Category Archives: Various

Ultra-cheap LED Spot Lights: Failure mode analysis, and some reverse engineering, and some concerns

Something amazing about the advent of LED technology for general lighting is not only the brightness, efficiency, and so on, but also the amazingly low price. Here, 20 light fixtures, including 3 LED elements each, 34 EUR total. That’s a bargain a friend of mine could not resist. But think twice, after about 1 year of occasional usage of these lights – several failed. Brightness is gone, some lightly flashing lights remains.


Still the price is amazing – considering the price of a singe 1 W LED element, with about 1 EUR retail. Plus the case, heat sink, aluminum circuit board, heat conduction paste, external case, 3 lenses!! No idea how this is made in China, for about 1.5 a piece delivered.


The first suspect – the drivers: each lamp has their own little driver box. Type S3W-0103.



The parts, and a good quality aluminum board, named CQ-LV8072. This is a universal board, found in many kinds of Chinese LED light fixtures.


Tested the LEDs – turns out, one of the LED elements is dead, and this ruins the whole thing, as all three LEDs are arranged in a series circuit. We can fix this easily by replacing the LED elements, all three, with some good quality elements. Albeit, at almost non-economic cost. Hint – the case and be unscrewed with the heatsink turning vs. the outer case. No need to apply brute force like I did, to open it up.


Some reverse engineering reveals a rather simple, but practical circuit. Using S8050 and MJE13003 TO-92 transistors, and a little transformer.


As you can see, no protection elements, what if the input capacitor shorts out, or if some overvoltage blows the transistor. Could it set your flat on fire? Well, my guess is, yes.

Major relocation, and numerous 230 V conversions…

Long time no post, not because there is nothing happening here, more to the opposite. Relocated from the US, East Coast, back to Germany, including the US section of my electronics shop, 40+ pieces of heavy test gear. All made it over the sea just fine, in a 20′ container. Now, changing all the fuses and converting everything to 230 VAC mains voltage. I will spare you the details, just a few impressions for some trusty HP power supplies. These actually require some re-wiring, you have to break to circuit traces, and install a wire bridge.

230v 6205c

230v 6209a

230v pcb traces

230v plugs cut off

Installing new plugs… wires properly protected.

230v plug

230v traces broken

Now just install a bridge between the middle solder points. Great that there are schematics and manuals, even for 50 year old devices!

230v schematic

230v fuse

…don’t forget to replace the fuse with one of the proper size for 230 VAC operation!

YIG Oscillator/YTO Analyzer: linearity, output power, hysteresis

With a good number of YIG oscillators (YTOs) around, time for a few tests. Rather than writing down all the numbers, a quick test setup, with two power supplies, an EIP 545A counter (with built-in power meter), an Agilent 66319D power supply (used as current source for the main tune and FM coils of the YTO), a 14.5 dB attenuator (just happens to be the value I had around) and a few cables.
When performing such test, make sure to put a 10 dB (or larger) attenuator of really good quality (low SWR) directly at the YTO output, without any adaptors or cables in between – some YTOs will show inaccuracies of not properly terminated in 50 Ohms.

This is a view of the setup:

yiga 1

yiga 2

One of the YTOs under test, a S081-0320 2 to 8 GHz Avantek part.

yiga s081-0320

yiga frq vs current

yiga pwr vs ghz
As you can see, the output frequency is pretty linear vs. main coil current.

yiga dev from lin vs frq

The tests are carried out first with increasing coil current, then with decreasing coil current, and the hysteresis is calculated (difference of output frequency, when a approaching from higher vs. lower current).

yiga hyst vs freq

One thing to watch out for are thermal effects, but let the YIG warm-up for 1 hour or so, with the main coil at half-range current. The effects aren’t all that big for the Avantek YTO discussed here, but you never now, for lower quality parts, other manufacturers, and so on.

HP 11683A Range Calibrator: no power meter calibration without it

With all the various HP power meters for repair, it would be really handy to have a range calibrator, HP Agilent Keysight 11683A. These have been around for 40+ years – any still not easy to find at any reasonble price – even used and non-calibrated units may be as much as 500 to 1000 USD. You can still buy it new:

11683a range calibrator

The internals, check out the picture provided by Keysight – there is a modified 8481a power head (using the same FET chopper assembly), a range switch using high quality 140 series Micro-Ohm non-inductive wire-wound resistors (0.1%, +-10 ppm temperature coefficient).

