Category Archives: Various

HP 3325B A26 Assembly: low byte SRAM, and an incorrect signature

A quick comment on the 3325B service manual – Thanks to a kind contributor.

3325b kernel sa test u27 pin10

The kernel SA test is a bit flaky on A26U27 PIN 10. This is an inverter output, and can’t be same as the input… correct signature is A70F. Appears to be a misprint in the HP service manual!

3325b a26u27 74as04

In the present case – the culprit was one of the SRAMs, the lower byte. Fortunately, not really a rare part, and easily sourced and replaced, once you know that this is the circuit at fault. You might want to check this very carefully, because there are multiple versions of the 3325A/3325B control boards.

3325b srams

HP 436A Power Meter: smoke and stench – X-rated cap failure

By coincidence, another HP 436A power meter – this one, emitting smoke and terrible stench! The culprit was easily found, a defective X-rated cap. One of the known-bad epoxy covered capacitors that tend to blow after about 30 or 40 years of service.

436a x2 capacitor 100 n

The residue, oily stuff, terrible smell. Use plenty isopropanol or methylated spirits to clean – otherwise, the stench will stay with the instrument for years, and I can’t say that it is a healthy smell.

436a 100n oily

The cap is of the well-known PME271M series. Still available, but hopefully, with improved construction.

436a pme 271 m 610

436a pme271 series

A replacement is easily found – taken from an old switchmode power supply. Make sure to take a “X2” cap, not an ordinary cap. Only X2 caps are specified for mains voltage service, and self-exinguishing, anything else will present a major fire hazard, don’t compromise on the choice of capacitor!

436a 100n x2 replacement

Fix complete – new cap soldered in, and insulated with some electrical tape. In general, I tend to avoid electrical tape where possible, but in this case, it appears to be the only viable solution.

436a fix complete

HP 436A Power Meter: a strange analog ground issue

This power meter had been received with strange defect, a permanent overrange error, irrespective of any settings or input to the sensor. Sure enough, in most cases, this would be because of a dead sensor – but not here.
The 436A is a really simple instrument, at first glance, but with its design dating back over 40 years (mid-1970s), it has a remarkable complex design to achieve the A/D conversion, and to use something close to a CPU, at the time, called a state controller.

What was wrong with this unit? Something with the analog ground driver.

Checking the A2 and A3 assemblies, it turned out that the analog ground was floating, at about -6 V. Strange! And, simple enough, grounding the analog ground on either A2 or A3 solved the issue! For a temporary fix, a wire was added, from the board edge connector, to chassis ground. Need to look at the analog ground driver…

436a analog ground wire

Using chassis ground for general grounding – an indication of the dated design, and some of these board use 3 or 4 separate grounding path to keep noise down…

436a analog gnd schematic

After this fix, working again (still need to check out what it wrong with the analog ground driver).

Update: found the issue – lower right and corner of above diagram, this is the analog ground driver (also supplying analog ground to the A3 assembly (via mother board) – transistor Q1 found dead, a 1854-0003 (which is equivalent to 1854-0637, JEDEC 2n2219A, or any other ordinary 0.8 W NPN transistor).

436a 1854-0003 2n2219a

Soldered in a 2n2219A, and removed the temporary ground wire. Fix done.

436a analog ground circuit

After a full calibration and extended test, the instrument is rock stable, both for zero point, and 1 mW input. Also checked linearity, and it appears to be better than any means available here to check… most likely, better than 0.1 dB.

Output of the 50 MHz 1 mW cal source – cross checked with a calibrated HP power meter, 437B , and in agreement within 0.01 dB – good enough!

436a pwr meter working

Some other issue with this unit – a stuck analog indicator. After disassembling the front panel, used a razor blade to open up the plastic case of the indicator, and some mechanical adjustment of the inner workings fixed the issue.

436a analog indicator

436a front panel

The 7 segment decoders, these use heat transfer compound, for some pretty unusual way for HP design – being pushed vs. the front panel for cooling. To make sure these stay cool, I added some fresh white stuff.

436a dm9374 7seg decoder driver latch

The decoders are quite remarkable anyway, for their time – these are latching decoders with constant current output, high level integration for the early 70s….

Kebaili CPG-500 Current Pulse Plating: reverse pulse plating power supply

A rather rare guest in the workshop, a specialized power supply, for reverse pulse plating. Most of these are huge boxes used in the plating industry, with current rated in 10s of Amps, but this device is made for the semiconductor field, for depositing metal on silicon, for MEMS (micro-electro-mechanical) devices, etc. It’s a rather lightweight device, in a handheld case, powered by an external DC power supply.

cpg-500 kebaili

The internals show a construction which is more like a prototype than a fully engineered device. Epoxy glue is used to hold in the buttons, and Capton tape at various places. Well, some prototypes last forever, and it is certainly build to a good standard. The folks building it were very concerned about their design it seems, and removed all the marking from the parts used! Keep in mind, this is just a pulse current supply – why all the effort, and why make it so difficult to repair! Even more striking is the fact that the company manufacturing these is now out of business, with no information available whatsover, and no service provided (at least, I was able to obtain the user manual from the owner of the device). Please, leave the markings on the circuits, they are there for good reason – to let others fix the devices, when your greed and protective nature has ruined your business.

cpg-500 board

A few words about reverse pulse plating. You all know the through-hole plated circuits boards, plated vias, etc. These are made by electrochemically depositing copper, a method which Faraday and others invented long ago, but only in recent decades refinements were made to the current profiles to get nicely through-plated (and also filled) holes. Being able to deposit copper in cavities of any shape, etched into other materials, opens up a tremendous number of applications (e.g., complicated patters can be etched into silicon using the well-established methods of the semiconductor industry, then filled with copped – after removal of the silicon by selective etching, the small copper parts remain, resembling all the detail etched into the silicon).

