HP/Agilent 6060A System DC Electronic Load: a quick repair

This is a 300 Watts, 60 Volts, 60 Amp electronic load, a quite handy device to have, especially, a HP/Agilent brand item. There are many cheap electronic loads, but I would rather recommend to get a good instrument, if you want to put some power supplies to real tests. Otherwise, you load may fail earlier than the supply.

The instrument we are dealing with here, a low cost auction fid – it had a bad front connector. These instrument use HP 60 Amp binding posts, these are quite rare and expensive (about EUR 40 per piece from Keysight), and the plastic gets brittle over time, and with overtightening it can break. The instrument had front and rear connections, I only need one set – so it will be an easy repair by just moving the good binding posts to the front.

Also, we find that all the X and Y rated capacitors have hair cracks, and are of RIFA brand, so these may fail soon – let’s replace them all.

The power is dissipated in several MOSFETs, all mounted to a large heatsink. Essentially, a small 300 Watts room heater, which is great to have these days in cold Japan.

The front connectors, after repair (just moved the rear connectors to the front, rear connectors, I don’t need them).

New caps soldered in – quite a difficult task because some vias are part of large copper fills, without thermal relieve, and I don’t want to preheat the whole board.

Finally managed to solder-in the X and Y capacitors.

A test at 40 Volts, 6 Amps, running for several hours with no issues at all!

Agilent 4352B VCO/PLL Signal Analyzer: working!

After a short xmas vacation, several spare parts arrived, including, 10 amp solder-in fuses, and thermal glue (704 silicon glue).

The glue is needed to mount the defective/blown thermal fuse to the power resistor. This resistor usually stays cool but will heat up in case of a power supply failure.

The fuse protects the primary of the switchmode transformer, it is a 10 Amp fuse, and it took a while to find it – it is located in a hidden place underneath the transformer.

Now, with the fuse installed, the thermal fuse glued to the resistor, and the two drive mostfets replaced, the Artsyn 24 Volts supply is starting up just fine. All self-tests passed!

Next step, let’s update the firmware, and do some tests.

The firmware version 2.11 is the latest one available, but it needs to be loaded from a 3.5 inch floppy – I have a USB floppy drive here, and one single disc which I purchased from Sri Lanka. Took a few attempts to convince the 4352B to read the disc and load the firmware. But finally, success!

Many tests could be done, here just a simple test with a 15 MHz signal from a 3585A vs. a 8642B generator. Seems to work well, and easy to use.

Now we can close the case, and use the device for VCO characterization, phase noise measurement, etc.

HP 6038A System Power Supply: all fixed!

After some weeks, the spare parts arrived – RIFA X2/Y2 rated capacitors (now made by Kemet), a full set (see earlier post, 6038a repair).

The new X2 capacitor, let’s hope RIFA has improved the resin and durability. Albeit, the old capacitors lasted for a long time…

And a fan, from China. The fan, upon close inspection, it has a broken frame, but fair enough, I will use this one while a replacement is on the way.

A lot of dust removed from the case and boards, all completely disassembled. The X and Y capacitors all replaced – the old capacitors are still working, but cracked and it is good practice to replace them, unless, you want to risk a lot of smoke and stench (usually, at least no fire risk).

Always good to use high quality tools – I only have low quality tools here, and bits that crack!

All cleaned and put together…

…finally, some testing. It is working, the fan is providing a substantial amount of cooling, it is definitely big enough for the unit.

HP 3326 Two-Channel Synthesizer: a bad lot of tantalum capacitors

This HP 3326A was found for a ridiculously low price, non-working, so I decided to pick it up, in case I need some spares for my good 3326A, or as a source for some HP parts. But when it arrived, it was in such good shape that a repair appeared worthwhile.

The symptom, it just doesn’t start up, the +15 Volt and -15 V rails shorted. Brief check showed that the power supply is working. There must be a short somewhere in one of the modules. How to find such short?

The 3326A has a cast aluminum cage construction, which houses all the modules in separate cavities, all heavy cast metal! To find the defective module, we first have to undo 100s of screws… and usually the last module will be the one at fault.

Half an hour later – found that both (!) phase detector boards – these are identical for channel A and B – have shorts.

Some more probing later – the reason a couple of shorted tantalum caps, 15 microfarads.

