Latching Coax Relais Driver: useful for all kinds of HP/Agilent/Keysight attenuators and switches

Driving a latching relais doesn’t sound all that complicated, and it isn’t, still, some thought is needed to get it working properly. This is what the circuit should do

(1) Accept one 5 V TTL input signal, LO signal will result in the relais switched to the rest position, HI signal to the activited position (“dash” position), the switching signals are called A and A’, for relais A.

(2) There should be an output-enable signal for all relais, to be able to de-energize the relais, because otherwise, the circuit can suffer if the relais is disconnected while powered; these latching relais only need a few 10s of Milliseconds to switch, power will be turned off, by the ATmega32L, 1 sec or so, after each switching event, just to avoid any unintentional short circuit or spike, and the positive supply line will carry a fuse.

(3) The circuit should be able to handle any common relais voltage, 5, 12, 15, 18, 24 and 28 V. The relais I am using here are 24 V rated, but work fine at 18-20 V, and up to 28 V.

(4) It must be avoided, at all cost that both the A and A’ signals are active at the same time. This can cause nasty relais oscillation, and damage/wear of the expensive relais. We don’t want to rely on software control here, because things can go wrong, during startup, program freeze, and so on.

coax relais driver schematic

The circuit is almost self-explanatory. The LS04 generates the complementary control signals, from the digital input. The LS373 is configured as a transparent latch, in fact, the latch function is not used, but the output control function is. If the OC input goes LO (and thus, the neg_OC high), the outputs of the LS373 will tri-state High-Z.

The output driver, a Toshiba ULN2803APG is used. These are very reliable circuits, Darlington transistor arrays with build-in diodes.


Make sure to put a good quality capacitor close to the COM pin, the return path for the snubber diodes, otherwise, the inductance of the traces will deteriorate the action of the diodes. Actually, the HP coax relais have build-in diodes, so I don’t expect and issues with voltage spikes here.

Note that the inputs of the ULN2803 have adequate resistors to ensure LO state when connected to a High-Z output; if other transistor arrays are used, the circuit might need some ~10 kOhm pull-down resistors to ensure noise immunity and defined signal levels.
The 15 kOhm input resistors serve the same purpose, they keep the relais de-energized if no logic input is connected.

This is the little board, with all space well used. Sure the logic could build using more modern devices, and I would definitely do so for a commercial device, or anything build in series, but LS logic is still popular, and it is cheap, at hand, and reliable.

coax relais board

That’s the setup, with the two 8673B transfer switches, and a 0.3 Hz test signal fed to the digital inputs by the brown wire shown above (removed for the picture below).

coax switch setup

The signal at one of the contacts, 24 V drive level, 1 V per vertical DIV, 10 ms per horizontal DIV. No spikes, high voltages, or similar undesirable features are present.

coax switching

After about 2000 switching events, two events per 3.3 seconds, everything is still working well, and all circuits, in particular, the ULN2803 stays cool.
As I said, not a very complex circuit, but may be useful for other purposes, and is a very cost-effective, reliable solution.

Tripple PLL Controller, for Micro-Tel SG-811/1295/1295: converting a ratsnest to a shiny box

Comming back to an earlier project, the frequency stabilizers/PLL controls for the Micro-Tel 1295 receiver (2 pcs, one for through, and one for reflected power), and the Micro-Tel SG-811 generator (0.01-18 GHz).
The setup has been working well, but it is an awful mess of cables, as a result of the development process.

18 ghz pll wire mess

Now is a good time to finalize the circuit, and to put it into a nice case.

pll controller case

This is the front panel – the back panel has numerous BNC connectors, for the “PLL Phase Lock” voltage, and the “Frequency Control”, i.e., coarse tune voltage. The latter voltages, 3x 0-10 Volts, are generated by a DAC board, which is now ready and tested. Not a thing of beauty, but it works, and no reason to assemble another one just to clean things up a bit.

dac board

The circuit is reasonably straightforward, a 2.5 V precision reference is used, and three MAX541 16 bit DACs, followed by OP284 amplifier to convert the signal to the 0-10 V range.
The MAX541, it’s a really great part, full 16 bits, +-1 bit integral nonlinearity, and very stable over temperature and time. Highly recommended for any precision application.
One of the channels has an additional TLC2201 opamp, which is there more for historic reasons than for anything else.

dac board schematic

The PLLs use three ADF41020 18 GHz chips, following the approach discussed earlier. This will give 100 kHz frequency resolution, which is more than sufficient for the intended purpose of measuring gain/attenuation and SWR over the full range of microwave frequencies up to 18 GHz, or 40 GHz, if no SWR measurement is required.