Here, the general pulse sequence: forward current pulse, reverse current pulse, idle time (no current).

cpg-500 reverse current plating generic

A plating example (not from my lab), (a) shows a not properly plated through-hole (cavity in center) resulting from conventional plating; (b) and (c) are pulse plated – one can see how nicely the copper is growing, even in the center. Needless to say, all can be adjusted by selecting the right mix of chemicals, and by optimizing the current levels and pulse duration for the desired results (filled hole, filled cavity, wall-plated hole, and so on). Typically pulse times are milliseconds. But can be 10s or 100s of milliseconds, in cases.

cpg-500 plating example generic

The CPG-500 has 3 current ranges, quite a flexible device, to cover applications from micrometers to square centimeters…. resolution is about 1:4000, about 12 effective bit.

cpg-500 current ranges

After some repair, no spectacular enought to write about, these are the output signals. Pretty clear how it works – lower trace is forward current, upper trace reverse current – during operation, the current sources is switched from one output to the other and back, grounding one of the outputs at a time. When switched off/when plating is finished, both outputs are switched off/high impedance, with no current flowing. Orange lines are 0 V ground potential of the power supply.

cpg-500 current test

Because of the way this bipolar operation is realized (by unipolar switching of current), probing the current requires a few tricks. A 100 Ohms resistor inserted in the current path (in series with an amp meter), and two oscilloscope probes.

cpg-500 current probing

The scope is then used in ‘add inverse’ mode to subtract the two signals, resulting in the display of the actual forward and reverse currents, as one trace.

cpg-500 plating current example

This is a quite typical waveform, the reverse current is 1.5-3x the forward current, at about 2-10% of the duty cycle. This example as 1 mA forward current, 3 mA reverse current, 40 ms forward, 2 ms reverse pulse (i.e., current ratio is 3, and time ratio is 20). This results in an average current of (40*1-2*3)/42=0.8095 mA. An, not really surprisingly, this is what the amp meter shows.

cpg-500 average current

Call it fixed!

HP 8565A Spectrum Analyzer: a ‘new’ CRT for a trusty unit

Not a very major repair, but certainly, noteworthy. I have a 8565A here on the bench, and actually use it quite a lot, along with the 8569B, because it saves you a lot of time not to have to wonder about all the buttons and switches of more sophisticated analyzers. Also, for its coverage (up to 22 GHz with no external mixers), it is rather affordable, and should it suffer from accidental abuse, which can always happen on a test bench with so many cables, it is rather easy to fix because of parts units being available, and because of its discrete construction. Needless to mention, full schematics are available from the HP service manuals.
The only downside of the unit, the CRT – it is working just fine, for all practical purposes, but has some ‘slow’ sections in the variable persistance and storage modes. These show up as green spots in the baseline region. Over time, you can make them disappear, but it takes multiple erase cycles, quite a tedious procedure. For the most part, I just did not care about these spots.

8565a old crt

Fortunately enough, I was able to get hold of a spare 5083-4471 CRT, which has been sitting in the storage room for quite a while. Condition was said to be ‘essentially new’ but never checked.

crt 5083-4471

8565a crt screen

8565a new crt

Last week, finally decided to give it a try, risking to have to install the old CRT again, in case the new one doesn’t turn out to be so new.

You be the judge…. conventional mode

8565a crt conventional mode

… stored display …

8565a new crt storage

… variable persistence mode …

8565a new crt variable persistence

…. good as new.

Clothes dryer repair: AKO 742336 control unit type EDR0692XAX

Repairs of household appliances aren’t generally my field of expertise, but in some cases, it can be a very rewarding activity. Some particular models of clothes dryers (“AEG”, “Whirlpool”) common in Europe use a control board, and this is not designed up to standard. The design flaw, it renders the machine inoperative in the very sense, you can’t switch it on, and it doesn’t show anything on the display. Typically, this occurs after roughly 3 years of service….

This is the control unit:
dryer control

dryer control board

dryer control board detail

edr0692xax board

After a quick inspection, it is fully clear that the defect rests with the control board, model EDR0692XAX. Easy fix, just get a spare! Well, easy, but outrageously expensive.

ako 742336 edr0692xax

The defect, it is related to a DC switch mode regulator converting mains voltage down to some useful voltage to run the controller. This is a step-down converter, NOT mains insulated!

lmk304 application

Two dead parts were found, the LNK304 switch mode converter circuit, and a 47 Ohms resistor.

dryer control lnk304gn 47r resistor

The LNK304GN is easily found, and replaced. The resistor: WATCH OUT! This is a fusible resistor, NEVER replace with just a common resistor – it will potentially set your house on fire, when the next switch mode failure happens.
The 47 Ohm resistor, it’s a 3 Watt resistor, and the ULW3 series, available from all major suppliers, should be a good replacement.

ulw3 fusible resistor

This is the failure charcteristics of the ULW3 resistors – it will go open circuit if overloaded, without release of any flames or particles. It a hybrid, partly fuse, partly resistor.

ulw3 fusing characteristics

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 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!