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 4192A LF Impedance Meter: Isolated power supply fixed

Working through the guts of the 4192A, I tried to operate the A8 assembly, which has several +-15 V isolated power supplies (for floating voltages, etc.), and a +-40 V bias supply, set by a pretty cerdip DAC.

Basically, the assembly consists of a set of switched power converters, converting the +-15 V input, to isolated +-15 V output. Running at about 900 kHz to 1 MHz.

The assembly is double shielded, to interference with the impedance measurements. It has a set of coils and capacitors, a number of parts! Definitely not a cheap unit to build at the time.

The issue – when connecting it to the 4192a power supply, it loads down the +-15 V rails, and the power supply assembly A7 of the 4192a soon goes into shut-down. Not a good sign.
Analyzed the circuit by touch, almost burning my finger – the two main switching transistors of the 4th supply are glowing hot!

Did some measurements, and the transistors, as well as two capacitors are dead. The other capacitors, I removed them because you never know, and better to fix it now than later.

Interestingly, the capacitors HP selected were 85°C rated, I decided on some Panasonic EB series, 105°C rated, low ESR-long lifetime caps. Hope these will last another 30 years of occasional service.

The transistors – difficult to find good substitutes. I believe some other reasonably high current NPN fast transistors like SS9050 or similar would work, but I don’t want to take chances and found some new-old-stock 2N3725 on eBay. Only it will take a while for these to arrive in Japan!
For the time being, I desoldered the working transistors of the #3 supply, to get the #4 supply going for the tests.

Surprisingly, one of the transformers has a damaged pot core, K5 ferrite, difficult to get. Strangely, there were no loose pieces inside the shields, and no traces of earlier repairs. Can it be that HP fitted a slightly defective transformer? But as the unit is working and outputting plenty of power, the core should be good enough.

To fix it, or at least make sure that it is not rattling or desintegrating any further, we have a choice of epoxy glue, super glue, or silicone. I decided on the latter, a 704 type head conductive silicone. It is available really, cheap, and it is good stuff, because it can be removed later without destroying the coil, should I ever come across some K5 pot cores.

All fixed and stable.

A quick test – powering from a current limited lab supply (about 80 mA on 5 V, about 0.11 Amps on +15 and -15 V with no load are needed for the A8) to avoid destruction.

Fortunately, the A8 assy is working again. I didn’t test the DAC, but no reason to believe it would not work.

Now just waiting on on the 2n3725A transistors, and then, it can be all put togethers with many screws and pieces of shielding metal.

HP 4192A LF Impedance Analyzer: some issues fixed, more unveiled

After a quick trip to Germany (and with a set of spare parts in my suitcase), I directed my attention to the sick 4192A again.

(1) First, the A7 power supply assembly. Fixed the defective signal diode, and ceramic capacitor. Discovered that the line frequency signal is not working, and that the 5 V supply (analog section) linear regulator isn’t regulating, but passing through about 6 volts. Also noticed that the digital 5 V supply is not working (running at 2-3 volts).

(2) After about 20 seconds of operation, there is some smell around the A7 power supply assembly. The input caps (10 uF, 350 volts) running hot.

(3) No display at all. Is the controller assy dead? Or the EPROMS corrupted?

Let’s tackle it step by step.

Fortunately, we are not alone here, the same instrument had similar issues elsewhere – seems the NiCd batteries weren’t fit for the purpose, and have all leakage issues, as can be seen in below screenshot of a Japanese blogger.

Unsoldered the 10 uF 350 V caps of the A7 power supply – all the positive connections are leaking. Fortunately, not as corrosive as the NiCd electrolyte.

Strangely, HP fitted 85 C capacitors, rather than long life types.

On the picture you can see the temporary fix with some capacitors I had handy. And, you can see the Y-rated capacitors (15 n) RIFA cracked capacitors replaced by WIMA brand new parts.

For replacement of the 10 uF Chemi-Con we will use CFX series capacitors.

These are rated for quite severe ripple current, which is necessary because that’s their purpose in the A7 circuit.

Of course, 2 parallel capacitor 10 uF will have better ESR than a single 22 uF, but fair enough for test purposes.

The digital supply, surprisingly, has no regulation, neither by linear nor by switchmode action (the switchmode circuit is regulated by an independent sense winding on the transformer (running at 29.5 kHz).

