs to be brushed of mechanically (use a very clean brush – otherwise, it will contaminate the surface). Had to repeat the caustic etch twice to get a uniform and shiny surface.

Step (4) – anodize. Mount the piece with heavy aluminum wire. For 7075 alloy, pure aluminum wire works. Alternatively, use thick titanium wire. Current needed is about 1.5-2.0 Amps per 100 cm2. I used 2 A, for the handle. As cathode, use a sheet of aluminum, lead, or titanium. I use just plain aluminum and it is working just fine. For the liquid, about 15-20% sulfuric acid (dilute 37% battery acid with destilled water 1:1 ratio). Keep at room temperature, cool with some ice (applied to the outside) if it heats up too much. Typical time needed is 30-60 minutes, depending on the temperature and layer thickness. Don’t let the acid heat up too much – the layer will stop growing.

Step (5) – densify by boiling in distilled water. Needs to be really boiling, not just hot!

After the first attempt – everything looked fine after step 4, but the handles turned pretty dark after densification.

Pretty much, a full failure.

So, etched off the oxide layer with 10% caustic soda, and repeated the process, with two modifications:

(1) Keeping the acid rather warm, about 30°C, and reduced anodizing time to 20 minutes. This will give a thinner layer.

(2) Added a bit of ammonium acetate to the water used for densification. You may also add a very small amount of acetic acid. Keeping the bath slightly acidic prevents darkening during the densification process for Zn/Cu containing alloys.

The final result:

It’s not perfectly silvery color, but a slight yellow-orange color (like lightly colored wood). And the layer is certainly not very thick. But good enough to protect the 7075 alloy from forming corrosion spots over time.

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Microwave Receiver: a set of new handles

2014/11/25 Simon Leave a comment

For a while I have been looking for a set of of spare handles for the MSR-904A, but to no avail; the unit was missing the handles – had some rough rack-mounting fixtures. Fortunately, most of the Micro-Tel equipment uses the same kind of handle, so at least I know how they should look like.

These handles are actually of a very nice design, and don’t look too complicated to fabricate. So I thought I would give it a try and machine a set of spares.

For the material I selected a piece of 7075 alloy T651 temper (fully hardened, stress relieved). This is a quite strong alloy, and it needs to be because the MSR-904A is heavy, and the full weight rests on handles if it is set down on the floor/carried around, etc.

How to machine such handles – well, it is best done using a CNC mill, and luckily, I have one in the basement, even if it is just a small machine.

It is a FKM349VL table-top mill, not a very heavy machine, but pretty capable if used correctly. This is not for heavy loads and throughput-optimized toolpaths, but it can yield very usable accuracy, and the surface finish is very nice, provided that good tools are used. Typically, I get parts that are reproducible to within 0.01 mm, and absolute accuracy typically better than 0.03 mm, depending on how the machining goes.

Starting from a plate, first, the opening of the handle was machines, and chamfered. The original handle uses a radius chamfer, but well, I only have tooling for 45° chamfers around, fair enough.

Then, the holes were drilled, and countersunk, followed by machining of the left and right (short) edge.

A bit more tricky, the piece where the handle is mounted to the case. This is fairly thin, and some excess metal was left to allow easy clamping of the piece. Sure, there are other ways of achieving the same result, but it saves a lot of time if the vice doesn’t need to be re-adjusted, and if everything can be done in two or three clamp positions.

As one of the last steps, the excess metal is taken off.

All in all, considerably more effort than I thought:

(1) Cut the plate to approximate size; machine one long side flat

(2) Machine handle cut-out

(3) Machine chamfer (upper side)

(4) Machine left and right edge (short sides)

(5) Spot drill, drill, and countersink holes

(6) Turn around – align

(7) Machine other chamfer

(8) Machine mounting piece

(9) Mount set of 4 pcs upside in vice

(10) Cut-off excess

(11) Cut recess of mount piece, left and right

(12) Machine front chamfer, 2x

(13) Deburr, finish

These were the tools used –

And, not to forget, EMC2 with the Axis interface. Thank You EMC2 (LinuxCNC) team for providing such great software, free of charge!

I might still anodize the handles – but first need to do some tests with 7075 alloy (never anodized this alloy before, and don’t want to waste the handles!).

Wiltron 6659A Programmable Sweep Generator

Wiltron 6659A Programmable Sweep Generator: EPROM images captured, some rubber items

2014/11/17 Simon Leave a comment

The 6659A is running on no more than 10 kilobytes of code, stored in 5 pcs 2716 EPROMs. These were programmed in 1984 – just about 30 years ago. Just in time to capture a copy. Well, not just of their exteriors.

Their contents –
wiltron6659a
-also contained are the data of the 4 frequency correction EPROMs.

Looking through the board, a very intriguing part: a DAC72C-CSB-I

It is a 16 bit DAC, 30 ppm INL, 7 ppm/K gain drift. Merely, a resistance ladder, with a build-in 6.3 V reference. Still going for about 40-50 USD each! Very impressive case, for 1984 vintage, and still working just fine, to the current day.

While silicon chips last, rubber is subject to aging – in particular, the 4 rubber stand-offs that were holding the fan. Removal was no easy task – they were glued in with some Loctite, but eventually, I could remove them, and they were replaced by regular screws – not too much vibration anyway.