These are in general high quality capacitors, but it must have been a bad batch. So, let’s desolder all the 15 uF caps, and solder in new ones.

Even with the dead caps cut-out, the 3326A is working again! No issues with any of the self tests, including the service self test (push button self test-%-6 to activate).

Some maintenance is also needed on the power supply. Checked all the transistors which are known to develop issues with the sockets. And added some thermal compound to the 5 Volt rail transistor which was running a bit hot.

Also, the connector to the transformer has the typical bad soldering and cracked solder joints, all now re-soldered with a generous amount of good old lead containing solder.

This unit even has option 001, the precision ovenized reference.

There is no manufacturer datasheet available for this Japanese OCXO, but the HP manual has all the data. It is not an ultra-stable timing standard, but by far good enough for a two tone synthesizer.

HP 8754A Network Analyzer: gold, sapphire and still low output

Some performance validation of the recently fixed 8754A revealed that the output is leveled at 0 dBm, but it doesn’t provide any more than 2 dBm, when you turn the knob to higher levels… it should provide at least +10 dBm leveled, and +13 dBm typical.

So, what is wrong? The signal source is mostly located on the A7 assembly, two VCOs, a mixer, and several amplifiers and levelers.

The osciallator is working, as we can see, but the amplifier circuit, a golden box, number 5086-7235, is not amplifying sufficiently. HP did not consider this field-repairable, so the manual only has some rough information about its contents.

To find out more, we need to crack it open – it is not welded, but glued with a generous amount of silver epoxy.

From right to left, the preamplifier, a filter (LC low pass to remove the VCOs and higher mixer products), and the power amplifier with detector and a -20 dB tap for the PLL-marker circuit.

I checked all the bias voltages and currents, these seem OK. The main amp substrate (sapphire?) has a crack, but is it not fully going through the material, and the gold layer is thick, and the crack is not cutting through the critical sections.

The filter inductances, gold traces on alumina, with some bonding wire. I assume, hand made… It is a pitty I don’t have a microfabrication facility at my disposal, and a wire bonding machine…

Various testing has been performed, to find out the power levels, using a 50 MHz precision source, and a fine tipped probe to check the levels on the substrate (using a microscope, and a steady hand to avoid damaging the bond wires).

From the data it is obvious that the preamp is not amplifying, but absorbing power. This is good and bad, because the final amp needs to provide clean amplification to avoid spurious, so I don’t want to mess with it, and there is also the detector diode, which is essential for the flatness of the unit, also something that is not easily fixed, if you introduce some parasitic resonances or the like.

The fix – scraped off the transistors and most of the gold from the preamp, and soldered a short wire across the substrate, I think it is about 50 ohms impedance. Then, I inserted a set of 2 integrated microwave amplifiers (a MSA-0505 and MSA-0386) to provide about 19 dB gain.

The maximum output +13 dBm at virtually all frequencies (a small dip around 1 GHz).

A test at various power levels, with a good spectrum analyzer (don’t have a calibrated power meter here, but this analyzer is pretty well calibrated). Amplitude is 1 dB per division. The 8754A is calibrated at 0 dBm and 10 dBm, at 50 MHz.

0 dBm leveled output, 1 to 1400 MHz… pretty good.

10 dBm output, also, great flatness.

Finally, at test at 5 dBm – it’s accurate and flat!

Now, we will let this run for several hours at maximum output, to see if the repair is permanent, then the amps will be sealed with epoxy (just plain epoxy, no silver epoxy).

HP 5086-7803 YIG tuned Filter and Switch: “SYTF” details and adjustment

With the repair of many spectrum analyzers, it turns out, the preselectors are usually not easily damaged, because of their self limiting characteristics, and because of the absence of active parts. The SYTF is diffient in that it has an active part, a switch.

It is already some years old, but no reason why such parts should have much aging at all.

The symptom of this unit, it is working in the low frequency region setting, LOW band out, but the high band is dead, loss in excess of 35 dB, at all frequencies, and independent of the tuning current supplied.

So my first assessment was, this unit needs replacing, and I found a replacement part online, from a US seller, not cheap, but OK, the 8561E analyzer is worth it, if it is working again after the repair. Unfortunately, several week of waiting were all for nothing – the seller shipped the wrong part and it took a while to get the money back, but finally all settled, except, we still have the defective filter.