The digital ground is connected to analog ground by a 10 Ohm resistor for noise isolation. Other than that, the circuit is rather simple and quickly found the defective part – surprisingly, one of the Schottky rectifiers, a 20FQ040 diode. Maybe it overheated, or it failed just because of age.

For its time, 1980s, it is a respectable diode, with very low forward voltage.

Unfortunately, replacements in DO-4 bolt-mount style Schottkys are very expensive and hard to get. Maybe there are some back home in Germany, in the junk pile waiting to be desoldered, but not sure about it, and no such diodes here in Japan. Anyway, after some measurement, the current of the digital supply will be somewhere around 2.5 to 3 Amps, not too much, for any common switchmode supply rectified diode. Decided on a SBL3040PT. 3 Amps at 0.2 Volts will be less than 1 Watt of dissipation, say, about 40 K temperature rise of a SBL3040PT double diode (such packages have about 40 K/W junction to ambient thermal resistance, probably I will fit a small metal part as heatsink (the existing heatsink may be used after drilling a mounting hole…).

Another issue fixed, a corroded spacer for the 5 V analog supply voltage-limiting Zener (a power Zener, still good!). The spaced could have been cleaned, but not worth the hazzle, so I machined a new one from aluminum alloy.

To the mains sensing and synchronizing circuit – some probing with a scope showed that the U1 comparator (a HP numbered LM339) didn’t work. Replacing it was no easy task, because this area had been affected by the NiCd electrolyte, rendering the solder pads difficult to desolder. Scratching off the corrosion layer from the solder with a screwdriver helped to get some contact with the head, and to finally desolder.

Further probing also showed a few damages to resistors, the wires had come off the main body (seems the corrosion damaged the weld between the resistor case, and the wire).

A quick test – there is a clean sync signal! About 120 Hz, double the mains frequency in this part of Japan. Perfect.

With these basics fixed (except the digital 5 V supply – still waiting for delivery of the diode – just fitted a temporary diode for test purposes), let’s have a look at the CPU board, which is a marvelous piece of engineering, with may TTL, EPROMs, RAMs, etc. – let’s hope we don’t need to fix this complex assembly.

Checking the clock and reset lines – no clock present! After some study of the manual, I understood that the CPU clock is generated from a 20 MHz signal, from the 40 MHz VCO and reference frequency assy. Why is there no 20 MHz? Easy answer, the referency assy has no power except -15 V.

Reason: corroded Molex connector, and broken contacts within. Not easily seen from the outside, but clearly, there is no contact.

Fixed the contacts, at least a few (and ordered more!), and voila, the 20 MHz and 40 MHz are back. No issues with any of the main counters (e.g., keyboard and display scan), good activity on the address bus, etc.

For proper tests, removed the CPU/controller assembly A6 from the unit, and powering it with a lab power supply, to make sure it gets stable power.

After all this, quite some relieve! The unit is starting up, at least as much as it can be told at this point (all the analog circuits disconnected from the A6 controller board – need to first fix their power connections). The startup passes all the RAM and EPROM tests, great! And finally stops at E-50, which means, it can’t find the line sync frequency – which is no surprise, because this signal is currently disconnected.

Next steps will be – (1) Fix the Molex connectors. Quite difficult because these are crimp connections, and the wired have some corrosion making them difficult to solder. (2) Finalize the A7 repairs – the 10 uF capacitors, the Schottky for the digital supply, test the digital supply. (3) Then proceed to the start up and repair-adjustment of the analog circuits, and synthesizer section, many, many, complex circuits, but these don’t have any visible corrosion or damage. Fingers crossed.

Micro-Tel SG-811 Swept Signal Generator: a hot (and golden) driver

Suddenly, my SG-811 microwave generator started to play up, running hot, and then, stopped working with power supply failing. Too bad! Fortunately, the Micro-Tel power supplies have a fairly consistent design for all their various instruments, and I have fixed already several of them, so it will be an easy task. In this case, just needed to replace the main transistors to get it working again (MJ12002 replaced by BU208A, which are much more easily available). But switching on, I immediately noted that something was wrong, too much heat and current around the oscillator control board. This board has a LH0021CK power (1.0 Amp) opamp that is driving the tuning coil of the YIG oscillators. The part seems to be shorted out.

Looking at the schematic, there are current sense resistors for each band, with the sense wires switched by an analog multiplexer.

The LH0021 are not quite rare, but expensive – fortunately, a kind guy from the US offered a pair for these on eBay, NOS or used, for a reasonable price. And some weeks later, they made it to Germany.