The 10 MHz reference circuit has also been completed, and is working well, converting a 10 MHz input signal (of more or less arbitray shape and amplitude) to 3x 10 MHz REF signals for the ADF41020, and two 5 MHz outputs, for the 1295 receivers, just in case I need to work with external downconversion-the 5 MHz signal is only affecting the 0.01-2 GHz range of frequencies. Well, why not having everything phase locked, if we can.

10 mhz 5 mhz ref circuit schematic

The 10 MHz circuit has been modified slighly, to accept 1 kOhm impedance 10 MHz input signals, which are rather common for instrumentation purposes. Some caps and protection circuitry has been added, output levels are just about 0 dBm. No need to go to stronger signals, it will only cause spurious response, and other trouble, because shielding inside of the PLL case won’t be very strong.

Some remaining work, before we can call this project complete:

(1) A power supply regulator, +5, +3, +3 (Vp low noise), +15, +20 supply has been build, but need to be mounted to the case, for heat sinking. This will need to wait for a few weeks, need to wait for some travel to Germany coming up, where the metal working machinery resides.

(2) Digital interface and cables for band control of the SG-811, and the two 1295s are complete, just need to be mounted and tested.

(3) Relais control module needs to be designed and build – there are two 18 GHz transfer switches (one is needed for basic functionality, a second one for the various unexpected test configurations). The transfer switches are HP 8763B, see earlier post. These are wonderfully precise and repeatable, albeit, list price is about USD 1k, each.

(4) Need to route the two serial signals (dB reading, signal strength) from the Micro-Tel 1295 to the ATMega32L, which only one USART input – no problem, because this is all TTL-level USART signals, but need to get a multiplexer/a few NAND gates to set this up. Currently, only one 1295 can be recorded at a time, which means, either gain or SWR measurement.

(5) Sure, the software, both firmware and instrument control software will need to be refined and consolidated. All PLL calculations and adjustments are done within the ATMega32L, still there is a need for a more convenient user interface than just command line.

… to be continued!

HP (Agilent Keysight) 8970A Noise Figure Meter: a makeshift noise source, and some test

With the 8970A back working, what would be the first thing to do with it – well, let’s measure some gains and noise figures. Unfortunately, the 8970A alone won’t be sufficient, because it uses a small, external noise source unit, commonly refered to as a 346A, B (or C model, if you need noise up to 26.5 GHz). These sources are still widely used, although Keysight has introduced a new series, the N4000 series, but still the 346 models are very common, and available – this product has been around for 30+ years, not bad. The only downside – most of them seem to get lost or damaged, so they are rare on the second hand market, at least, if you don’t want to pay more then USD 0.5k for a used, out of cal, and scratched item, for a device that sells for USD 2.5k brand new.

Key characteristics of a noise source for noise figure measurements, and related tasks:

(1) The connector, preferably, get a 3.5 mm APC, then you can add a connector saver, and most of the small devices being characterized are SMA or 3.5 mm design; sure, have a few adapters at hand, or a SMA to N cable. A noise source with N connector is more sturdy, but also these connectors wear out, and aren’t all that hand except for directly connecting the noise source to the analyzer, which is not often done. Typically, the device-under-test (DUT) is connected with some short test cables anyway, and for calibration, you just remove the DUT, rather than all the cables.

(2) Flatness. The noise output needs to be so-called white noise, absolutely flat with frequency.

(3) Related to flatness, very low SWR. The various common DUTs, amplifier, mixers tend to have not too good SWR, so at least the noise source needs to have low SWR, otherwise, measurement errors will be enormous. Also, the SWR needs to be close, or the same, irrespective of the on or off state of the noise source.