Let’s try to investigate the nature of the defect, and open it up. Fortunately, these filters are not hermetically welded like some other YIG parts.

You can clearly see the coil, the inlet and outlets (low and high band) by rigid SMA cable (1 mm size!).

First, let’s study the switch. It is not actually switching the high band, as I originally assumed, but it is switching on and off the low band.

It is a series-shunt-series type FET switch, controlled by about -10 V negative voltage (1 kOHM vs. -15 V connection is the usual control method, floating or ground to switch off).

I could not find the exact die and model for this switch, but there are many similar models that clearly show the structure. The shunt and double series construction will provide very high isolation.

After removing carefully the gold mesh (it is only lightly glued on, I will use some tiny traces of epoxy to stick it back on), some study under the microscope.

Clearly, the spheres are misaligned! The spheres must be placed in the center of the coupling loop, to allow for RF to couple. Generally speaking, during alignment, the sphere is only turned, and then the position fixed by some epoxy – which all seems to be intact, and solidly fixed. So what has happended? I think it has to do with the mounting blocks, which are of different material compared to the based (which needs to be magnetic Fe-Nickel alloy). With frequent temperature cycling, I believe there is some migration of the mounting blocks, fractions of a micrometer every time (keep in mind, the YIG spheres are heated during operation). While we can speculate about the reason of the migration, the result is clear, and the action as well: we need to realign the spheres.

I decided not to undo the screws because the coupling loops can be easily damaged, and used a screwdriver to carefully push the mounting blocks away from the coupling section, bit by bit, under control with a microscope.

Finally managed to get all the sphere properly aligned. If you don’t know how it works, never turn the YIG spheres! These need to be aligned for thermal stability effects, not only amplitude – something which you may have trouble doing at home.

After all the alignment, a quick test setup, with a current source for the main coil, and another supply for the heater and switch connection. Note that the current source is set in parallel with a capacitor (22 uF) to allow for stable regulation with the strongly inductive load.

The insertion loss test – done by checking at several frequencies, using the lines of a good comb generator.

The insertion loss, in my simple setup, it is about 4-6 dB for a 3 stage filter, not bad. I don’t know the original performance spec, but it is definitely in the typical range of such filters, and good enough for a spectrum analyzer in any case (in the worst case, we will loose 1-2 dB of sensitivity). Maybe I will eventually find a new filter at a reasonable price, to check it – it could also have moved spheres.

The tuning current is very linear, I don’t expect any issues with using this part in the analyzer (the tuning characteristics can be programmed and stored in the EEPROM of the analyzer, to control its DAC appropriately, also, we will need to recalibrate the flatness).

HP 8754A 4 MHz to 1300 MHz Network Analyzer: an analog computer, and a few rusted transistors

There is no specific need for a 1.3 GHz Network Analyzer in my workshop, because there are already several more modern instruments, but this HP 8754A is a real marvel, it was original designed as a “moderately priced, compact” type network analyzer, whatever was considered moderate by HP at the time (maybe the value of two or three small cars?). Finding the offer for a rediculouly low price, for a non-working unit, on Yahoo Japan, I could not resist to place a very moderate bid. Turns out, I was to only bidder, in whole Japan. My original thought was to use it for some experiments, and then, use it a as a source of HP spare parts (there are many FETs, Opamps, transitors, etc. in this machine).

Once the unit arrived, I powered it up, only to find out two things – the -10 Volt and +5 Volt power supplies are not working. And the CRT is very good and sharp. Maybe not many hours of use. Also, the unit is generally clean and in original condition – no other repair attemped. Even the HP instrument feet were included.

The -10 Volt, it required some troubleshooting of the low voltage assembly (corroded transistor legs), see below. The +5 supply, the issue could be traced to a defective TO-3 HP 1820-0430 integrated regulator, alias LM309K.

This regulator is mounted on an aluminum plate in the chassis, with some rather thick ceramic insulator, and what appeared to be only traced of thermal grease. Usually, these are protected against short and overheating, maybe, it was just running a bit hot for year, eventually, accelerating aging and finally triggering natural/random failure with no external even. We will never know, we only know, we have to fix it.