These are really nice parts, all gold plated and solid pins.

Replaced the LH0021, and the SG-811 is basically working, but still too much current on the LH0021. What is going wrong? Turns out, there is a oscillator control board inside the RF unit, which is switching the coils depending on the band selected. Probing around, this doesn’t seem to work, because one of the coils, band 3, stays energized all the time.

Easy to find the troublemaker – a shorted switching transistor, a medium power PNP, 2N5193. These 2N5192 are not very common, so let’s do a search for similar parts in by basement archive of obsolete parts – and, in fact, there is a bin of BD438, including a note about their characteristics, and a not from the former owner (a generous old man who didn’t need any electronic parts any more, and had several lifetimes’ worth of supplies).

With the RF unit open and the board pulled out, it’s a good idea to check all the transistors and diodes, and in fact, another one found shorted as well (not a tuning switch, but the main power for one of the oscillators).

With both of these transistors replaced, the SG-811 can be put back into service. Didn’t take all that long to fix, not much longer than to deal with repair quotes, shipment, and other hazzles when repairing more modern units.

…plenty of power, up to 18 GHz…

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.

HP 8642A Signal Generator: Further repairs

After quite a bit of traveling and other time consuming endeavors, back to the 8642A. After the repairs described earlier, the unit was working fine, until some some escaped from the A2 assembly, which has the low frequency source – one of the tantalums blew. No problem, this can happen for a 30 year old unit that hasn’t been used for while. With close inspection, I discovered not only the defective tantalum, but also two resistors to be defective-burnt.

These resistors are in the supply circuits of some of the amplifiers, to remove ripple from the power supplies.

These are the parts, R17 and the blown C17, and R23. Maybe C27 has some issues as well?

The burnt parts marked in red:

… same case for R23. Note that the resistors had about 12 ohms and 15 ohms, so they did not fail open, and this is why the A2 assembly had no functional defect.

Further tests showed that C27 is perfectly fine. Maybe some overload occurred out the output, or the resistor R23 has some weak design (these are only 50 mW resistors!).

The repair is easy, just put in 22 Ohm resistors (0.4 Watt metal film – don’t have any 19.6 Ohm resistors here, but for the given purpose, 22 Ohms is good enough). The 10 uF tantalums C17 and C27, I replaced by 10 uF multilayer ceramic capacitors.

Carrying out the full self test (Shift-Preset-Shift-SPCL-330-Hz).

It takes a while to test the unit, several minutes, but all tests passed with no issue. Modulation output and amplitudes are fine as well.

As you can tell, I have also fixed the backlight (same as before – some high efficiency white LEDs and two 120 Ohm resistors; keep in mind, the common pin for the backlight is ground, and the bulbs are driven by -5 Volts, so, make sure to get the polarity right).

Last but not least – I also replaced the front frame of the instrument, I had a spare back home in Germany, and carried it over to Japan at the occasion of the last visit in Germany, end of March.

HP 8642A Signal Generator: To be, or not to be a parts unit

The HP 8642A is the cheaper brother (or sister) of the 8642B (see here, I have two of the 8642B around and still use them quite a lot, one in Japan, one in Germany), it is essentially the same unit, but the “B” has a built-in doubler to effectively double the frequency range. The 8642A works up to 1057 MHz, good enough for most HAM purposes. It has all the desirable HP high frequency goodies inside, including, a set of precision 140 dB attenuators, and a huge number of parts that would come in handy for repair of other RF gear, everything of highest quality, low noise transistor, high reliability tantalums, a box of cables and connectors, and at least 20 kg of case aluminum. So, I did not hesitate to buy this unit for the scrap price of the aluminum contained. It also has a low distortion modulation source, which is also very useful, and has a may high quality relais and opamps.

The unit is somewhat dirty, seems it had been sitting in some storage room for a while, and looking at the fan inside, it also has seen some hours of operation (which is not necessarily a bad thing).
The front frame has a mechanical damage, some part is missing – fortunately, no damage to the front panel. But I have some spare HP System II frames, let’s see.

Strangely enough, one module is quite shiny, the case aluminum had some other surface treatment – also, it has a later date code (1989), compared to the other units (1985-87). Upon close inspection of the connectors there are slight scratches – seems this module has been replaced. The 8642A had a field repair program based on module exchange (even the specs were guaranteed after such exchange), quite likely that this module had failed after a couple years of service.