11683a internals

11683 schematic

Note that the schematic shows the H01 option – which allows an external DC connection, from a calibrated DC source. This is much preferred over the build-in power supply and resistive divider (which has known issues at low output voltages). The design of the 11683a also has some ground loop issues, better to just leave it disconnected from mains, and supply the DC voltage from a known-good source.

11683a calibrator instrument

These issues are known to the experts of the field, see, e.g., this comment from the Keysight EPM-P power meter manual.

11683a accuracy

Now, a very complicated issues with the range calibrator – it’s output isn’t strictly linear over the dB range, because the power meters have a shaping circuit, to compensate for the somewhat high output of the 8481A and similar sensors, above about 5 dBm of input power. Accordingly, the sensitivity is reduced for this range.

11683a 436a pwr meter high input signal adjustment

Furthermore, the 11683a has ranges labelled in mW, e.g., 3 mW, but the output actually is calibrated in 5 dB steps…. so the output power is more like 3.16 mW, etc.
To figure this all out, a thorough calculation has been done, considering the FET input impedance, the resistive network, and the range switch.

11683 nominal output

11683 dc calibrator input

At the 10 mW and 100 mW ranges, calibrations applied in the 11683A (and the 43x series power meters) were determined to be different from the newer EPM-P meters – quite surprising. The reason for this difference of the older meters, to the new EPM-P meters is rather hard to guess, but thanks to a kind engineer at Keysight, we now know: the EPM-P meter reacts differently to the 11683A (because it measures in virtually one range), in contrast to the 43x series meters that have several ranges. So, there is no difference in the actual power meter calibration, it is just a difference needed when considering using the 11683A for either 43x or EPM-P meters, because of the different response to the level calibration, but not actually different response to the power head when measuring actual RF power.

11683 correction

This table has the voltages that should be provided to the calibrator, depending what you want to do – (1) calibrate a EPM-P meter, (2) calibrate a meter “simulating” the acutal 11638A range switch voltages, (3) calibrate an old 43x power meter, with corresponding scaling factors for 10 mW and 100 mW ranges.

11683a ideal voltages

A quick scheme of the 11683A power supply, and the clear-written resistor values, which are not so clearly seen in some of the schematic copies.

11683 pwr supply

Now, how to get a 11683A range calibrator at reasonable cost? Turns out, you can build your own from one of the many defective 8481A that are around in most labs, and on xbay. Well, in fact, most “working” powerheads sold only for below USD 100 are dead anyway… but this is different story. These powerheads hardly ever have any issue with the copper and FET boards, but in most cases, the thermistor is dead, blown by too much input power.

11683 436a voltage check

The modification – a wire has to be added to connect signal and guard ground (brown wire), and a 196 ohms resistor soldered over the FET input (I used a 220 ohm resistor for the test, but will replace one 196 ohm on hand). Also, you need to add a 196k resistor to the input, according to the 11638A schematic (this can be assembled from some other resistors, if no 196k in stock).

11683 8481a modification

Make sure not to bend the wires – this can affect the FET chopper balance (see 8481A or 11683A service manual to re-adjust if needed).

11683 8481a board

The input is currently still arranged with open wires, but I will fit a 1n feedthrough cap soon – will modify the original N-connector (the golden part holding it). But this will need to be done back at the main workshop in Germany – need to use a lathe for it.

8481a n conx disassembled

Some test results will follow soon – but so far, everything is working just fine.

HP 11708A 30 dB Reference Attenuator: less than 0.0005 dB drift per year?

One of the products that have been in the HP/Agilent/Keysight catalog for 3 or 4 decades, or more, the 11708A reference attenuator. Specified at +-0.05 dB, it is a remarkably simple device – it just provides 1:1000 attenuation, chiefly, 30 dB. It’s main application is the calibration of 8484A power sensors, from a 1 mW source – the 8484A needs a 1 µW reference level.

Unfortunately, it doesn’t come cheap, when ordered from Keysight today, at least for a hobbyist’s budget. So I got mine used, aged (30 years?), and at a minor fraction of the cost.

11708a keysight

Before using it for a considerable number of power measurements, it is a good idea to confirm it’s performance. Measuring attenuation to +-0.05 or better is no easy tasks, but fortunately enough, a tractable one, with a 8642A signal generator, and a Micro-Tel 1295 precision attenuation measurement receiver. The Micro-Tel is specified to +-0.02 dB, plus +-0.02 dB for each 10 dB, say, +-0.08 dB. Actual performance, of a well-calibrated and well-heated-up unit is considerably better, but only in combination of other high quality components, like, a stable source (the 8642A has virtually no measurable drift), and, good test cables (using Suhner Sucoflex).