The LM309K tends to become rare and expensive, I still have some back at the Ludwigshafen, Germany workshop, but not here in my temporary Japanese workshop. Checking the offers, I found some very inexpensive LM323K.

The LM323K, it is a very similar device – just higher current capability. It is not critical for the 8754A, the 5 V rail is only loaded by about 0.25-0.3 Amp (as checked with a power supply).

Now, to the corroded transistors. This only seems to affect the boards thats are close to the air inlet, maybe some contamination from ambient air (salt?) is accelerating the effect, related to gold plated steel wire leg transistor. Other transistors have copper, or special alloy wire, but especially the “4-404” and 2N2222A transistors used by HP in the late 1970s seems to be affected by this phenomenon. Not so much in dry countries, but in instruments subject to humind and salty (sea?) conditions here in Japan – just a few km from the cost in most cases.

The 4-404 transistor, alias SS9333, 1854-0404 HP part number, it is a kind of mystery, no data available, and I have seen this part in may Hp instruments, always replacing them with some 4-404 scavenged from part units, etc. But for the 8754A, should we really buy some expensive old HP parts or wait for a long time to go back to Germany to the parts storage? Time for some characterization – found one good, only slightly rusted 4-404, and did some gain, DC performance and frequency response tests at typical currents.

Some basic data could be found – nothing special, the voltage rating rather moderate, and power rating, as well.

Some tests and calculations, it is medium to high gain NPN transistor. BC337-25 or BC337-40 can be valid replacments, I used BC337-25, selected for a gain of 250-300.

High frequency performance is nothing special, it can be easily met by a BC337.

Several 2N2222A are a bit easier to replace – just replaced the TO-18 metal can units with some generic TO-92 2N2222A (or whatever silicon fragments the mass producers put into the 2N2222A case nowadays).

After these initial fixes, the power is up, and the transistors all good. Initial assessment –
(1) front panel “analog computer” is working, some contact cleaner will do the trick, there is no mechanical damage
(2) the CRT will need a filter, it is missing.
(3) the RF output seems to work, at least there is power – need to check with a counter and properly align the linearity, etc.
(4) The VCO and PLL of the receivers seems to have some trouble, but the samplers are working! That’s a relieve.

See below this is a 35 MHz input signal, sampled with the VCO at about 33 MHz, giving a 1 MHz frequency.

So, what is wrong with the PLL? The PLL, it’s purpose is to have a line of a comb generator/multiplier (which is generating the sampler pulses by a step recovery diode) always 1 MHz away from the RF, to give a 1 MHz IF for the R, A and B channels.

This is achieved by first pretuning the VCO, by setting a frequency close to the needed multiple of the VCO, then the PLL is activated and phase lock achieved.

The phase detector, first, I thought is not working, because there is no proper output. But once desoldered, all the transistors tested OK.

The pretune, also this seems to be working, but hold – it is working too well! It is overruling the phase detector.

Further study shows that there are FET switched controlled by a logic signal, via a LM339 comparator. And, as it turns out, the LM339 is dead (both switches on)…

Temporarily fixed the issue by disabling the pre-tune, and enabling the PLL – and, it does lock (albeit, not a fast sweeps – which needs the pretune). But it works of you slowly increase the frequency starting from 0 MHz (this way, you can even measure as 1000 MHz, phase locked!).

After the PLL had been fixed, still some more issues – the R channel detector is not giving a proper output (switching the A8 and A11 boards showed, that the A8 board, which is the same board but used for the A-B channel is working!). Also some issues with the IF switching of the A/B channel, let’s fix this first. The IF switch is part of the A6 assembly mentioned before (which has the VCO and comb generator-diode pulser). To check it, without any fancy extender boards, you can just solder a few wires to the board. I generally prefer solid core telephone wire, this has a very strong and thin insulation, and doesn’t cause shorts easily, because of the single, solid core.

Also here, a dead LM339! Hardwired it for now to conduct the A channel IF.Ordered some LM339N, 10 pcs for USD 1.37.

Now, a few general views, top view:

You can see the card cages, power supply, and the RF sections with oscillators, mixers, samplers.

The bottom side, there are several dangerous DC voltages exposed, don’t touch!