HP 3455A Digital Voltmeter

HPAK 3455A Digital Voltmeter, 34401A Digital Multimeter: switch-on behavior

2014/11/16 Simon Leave a comment

It is often assumed that high precision instruments are better not switched off but left on running 24/7, just because it is to tedious to wait for the specified “warm-up” period. Mostly, this seems to be more of a habit than a real requirement, at least for multimeters and their build-in references. Let put it to a test.

With all good references back in Germany, we first need to fabricate a little substitute, a +5 V reference based on a MAX6350. This has roughly 0 ppm tempco around ambient temperatures, and +-0.02% initial accuracy.

Some caps were added to keep noise down.

With this reference fully ‘warmed up’, two instruments were used to monitor the output, a 34401A, with current calibration, and the recently repaired 3455A, with about 4 years old calibration.

The result:

The 3455A provides 10 µV resolution, in the 10 V range. The 34401A – the displays shows 6.5 digits, same as for the 3455A, but the build in resolution, when read through GPIB, is about 1.3 µV.

As you can see, there is really no warm up. It is stable to ppm level, after a few seconds.

The offset of the two meters:

About 4 ppm difference – not too bad; certainly, well within spec of these two instruments.

HP 3455A Digital Voltmeter

HPAK 3455A Digital Voltmeter: some cleaning, and a really low drift reference

2014/11/15 Simon Leave a comment

The 3455A is a really nice unit if you are looking for a digital voltmeter/multimeter that is really accurate, and if you don’t want to spend too much. These units go for about USD 100 to 150, not bad, considering their performance – 6.5 digits.

This unit just needed some de-dusting, and was missing some screws. Build some time early 1979 (judging from the data codes, rather than the serial number), this unit doesn’t seem to have hand any issues over the last 35 years – while in service at Northrop Grumman.

Last calibration:

Did a quick check vs the best reference I have in the lab, a 34401A.

For a bit more through test, the 3455A was monitored for about 1 hour, with a low noise 9 V source at the input, and the differences logged.

Apparently, the 3455A is about 6.7 ppm low, vs. the 34401A. Which one is right, I can’t tell – the 34401A stability specification is about 20 ppm, after 90 days, far worse than the difference of the two meters.

The 3455A has a 11177B reference assembly. It has option S01 – no idea what option this is. HP P/N 03455-66520 Rev B.

The active part, a HP 1902-0926 thermostated Zener – very similar to the LM399, but not in the typical isolated (white) case. One thing is for sure, this part is now well aged!

The reference assembly has some remarkable 0.01% tracking resistors, with 1 ppm tempco – it doesn’t get much better even nowadays, and the resistors are in hermetically sealed cans, with glass seal, except one 1.0020 k +-0.01% resistor in the red resin package.

AC measurements were also carried out in the 10 Hz to 500 kHz range – it seems the 3455A even outperforms the 344401A for flatness, especially at the low end of frequencies.

Wiltron 6659A Programmable Sweep Generator

Wiltron 6659A Programmable Sweep Generator: 10 MHz to 26.5 GHz, in a single sweep…

2014/11/09 Simon 4 Comments

This instrument just needed a few adjustments, still, very interesting to look at.
It is of a very classical design – 4 YIG oscillators (2-8, 8-12, 12-18, 18-26.5 GHz), a coupler, a detector, and all the driver and ALC circuitry to make this work.
There is also a Wiltron-brand downconverter that provides the 10 MHz to 2 GHz output, by conversion of a 4610-6600 MHz input, with a 4600 MHz LO.

The YIGs are all of best quality, Avantek parts.

The coupler, for the ALC loop, a Krytar ultra-broadband part, with a biased detector.

The generator provides ample power, 10 dBm or more, over most of the bands; about +6 dBm, at above 18 GHz – and, around 23 GHz, there is a dip in the power curve (see scope screen, showing the ALC/power signal vs. horizontal sweep). Checked the bias (was set at a constant +11 V) – changing it, and going up to +15 V, no change. Also checked the power directly at the YIG output – still, the dip. So it seems, nothing we can do about it, but for most practical purposes, about 0 dBm will be plenty, at any frequency.

After some more alingments, the frequencies and bands are spot-on (no need to re-programm the linearizer EPROMs – all YIGs are still tracking perfectly fine), some some cleaning, using 50% isopropyl alcohol – done.

The only thing left to be done – a back-up of the 2716 EPROMs that still hold the firmware, after about 30 years.

SimonsDialogs

Category Archives: Test Equipment Repairs

5372A Frequency and Time Interval Analyzer: out of sensitivity cal. error – easy fix

8663A Synthesized Signal Generator: a 1 year long repair story

Micro-Tel MSR-904A Microwave Receiver: AM detector/AGC circuit fix, A3B5 assy

HPAK 8642B Synthesized Signal Generator: backlight replacement

Anodizing 7075 Alloy: Micro-Tel handles

Micro-Tel MSR-904A Microwave Receiver: a set of new handles

Wiltron 6659A Programmable Sweep Generator: EPROM images captured, some rubber items

HPAK 3455A Digital Voltmeter, 34401A Digital Multimeter: switch-on behavior

HPAK 3455A Digital Voltmeter: some cleaning, and a really low drift reference

Wiltron 6659A Programmable Sweep Generator: 10 MHz to 26.5 GHz, in a single sweep…

SimonsDialogs – A wild collection of random thoughts, observations and learnings. Presented by Simon.