After a quick power up test – nothing to report, the unit is not powering up at all. Took all the panels off, and checked the voltages – nothing present. Checking around the rectifiers and capacitors – all is good here, but the voltage regulators (+-5.2 V, +-15 V, and +-50 V) won’t start up, even when I try to force them. Checked the rails – disconnected the cable (ribbon cable) from the supply assy (A17) to the power distribution board. The 15 V line has a hard 0 Ohms short!

15 Minutes later – checked each module. And the shiny one has the short! A bad 10 uF Tantalum (25 V rated, running at 15 V – should usually be good enough). Replaced it with a 15 uF, 25 V Kemet – no 10 uF Tantalums here in my Japan workshop.

Still, before we proceed, let’s be careful with the power supply. Not that it starts up, and has some issues, and all the modules are gone. Easier said than done – there are sense wires going to the power distribution board, and, a ground sense wire going to the rectifier board (A18). I didn’t bother to study the schematic and notes too precisely, there it says: sense ground, connected to a screw and to the chassis. Of course, I had removed this screw, and now wonding why the supply won’t work…

That’s how this screw and trace looks on the schematic.

Fixed it with a jumper wire, still no success.

Fortunately, only minor trouble, a dead Zener in the 15 V crowbar (using a Zener-Thyristor-SCR circuit, marked red below). And, by design of the supply, if the 15 V is dead, all supplies stop.

After this fix, the supply is starting up, and all voltages are accurate to 5 mV, with no adjustments… this is real quality. And with the supply, the unit is starting up, and passing the start up self test, and even the extended self test (preset-shift-330-Hz), no issues.

Not so high quality are the elastomeric materials used – two kind of foam, one of low density, which completly desintegrated to a black glue like substance (same applies to the 8642Bs I have, so it is a material age issue, not related to the storage or use condition). First, scratching off all the old stuff with a credit card. The bottom cover was a mess, so I don’t show pictures (couldn’t touch the camera with the gloves).

Everything cleaned off. Below, these are the craps (including a chocolate bar cover, which you will need after this messy work).

The new foam pieces (not shown), were all cut to the precise shapes, and mounted with double-side tape (carpet tape).

There will be some further repairs needed (the backlight is not working, and I need to get a good front frame from my German junk pile), but some initial tests were done. Phase noise is good, at least as much as I can check, vs. a 8662A, tested at some random frequencies (10 and 56-odd below).

Note, at above 10 kHz, the 8662A has higher phase noise than the 8642A, so the test can only show the overall function and absence of phase noise issues (for the 8642A) above these frequencies.

There are issues with the attenuators. And flatness, see below. Even with a rather crude spectrum analyzer as flatness indicator, all within 1 dB easily, over the full span.

All in all, still a good unit, and I won’t yet use it for parts and spares.

HP 6634A System Power Supply: A few almost-bad RIFA caps, and a 100 Volt, 1 Amp, source-sink supply, and a generous load of transistors

A quick look at a really nice piece of kit, a 100 V, precision regulated power supply, can be floated to +-240 V, and can provide 100 Watts of power, or sink power, about the same range.

The front panel and handling is like any other HP system power supply from this era, and there are models 6632A (20 Volts) and 6633A (50 Volts) that share virtually the same control circuit. All is GPIB controlled, of course, and this unit has front and read outputs. I am going to use it for a capacitor tester (to study the voltage bias effect and hysteresis of ceramic capacitor materials), so I need a fairly reliable unit because it will run unattended for a while.

The top view, there is a massive heatsink, for 100 Watts of dissipation…

The transformer, it is the highest standard and insulation I have ever seen.

There are 8 power transistor, in a really massive output stage (4 complementary pairs, 2 each on each heatsink-the heatsink is sub-divided in two sections), each of them capable of handling 250 Watts of dissipation.

The output stage, it is a really generous design, considering that this is a 1 Amp supply (most designers would handle it with two transistors).

The only thing I don’t like about the unit, the RIFA X and Y rated caps. These are all cracked (still not shorted, but I don’t want to take a chance). So these will need to be replaced.

Otherwise, all is good with this unit, almost no dust inside – I believe this instrument had very low hours, or has been used in a very clean environment (not even a trace of dust on the fan).

HP 3326A Two-Channel Synthesizer: replaced, and replaced again!

