Tag Archives: power supply

HP 6205C Dual DC Power Supply: a generous binding posts fix

The repair itself, it is not particularly noteworthy, because this supply has served me well in the last years, in fact, it had been switched “ON” all the time to power an experimental setup.
The initial repair of this supply has been documented before, and on the pictures there it is quite visible that this supply had damaged binding posts. Seems that the prior user dropped it on the front panel.

Now the noteworthy facts, a kind reader of this blog, an American fellow, had a few of these posts at hand, from a HP plotter. He kindly sent them to me, free of charge!

So, as a result of the kindness of the reader, and the standardization of the parts HP used in their equipment, the power supply is now in better shape than ever before.

Did a few tests, like, checking ripple current at full load, and electrical safety – ground resistance, but all looking good.

HP 8662A 8663A Power Supply A7A3 Assy: base transformer defect

A little note, Thanks to a kind fellow sharing this repair info with me, related to the A7A3 assy of the 8662/8663A generators: the 9100-4018 base drive transformer. There are two of these on the 08862-60289 board, protected by a fuse and diodes, nothing should acutally happen to them, but things can go wrong.

Below, the two versions of the A7A3 boards, left, the older 08662-60289, and the more recent 08662-60604. This post only refers to the -60289, the base drive transformers of the newer units look the same, but have part number 9100-5291 – don’t know if the can be exchanged.

8662 8663 pwr supply

A snipet of the power supply circuit, there are two identical base drive transformers, center tapped on one side.

08662-60289 base drive schematic

Cross reference – this is a NSN part, the most prominent manufacturer seems to have been Fil-Mag, which use to be a Sprague company long ago.

9100-4018 master cross ref

5950-01-267-1279 aka 9100-4018 transformer

Note that these little beasts come at a quite hefty price! That’s well over USD 100, just for the two base drive transformers – I hope, HP did not pay the list price, or anywhere close to it.

Here is a rare view of the interals, after heavy work with sandpaper and other means (these transformers are potted, but as with all potting compounds it can be removed, if you have plenty of time, a good supply of tools and don’t mind the dust and dirt).

8662 8663 pwr fil-mag 42z994 internals

It uses an OJ41408 bobin/pot core, PC14/8 size. Wire is about 0.1 mm size. The pot cores are still available from Magnetics Corporation, mag-inc.com, and the material is just a regular mid-frequency ferrite. So you might be lucky with just using any average good pot core with about 5000 permittivity (e.g., N30 ferrite).

8662 8663 pwr oj41408

mag-inc 41408ug pot core

mag-inc j material

Fingers crossed that you will never need this information, because it is quite a laborious effort to reverse-engineer the internals, and to fabricate a new transformer manually. Tempted to say, I could manufactur them well below list price, if someone would need a 1000 pieces….

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!

Tesla/Voltcraft BK127C Power Supply: a trusty fellow

One of the first pieces of electronic equipment I have ever owned, maybe the very first, a 0-20 V power supply, 1 A max. current. Made for Voltcraft (brand of the “Conrad” electronic mail-order company, popular in Germany), by Tesla, “Czechoslovakia”.

In the mean time, I have 3 of these, and despite the “1 Amp” limit, these are very useful supplies, and there are hardly any circuits that need more than 1 Amp. The output is reasonably low-noise – very similar to other DC supplies or power packs.


Build quality is very sturdy, folded steel – and a basic but very reliable circuit, designed around a uA723.

bk127c schematic

Years ago, I had one of the supplies fail on me, when powering a high voltage circuit – this caused the power transistor, a KD606, to fail. Replaced it with a BD317 – working perfectly fine.

The manual – sorry, in German only.
tesla bk127c pwr supply

Harrison HP 6202B Power Supply

A quick repair job – an old but trustworthy Harrison (HP) 6202B powder supply. 0-40 Volts, 0-0.75 A.
This is quite a useful range, has overrange to about 45 V, and a good compagnion for a 6200B supply that I have been owning for a long time.

This is it:
harrison 6202B

A suspicious label:
6202B label

It worked well, but had excess noise and some oscillation, when loaded. BTW – no fancy dummy load here:
dummy load

The inner workings –
620BB circuit

After some clearing, more or less washing it with isopropyl alcohol, I found this capacitor, date code 6708 – February, 1967, not quite 50 years.
6202B defective capacitor

After a few hours of test run, under about 50% load, a quick check – at various loads, with and with out current limitation. That’s the kind of noise I’m getting. It is mainly 60 Hz ripple, about 2-2.5 mV p-p; maybe 1 mVrms – a bit more than the 0.2 mVrms specified by Harrison – but still in the same range as the Vrms noise of modern power supplies.
6202B noise