(4) Well-known absolute noise power, measured in ENR, which is noise above a 290 K floor, -174 dBm/Hz (a 1 Hz bandwidth power density). 290 K is the Kelvin temperature of an average antenna on the surface of an average place on earth. Well, where are these average places that are at these constant 290 K…

(5) The driver input, commonly, a BNC connector that is driven by a 28 V DC signal. Most sources adopt this style of input.

This is one of these desirable items, in the typical used condition. Very similar device are available from Anritsu, NoiseCom, and others. The 346B has 15 ENR output, which is a good amount for general purpose application, maybe a bit too much for certain GaAs preamps, or other low level low noise applications; then you can just add a good (really low SWR) 10 dB attenuator.
346b noise source used

…unfortunatly, I currently don’t own any of these extremely broad-band calibrated and well-working sources, and need to deal with less fancy apparatus, but let’s at least investigate what it is all about.

The block diagram (taken from the April 1983 issue, of the HP Journal,, shows the internal construction, still looking for a schematic of the current source, it seems to convert the positive 28 V signal, to a negative current, looking at the polarity of the noise diode. Maybe more about this later; to get proper accuracy and repeatability, it is a must to have a very constant bias current supply, on the order of 8 to 10 mA. It should provide a low noise DC current, without any large buffer caps, because the 8970A will switch it on and off periodically, to do the actual noise figure measurement. But there plenty of circuits around to accomplish this.

346b noise source block diagr

Most interesting, the matching network. Noise diodes have about 15-30 Ohms impedance, so this all makes sense. The strange stub is one of the secrets (the major secret) that ensures the 18 GHz flat output. The 6 dB attenuator improves the output SWR and SWR change from on to off condition. In fact, it is a good idea for any noise source design to have a high quality attenuator at its output, with at least 6 dB, or a bit more.
346b noise source matching

To replicate the 346B design, or at least a similar design that is good to a few GHz will remain a venture for future cold winters (good designs have been published by others but they all appear to lack flatness, and some use pretty costly noise diodes, and all need calibration that is not easily achieved unless you have access to a calibrated source).

For work demanding less accuracy, many design are pretty suitable to get reasonably flat noise of the desired power, in the 10s to 100s of MHz range.
This is one of the circuits that I have successfully employed.

simple rf noise source

It uses the breakdown of the emitter-base diode, according to the datasheet, about 5 to 6 Volts for any common NPN transistor (minimum values, actual breakdown might not occur up to 8-10 Volts). The noise source currently in use has a BC238B transistor, because it was the first one to grabbed from the junk box. Others will work as well, including BC107, BC548, 2N2222, 2N2904, and so on. The latter two appear to have a higher breakdown voltage. Obiously, there is no bias current regulator, and the 5k6 resistor will need to be adjusted to get the right level and flatness of noise in the desired range of frequencies. Sure, better results can result from a RF transistor like the BFR93, or other 4 GHz, or even 22 GHz type transistors – will give it a try back home in Germany because it really only makes sense in a proper RF setup, and on a small test board.

noise bc238

A quick test, to determine the gain and noise figure of a 6 dB attenuator. Attenuators have negative gain, equal to their attenuation value, and increase the noise figure of a system by the same magnitude.

8970a 6 db atten test 2

For the time being, let’s call it close enough. With the simple noise source, calibration works perfectly fine from well below 100 MHz, to above 1 GHz, making it suitable for various general purpose application.

HP (Agilent Keysight) 8970A Noise Figure Meter: getting started, Error E26

Gain and noise figure are criticial performance parameters for amplifier, mixers, and so on. In my lab, these parameters have so far been determined mostly to a qualitative (i.e., approximate) degree of accuracy, by using either general purpose equipment, or specially build test setups, for the microwave region. Such approach comes with a lot of time lost, difficulty, and lack of absolute measure. Time to improve the noise figure measurement capabilities.

Which instrument to take, well, there are a few options, mainly HP 8970A/8970B and Eaton/Ailtech/Maury (the 2075 model). Remarkably, the 2075 is actually superior the 8970x units, and also had a lower list price, considering that the HP 8970x sold for about USD 10k.
The downside of the 2075, and in particular the more advanced B and C models, there doesn’t seem to be any documentation around, no schematics could be found (if you have a service manual of an Eaton 2075B, or Maury 2075C, please let me know); for the HP 8970A, full manuals and several application notes are around.