The remaining issue, fixing the R detector and log amplifier, assembly A11. After some probing and thanks to having a working assembly (the A8 A/B detector assembly), the fault could be traced to the log amplifier, and furtunately, not to the transistor pair, which would be very difficult to source or replace, but to the reference amplifier, U2. This is a simple LM301, alias HP 1820-0223.

The LM301, a really early Signetics model! Unfortunately, it is dead, the inputs are somehow leaking negative current.

I already have some LM301 on order, but for the time being, used an old LM301, slightly rusted that I had desoldered recently from a 4191A power supply.

After all these fixed, the unit’s basic functions have all been restored. Sure, there will be through alignement and check, but I will do this once the LM339 and LM301 have made it to Japan. Checked for other issues, by running the units for several hours – very stable. To track the frequency stability, I used a 830 MHz bandpass filter.

Working!!

Also, the instrument originally came with a plastic printed Smith chart that can be attached to the CRT. Wanted to print one, or have one made by photo printing on lightsetting film. But this is more for decorative purposes, and can be done later.

HP 6038A System Power Supply: A 150 Watt Option (Option 100)

The 6038A is a very capable switchmode power supply, which features great reliability, 10 Amps of current, up to 60 Volts, and 200 Watts – for the regular unit. The unit discussed here is an Option “100” unit, this means, it can work here in Japan, with 100 Volts AC mains voltage (50 or 60 Hz, depends on where you are in Japan…) – at 75% of the rated power, say 150 Watts max.

Well, 150 Watts is fair enough for my purposes, and I got this unit for next to nothing, “doesn’t power on, blows fuse”.

Indeed, the fuse was blown. An it has low resistance when measuring across the mains. Difficult to find the issue.

Looking at the datasheet, it is definitely worth the repair. New, it was around USD 3000 list price (maybe more like USD 5k in nowadays dollars)!

Somehow, I could not find anything wrong with the power board. Maybe a short on the main board?

That’s the main power board – checked all components, no issues, no specific signs of heat or excessive aging. Anyway, we have to take this thing apart for thorough cleaning.

Finally, after a lot of probing – the short disappeared. How can it be? After even more probing – it turned out that the fan (!!) had a hard short. This fan is a SU2A5 fan, quite common in HP equipment of that time, but pretty rare and expensive nowadays, and I really don’t want to fit an old fan, but rather a new part with new bearings.

After quite some study, I found a good offer for a NMB B30 fan, which is quite similar.

Best to compare not only the numbers, but the full pressure vs. flow curve, because the instruments has many cavities and corners, so the flow resistance can be quite substantial. But as it turns out, the B30 design is a high pressure fan, it will meet or exceed the performance of the original part.

Now, we have to wait for the delivery of the fan – at least, I tested the supply without the fan, and it does work and start up. So it is confirmed, the repair will be worth the effort.

Additionally, all the X and Y rated capacitors are RIFA type, of the cracking epoxy-coated series. They will all need to go, and will be replaced with new RIFA caps – hope they have improved the design – at least, these will last another 20 years.

HP 4276A LCR Meter: several issues, and several animals found

The HP Yokogawa (Made in Japan) LCR meters are still very useful, these are 0.1% basic accuracy LCR meters – with various test frequencies from 100 Hz to 20 kHz. And, you can apply DC bias as needed to test ceramic capacitors, diodes, etc.

I would this meter for a marginal price, in Japan. It arrived in good overall state and well packaged, but very dirty inside. A quick review:

(1) Power supply seems OK, but the fan is not working.
(2) Display shows overrange and self test shows errors in the analog circuit. In some ranges, I get a reading but it is far off.
(3) Dirt, dirt, dirt – this instrument needs to be fully disassembled and cleaned, no doubt, and the front panel needs special treatment to remove all stickers and residues.

The top view – a lot of dust, and of the sticky kind.

In the corners and gaps, a wasp, and a dried frog. Probably, this instrument has been stored in some shed?

The main board – first, brushed of the dirt, then soaked it in 50% isopropanol. Take care of the fan hybrid, which is a thin ceramic plate – overall, a rather delicate construction. Not sure why they did’t just use regular components rather than a hybrid for the fan drive – there is a huge amount of unused space in the case!

The fan, it is a brushless fan, Buehler, which needs an external driver to commutate the phase.