Recently, two assemblies of a non-working HP 3326A were fixed by replacing their 15 uF tantalum caps – a good number of them had failed, presumably, because of a bad production lot of these capacitors (see earlier post)

Unfortunately, during the test run, some sporadic failures of the power supply, with overcurrent indication flashing. Then, permanent failure of the -15 V rail – as it turns out, by a short in the assemblies we had just fixed! An again, a discolored tantalum capacitor. Replaced it, and a few hours later, the same issue, with another capacitor of the same kind.
My mistake, I had use a bag of cheap China-sourced 15 uF, 25 Volts dipped tantalums, but these seem to be no good (unlike other Chinese electronics good that have attained good quality in recent years, provided you don’t by the cheapest kind). Maybe it was my mistake to buy the cheapest tantalum capacitors, but not much choice if you need 34 pieces to fix some old equipment – I don’t want to pay EUR 1.45 each from top brand parts from Mouser or similar sources.

With some luck, I found reasonable prices KEMET T350 series Ultradip II capacitors, these are known to be reliable.

You can see the size differences – the KEMET part is much bigger than the Chinese 25 Volts part – it is more similar in size to a 15 Volts KEMET part. Probably, the design was put a bit to the limit.

With the capacitors replaced and another 48 hours of run in test – no issues at all and the 3326A can be considered fixed and working for now.

HP 4191A Impedance Analyzer: power supply fixed

In a recent post,HP 4191A, I have introduced a 4191A with defective power supply. In the meantime, a spare power supply regulator assembly arrived, and the repairs of the originally fitted, defective supply have been completed.

First, some research into the HP p/n 1826-0043 Opamp, which is considered to be a LM307 comparable part. But what is acutally inside the can? Let’s crack it open.

The die has a small marking, reading “LM107C”. The LM107 is very similar to the LM307, except for a wider temperature range, and somewhat extended power supply limits. In the current application, we can safely drop in LM307N opamps. It is also clearly visible that there are no bonds for external frequency compensation (like for the LM301).

The replacement power transistors have been discussed in the earlier post, and now all has been cleaned and new heat conducting grease added.

All the replaced parts marked, after all, I should have also replaced the fuses and fuse holds which show signs of corrosion and bad contact.

A quick test, on the 4191A, left hand side is the working spare, the right hand side, the repaired original supply. I didn’t connect it to the 4191A board, but just to the transformer to confirm the basic working condition without putting the 4191A to any dangers – the repaired supply with but just a spare, and probably it will never be needed… at least not for this unit.

There were no other defects with the 4191A, and now some tests with the 4191A working instrument. It is essentially a very precise one-port vector network analyzer, with a thermostated reflection bridge and some other features that make it suitable for component measurements. There are some (expensive) HP test fixtures, but essentially, you don’t need these because most of the components will need to be tested on some custom boards, or directly soldered to a test connector, to avoid parasitic effects at the frequencies of interest. I just used a set of SMA flange-type connectors.

First some standards were fabricated – a short, by applying a generous amount of solder the connector to completely short it, an open, by cutting off the center pin and machining it flat (maybe need to add a cap or other structure later, to avoid any parasitic capacities, for now, during calibration, you just need to keep the hand away (and any other ground planes). The load 50 Ohms – from two 100 Ohms SMS resistors in parallel.

Some test objects, a 680 pF SMD 0805 capacitor (NP0), a 100 Ohm SMD 0805 resistor, and a wired 100 Ohm metal film transistor.

The test itself, run with some excellent Excel software (by a certain Harry Percival) and Zplots (by Dan Maguire, AC6LA) via GPIB bus.

The 100 Ohm SMD resistor, it has pretty good performance out to 1 GHz.

The wired transistor, inductance is adding to the parasitic behavior.

… same in linear frequency scale.

The 680 pF capacitor.

Also did some drift checks, over 10 hours (after about 2 hours of warm up), I could not find any detectable drift of the Z values, so the instrument seems very stable at least with reference to the stability of the 50 ohms load measurement.

HP 8754A 4 MHz to 1300 MHz Network Analyzer: final repairs, and a function test

Finally, the spare parts arrived, and the repairs of the HP 8754A could be finalized. The LM339 comparators, fitted to the boards…

The cap of the mains filter had many small cracks – replaced. For some reasons, the original filter had a Y-rate cap across the mains supply – Y rating is usually for connection from mains to earth. So I replaced it with a X2 rated cap for service parallel to mains.

Some tests – the 8754a is a very nice unit, because of its instantaneous response to the dial settings, rather than the delay of any digital network analyzer. Even the most modern of all units still don’t such a direct feel compared to the fully-analog 8754a.