As luck would have it, I recently found a unit on xbay, a 8970A, non-working condition, error E26. This error indicated that the IF attentuator calibration has been lost, typically, it is associated with a low battery of the non-volatile memory.

8970a error E26

Note the Keysight cal seal – the first one I have ever seen. To proceed with repair, this will be the first thing to be removed. Never mind!
8970a cal seal

There are multiple versions of the 8970A controller boards, earlier boards use a 3.6 V NiCd rechargeable battery, but these tend to leak, and are now no longer available. The unit I am dealing with has a more recent control boards, using a BR2325 Lithium cell (3.0 V).

8970a 2325 holder

Rather than dealing with BR2325 cells, decision was made to replace it with a more permanent solution, which might last 10 years, or longer. A 3.6 V Li-SOCl2 cell, of prime quality, SAFT, Made in France. These cells are of excellent quality, and have very small self discharge.

This is the battery, with some (transparent) heat shrink tubing around the leads.
8970a ls14500 battery

…a few minutes later, the battery fitted to the board.
8970a battey fitted

Note the “BR” rather than “CR” coin cell! CR uses Mn-oxide (MnO2) vs Li, whereas the BR type is using a carbonmonofluoride compound, it typically has lower capacity but better long-term performance, which is important for memory back-up applications, where self-dicharge is often more limiting than actual current drain.

br2325 li coin battery

8970a ls14500 li-socl2 battery

The new cell has 2.6 Ah capacity, 15 times more than the coin cell!

Having the battery replaced, now the tricky part. The actual attenuator calibration, to get the proper cal coefficients back into the 8970A. The calibration itself is just a matter of 1 or 2 seconds (initiated by special function SP 33.0), however, it requires a 346B noise source, with 15 ENR output, unfortunately, I don’t own any of these wonderfully noisy things.
Well, there is a work-around. Just push the green PRESET button, and connect a 30 MHz generator (adjustable down to -100 dBm) to the 8970A. Then, we need to adjust power to approximately equal 15 ENR in a 4 MHz bandwidth, the bandwidth of the 8970A.
15 ENR, that’s about 174 dBm/Hz+15 dB+10*log(4 MHz)=-93 dBm on the 30 MHz generator, to get the same total power into the 8970A detector, roughly. Using special function SP 9.2, the 8970A can actually measure absolute noise power, and the generator can be adjusted to result in a reading of about 15-16 dB (which is dB above a -174 dBm/Hz noise floor). Adjusted the generator to -95.5 dBm, which gave just about 15 dB.
Then, execute SP 33.0 to calibrate the attenuators, and all should be fine. In fact, it worked, and the unit is back alive.

8970a working

The inner construction, it is a thing a beauty, and a pleasure to the eye.
8970a top view

Note the wire above the input section resistor – is this for adjusting their parasitic properties? This I will need to put to a test later, using a few resistors, and measuring the SWR of such PI-type attenuators, with and without wires….
8970a input section

8970a last converter detector

The 1st LO, a YIG oscillator. The same as used in the 8558B spectrum analyzer!
8970a yig 5086-7080

8970a 1st and 2nd converter
… the 2nd converter is idential, or nearly (fully?) identical to the 2nd converter of th 8565A, 8569A, 8569B spectrum analyzer units (P/N 08565-60216, the A35 assembly of these analyzers). Another example of HP cleverly re-using some of their most ingenious designs.

The shielding of the 8970A assemblies, this seems to have been developed for the 8970 series exclusively, a cast aluminum body, separate, spring-contacted aluminum covers, and a more rugged overall cover with an elastomer/metal mesh seal in the corners to prevent any of the waves from escaping, and to ensure that no spurious signals are getting into the chain of modules. In the end, this is a highly sensitive receiver, working at the thermal noise level. So we don’t want any high-level radio stations radiating into there.
8970a shielding

HP 3326A Two Channel Synthesizer: finalization of repairs

The 3326A has been working for several days now, with no problem, but the current repairs were only temporary, with some parts that were at hand, but not quite up to the requirements.
Still, I didn’t have the service manual, and no luck on the internet, except for some companies that want to charge me 20 or even 50 USD, for a 30+ year old manual. Well, these manual dealers somehow have to make a living, but I have no dollars to give away, just to get some part numbers. After a few requests sent around to various people that might be proud owners of a 3326a service manual. Finally, some luck: a generous engineer out there provided me with a pdf copy today, of the service manual HP 03326-90010.