The test setup – with an external 8 Volts supply. Turns out, the fan starts if we move it a bit. Probably, some issue with the driver.

A good phase – you clearly see the switching action of the transistor, grounding the phase wire for some time to move the fan forward.

After some probing, it is clear that one of the 3 phases isn’t working!

Tested all the timing capacitors of the driver – all good!

But – one of the transistors, SMD, marked “B15” is not OK! B15, this is a 2n2222a equivalent transistor – watch out, there are different pinouts.

Unfortunately, I had no SMD NPN transistor of suitable kind at hand, so I soldered in a 2N5551, for test purposes.

It is working – the transistor has a bit less strong/fast switching action, but no problem, the fan is working again!

Next – we have to fix the analog circuit. This is a nice board, with several hybrids. First, we check the power supplies – HP designed this to have all individual +-12 Volt regulators, for each sub-section (all sections are nicely labeled, and clearly identified).

After some difficult probing – found one bad 12 V regulator! It even had some traces of corrosion – maybe it was running hot or some other failure.

Some test – still not working – at least not in all ranges. Seems the input amplifier is clipping the signal. Clearly, something is wrong with the range switching.

… about 1 hour later, traced the signal back to the range selector logic, one bit is not getting through – it got stuck at the edge connector of the CPU board – the edge connector shows some residue and oxidation. Cleaned it thoroughly with an eraser, and alcohol.

Finally, self test passed!

… testing a 22 nF capacitor.

It is a really great instrument to test ceramic capacitors, and the impact of DC bias on their capacity!

As a last measure, inspected all the X and Y rated capacitors – it good practice to do so with any old instrument. These are safety capacitors, and they often fail with age and use.

The power supply cover is connected to the mains with a Y rated capacitor, and these don’t look good.

.. fitted new Y capacitors – feeling much safer now!

HP 8561E Spectrum Analyzer: 100/300/600 MHz system fixed!

Progress with the HP 8561E – the 100 MHz to 600 MHz system, driving the 2nd converter, and other sub-system including the 300 MHz output.

Initially, it looked like a failure of the 600 MHz doubler, so I decided to open up the RF case of the A15 assembly, and to go to component level troubleshooting.

According to the block diagram, the 8561E first uses a trippler to convert the 100 MHz VCO output to 300 MHz, and then a doubler to convert the 300 MHz to 600 MHz. Note that there are various revisions of this board, not all use the same frequency multiplication scheme.

Very soon it became clear the the double is not getting enough 300 MHz power to work. So, to check it, I injected a 300 MHz signal after opening the signal chain after the tripler (there is an attenuator, just desoldered the middle resistor of the PI configuration attenuator, and checked all the components around this area)

Only about 2 dBm are needed to drive the tripler, there is an amp stage in front of the doubler. A quick test – the doubler is working just fine!

So, probably a fault in the trippler? At least, there is 300 MHz present. What is going on? Let’s go one step further back – removed the tripler transistor, marked “Hb” which is a NE85635 transistor.

Let’s drive the 300 MHz circuit from an external generator. This is running at quite high power – about 18 dBm!

With the injected signal, all is working fine! So, the 300 MHz transistor probably failed? By luck and coincidence, I found a spare 2sc3603 transistor, marked “Oq”, and soldered it in.
Surprisingly, the old transistor, once desoldered, tested just fine. And, to confirm this, the 8561E still not working!

The 300 MHz system can be conveniently monitor by checking the 300 MHz cal output with another (working) spectrum analyzer.

Well, we need to go back one more step – to the 100 MHz amp.
The tricky fact – the 100 MHz system is working, but after some careful measurements and calculations (I don’t have a precise active probe here), the power at the output of the 100 MHz amp is clearly low. This needs to deliver well over 10 dBm of power, otherwise, all the following systems won’t work properly.

Fortunately, the 100 MHz amp is a fairly common part, a MSA-0505 gain block.

These MSA-0505 are used in many HP circuits, just took one from an old board:

The A15 board, still with the 100 MHz gain block, and the tripler transistor replaced…

A last step – replaced the Oq transistor, with the old/original HP part (300 MHz circuit).

Finally, some tests – the 8561E (at least the low band up to 3 GHz is working again, and the CAL output is in spec (0.02 dB difference to another calibrated 300 MHz source!).

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