Let’s get started on the remaining items to call this repair complete:

(1) Replace the input cap with a 100n X7R, 100 Voltage rating – so far, only 50 V was in stock, but recently, a package of Kemet brand, mil-spec C430C104M1R5CA arrived, hope these will last!

3326a x7r 100 v c430c104m1r5ca kemet

(2) The over voltage SCR crow-bar circuit (currently with a 1884-0261 installed, 4 Amp rms). Looking at the parts list, this needs to be a 1884-0231, a very rare HP numbered SCR. Apparently, it is a TIP116A, 100 V, 8 Amp rms, 20 mA max. gate trigger current equivalent. So the 4 Amp part currently in there might be a bit too weak.


3326a scr 72048 cr800 1884-0231

The only SCRs around are a 1200 V, 25 Amp, 40 mA max. gate trigger current.

tyn1225 bag

Question – are there any issues to be expected, when replacing a 20 mA max. gate current, with a 40 mA gate current part? Well, there could be. After a more careful look at the TYN1225 datasheet, there is some hope that it could work. 4 to 40 mA, that’s a large range. Maybe the part will trigger already at much lower current, given that I don’t intend to operate the instrument at very low temperatures, below freezing, where the SCR gate trigger current typically increases.

tyn1225 gate current

After a quick test with a power supply, a resistor, and a current source – the particular TYN1225 which I randomly picked from the bag triggers at about 4.5 mA. All should be fine.

3326a overvolt circuit

Note that the SCR is triggered via the Q800 transistor (which has been checked and found OK), and this circuit could supply well over 30 mA to the gate, if needed… also, there is connection via the mother board, so the over-voltage circuit will only work when the power supply assembly is mounted in the 3326A. Otherwise, the gate trigger will be open circuit.

03326-66570 tyn1225

… with the SCR soldered in, the 3326A repair is now complete!

HP 8569B (8565A) Spectrum Analyzer: curing the knob disease

Having seen quite a few of the 8569A, 8569B, and 8565A analyzers in the shop recently, once common issue are the controls. The 3-knob operation is one of the features that makes these long-established machines still desirable for today’s work, in particular, for general test and troubleshooting in the microwave region, up to 22 GHz, and above. At the same time, these knobs were made of plastic, and they age – most of they show cracks, which will sooner or later require difficult repair. In the current case, I am dealing with the unit described earlier, it is a 8569B, and someone had fit a 8565A control pad – all of the knobs more or less cracked and useless – I might use their remains to fix upcoming instruments.

So, what do to? Well, decided to go for a rigorous approach, and provide completely new knobs. These use a coaxial design, with 1/4″ outer shaft, and 1/8″ inner shaft. For the small controls, just ordered a few Augat/Alco knobs, which are quite sturdy and easy to mount.

8569b augat alco knob

The large knobs, I custom machined back home in the main workshop, using my little CNC lathe (kind of an overkill) – they are made from POM/Delrin plastic, with some brass inserts, and 4-40 set screws. The knobs are knurled at the outside – one turned out a bit black, because I didn’t clean the tool properly… fair enough.

8569b large knobs

The only downside – there are no markings on the knobs, like for the originals, but such markings aren’t really needed for the 8569B which has a full on-screen display of all settings. Also the ‘lock’ positions for coupled BW-Span still works!

8569b new knobs detail

8569b new knobs mounted

All in all, I’m pretty happy with this repair, estimated useful life of the new knobs – 30 years, and replacements can be made, as we go. But maybe, by then, the unit might be really obsolete, although that’s the thing I am least sure about!

HP 3326A Two Channel Synthesizer: a engineering mystery, and a fuse mystery

…coming back to the 3326A repair, a slight mystery remains – at one time in the past the over-voltage protection circuit must have been triggered, destroying not only the SCR, but also damaging some quite large traces on the board. Typically, the SCR is just switched on for a few milliseconds, before the mains fuse will blow, with no damage of any trace. Typically, these HP devices are well engineered, so there is only a small chance that they didn’t design the system right, to the extend that power would be provided to the board for any length of time even with the over voltage protection circuit triggered.

3326a over volt protection hot trace

Now, the 3326A has passed a 24 hour run-in and multiple power cycles, time for the final safety check which I perform on any instrument repaired, like, checking the ground resistance, checking the isolation resistance, checking the mains fuse rating, etc.


3326a fuse 20 amp
That’s why the over voltage circuit didn’t work, and presented a fair risk of fire and instrument damage, of an instrument, sold at over USD 10k, 1990 dollars.

The fuse rating – 3 Amp, normal blow.
3326a fuse rating

And a big warning sign, hard to overlook.
3326a fuse warning

But what is a normal blow fuse? In the US, fuses are actually rated by the manufacturers, and standardization is well above the level of fusing time, etc. – two types are typically sold, ‘Slow Blow’, and ‘Fast Acting’. Slow blow fuses are easily idenfied, they either have the wire wound around a ceramic core, or are filled with sand.

Found this document, related to ‘normal blow’ – as it turns out, these fuses are actually identical in rating to the typcial ‘Fast Acting’ fuses that I have in stock here.
normal blow fuse

agc fuse
Well, there might still be some tiny difference of the original, HP branded ‘normal blow’ fuse, but test shows that a regular 3 Amp AGC ‘Fast Acting’ fuse is working perfectly find. So for the time being, we can just consider ‘Normal Blow’ and ‘Fast Acting’ fuses to be identical. How easy would be the world, if all things were build according to common, well-established standards. At least, for the fuses.

HPAK (HP Agilent Keysight) 3326A Two Channel Synthesizer: power supply trouble

Should you ever send in any instruments for repair, please ensure it is properly packaged!
3326a packaging
This defective 3326A dual channel synthesizer arrived with no major transport damage, but only due to luck, not due to proper packaging.

First, let’s open up the top panel, and have a look inside. There are two complete synthesizers in the box, similar to the massively popular 3325A design. The synthesizers can be combined, for various two-tone operation modes, phase-shift and PWR modes, two-tone sweep sources, etc. This makes the 3326A a very hand instrument to test all kinds of mixers, receivers, amplifiers.

3326a top view

The outputs are extremely precisely frequency settable, down to 10-6 Hz in the kHz region, and 10-3 in the MHz region… that’s 1 part in 10+9, so you can simulate small oscillator drifts – the frequency stability of the current unit is excellent, it features an option 001 OCXO, +-10-7 per months drift.
The 3326A is also great sources for modulated signals, having all kinds of internal and external modulation sources, including phase modulation. This makes it very useful for PLL characterization, phase detector characterization, or similar tasks.

Well, in priciple. The current unit arrived in dead condition. Plugged it in – a bit of smoke, and bad smell, that’s it.
The faulty assembly: the 03326-66570 power supply.
Another issue: No service manual!!! There are 100s of HP service manuals around, but none of the 3326A!!! Very disappointing – if you have one, PLEASE LET ME KNOW! Your help will be highly appreciated!

3326a  03326-66570 pwr supply

Someone must have tried to fix it before, because a few parts are missing – a screw, attaching the capacitors to the case, and a SCR (aka, thyristor), of the over-voltage protection circuit, CR800.

First fix – the ‘smoking’ capacitor, C706. A 100 nF ceramic cap, at the input of the rectifier – actually, running with about 50 V AC, and a cap, rated at 50 Volts… no idea why HP was doing this – typically, they employ a large safety margin, when designing the circuits. Not it this case, and not to the benefit of reliability.

3326a pwr supply c706 defect

Unfortunately, the cap heated up the traces, and damaged the board – so I removed to loose traces, cleaned it up, and soldered the a replacement cap to the bottom of the board.

The protection circuit – the board was missing the CR800 SCR when received – I can’t find anything wrong with the voltage sense circuits, formed around two LM339 comparators. But there are burnt traces that show that high current must have been flowing throught the SCR at some occasion in the past, possibly due to an over-voltage condition on some of the rails. And the former owner of the device didn’t bother to put a new SCR back in.

3326a over voltage protection 1884-0261 scr

Fair enough, put a spare 1884-0261 back in, a 100 V, 4 Amp on-state RMS current. Will replace it later, either once I found out the original part number from the service manual, or once I get hold of a 100 V, 16+ Amp, TO220 device (which rest back in the main workshop, in Germany, while I have to get the 3326A going here at the US East Coast).

While inspecting the power supply, also noticed that the J101 connector – the main connector to the transformer – had several bad solder joints, seems the plating has come off the pins, making bad contact, even leading to head being generated. Resoldered the pins with big blobs of solder, not my usual style, but should work fine here to distribute the current more evenly.
3326a pwr j101 connector soldered

Now, the moment of truth…. switched it on, and, all rails are up (you can use the little jumper on the board to operate the supply outside the slot – don’t destroy your instrument by putting back in an untested power supply assembly!).
3326a test
…it works! Seems we have won, and still some years to go before this instrument will turn into a paperweight, or, well, a doorstop.

As usual after repair, now, running it for a few hours, switching it on and off a few times – checking the stabilty of the output. Not so good news. Sometimes, instabilities show up, and after a few power cycles, it doesn’t come on any more. Then, it comes on again – an intermittent fault! Never good!
Good advice, in case of intermittent faults – let them develop into permanent faults, and in this case, watch the ‘power good’ LEDs of the various rails.

After a bit of probing, knocking, knocking, pushing – found the issue to reside with the 5 V rail. Even without the service manual, a few tests of the voltage regulator shows that the regulator working, what is not working, is the series pass transistor, a HP 1854-0618. This is a re-branded Motorola MJ3000.

3326a 1854-0618 transistor 5 volt rail

3326a mj3000 transistor

A dead transistor that has intermittent function, very strange. Look at the way it is mounted – using a pcb-mount TO-3 socket. Let’s remove the transistor, and check it out…

3326a to-3 contact

3326a transistor oxidized pin

Now, things are clear – the 5 V rail is quite high current, and the pin-socket combination (for the emitter pin) just isn’t made for it, well, at least not after 30 years of service, oxidation, and so on. One day, it must have heated up quite a bit, judging from the state of the contact. No way to fix this by just cleaning it up – the contact is all soft, and won’t provide a low resistance path. So, I removed it alltogether, and soldered in the pin, using some tin plated copper wire.

3326a to-3 print mount

Also noticed some discoloration of the via at the emitter pin – the heat caused some damaged, but not too much, and also here, added a large blob of solder, to ensure good contact both sides of the via.

Talking about the obvious engineering weaknesses of the power supply, also some good things – it actually has several protection circuits, all rails are protected by heavy Zeners (which will short when overloaded), plus the active monitoring-SCR circuit.

3326a power supply monitor

For the 5 V rail, even the current is monitored, by this rather fancy shunt.

3326a pwr supply shunt 5 v rail

Gave it another few hours of run-in, and numberous power cycles, still, all is working just fine.

Now, check out what it can do:
3326a working

3326a 10 khz xy

HPAK (HP Agilent Keysight) 6205C Dual DC Power Supply: a mechanical fix

Today, a package arrived, containing, a defective 6205C dual power supply. This model is capable of 0-20 V, at 0.6 Amps, or 0-40 V, at 0.3 Amps.

6205c front

The ranges, as well as the meter indications (V or A, x1 or x10 scale) are selectable by two groups of pushbutton switches, and someone figured out earlier that the switches for the V2 output are defective….

6205c bad switches

… well, not quite. The switches work, but they don’t stay pushed in. A mechanical failure?

6205c top view

Fortunately, it is quite clear what had happened. Someone dropped the instrument, and the front panel was hit – bending it inwards, reducing the gap from the switches to the circuit board. With insufficient room to work, the switches appear inoperative.

6205c panel

To fix this, no soldering iron is needed, just a hammer, and a piece of wood, to get the front panel back in shape and aligned.

6205c tools

6205c connectors

The front terminals are a bit damaged, but they work, and I will have a look around for a few spares (these are 1510-0091 binding post – let me know, if you have one around), or try to fix them by some custom-made red plastic inserts – this will have to wait for the next winter!