The Micro-Tel 1295 employs IF substitution to determine attenuation, and the IF attenuator works in 10 dB steps. Therefore, for best accuracy, the tests should be done at various power levels, to use various combinations of x0 dB segments, of the IF attenuator.

The results, quite remarkable!

11708a low level

11708a low level2

11708a high level2

One thing to consider for the test – the input and output matching losses. Neiter the source nor the cable/receiver are perfect 50 Ohm terminations – but the 6 dB pads will ensure only very minor losses. Obviously, you need to use high quality pads here, specified to small return loss, 18 GHz parts preferred.

First step – reference measurement is taken without the attenuator-under-test:

11708a test atten 1

Second step – actual measurement is taken with the attenuator-under-test installed between the two 6 dB pads:

11708a test atten 2

Before the start – best to check reproducibility and repeatability. With good cables and hardware, +-0.005 to +0.01 is achievable with the current setup.

Well, let’s say, chances are that the 11708A is +-0.02 off its nominal value, most likely, it didn’t drift at all over the last 30 years.

A13 30 MHz Reference Oscillator: a reasonably quiet oscillator, and a noise cable

A nice little oscillator assembly came my way, supposed to generate about 17 dBm at 30 MHz. Nothing special at first glance, but after checking out its internals, it appeared to be worth a more careful look.

a13 ref osc

A hand-made box, and even more labor intensive assembly work inside. All build by point-to-point wiring, using only the best components available, glass trimmer caps, filters, mica caps – most of these parts are still available today – about 100 USD bom, at least.

a13 upper side

a13 lower side

After a bit of reverse engineering, here the schematic, a modified Colpitts oscillator. Note: base resistor of 2N5109 is 150 Ohms.

a13 schematic

To measure phase noise, connected it to my HP 3585A spectrum analyzer (this is really a great piece of equipment, a bit heavy, but still best of class noise performance and holding this title for the last 35 years….). Connected the oscillator via a 6 dB attenuator, to provide a clean load to the output, rather than dealing with the imperfections of cables, adapters, and the analyzer input.

30 mhz ref osc floor0

Quite shocking, all this noise. The green trace shows the analyzer noise floor. Check, and re-check, still a lot of noise. Too much to be true. After 3 hours of tests, found the issue: a defective BNC cable. Center connector was fine, but both shields were non-connected.

a13 bnc plug

A bit more examination of these cable shows their lousy construction. Not bad for 2 dollars a piece, but you get what you pay for…. the shield is not even reaching to the plug – there is a 5 mm gap from the screen end, to the actual plug. So even if all would have been connected fine, the would still be a lot of leaking, from inside out, and outside in.

a13 rg-58u cable

Notice the BNC plugs – these have a somewhat uncommon construction, the dielectric is covered at the front… not quite according to BNC standard.

a13 bnc cable assy

Clearly visible, the cold solder joint…. Turns out, both ends were open-circuit at the shield.

a13 bnc cold solde

Finally, using a good quality BNC cable (also, using LMR-195 double-screened cable). Looking much better. Noise is down -115 dBc at 10 kHz from carrier. It’s good, but not great. I think one could do better, especially, considering all the pricy parts, and high-quality construction. A good target for a Colpitts osciallator would be better than -130 dBc, at 10 kHz separation.

30 mhz ref osc recheck1

Note the pink trace – this is the bad cable, terminated with a 50 Ohm resistor (with the shield broken at both sides, it is actually a 1 meter wire antenna, with an open-circuit 50 ohm resistor at the end).

440 MHz ISM Band Amplifier: a few extra milliwatts….

There are quite a few devices that use the ISM (industrial-scientific-medical) band at around 440 MHz to transmit information, like, remote thermometers, or to control some installations, like, garage doors, or for personal communications, like, LPD radios, or cordless headphones. Another general term for these devices is SRD – short range devices, and the short range is ensured by a typical maximum power of 10 mW, chiefly, 10 dBm.

In some cases, it may be desirable to boost the output a bit, especially, if you are out in the woods, or for some experimentation of various kind. Be aware, depending on your country of residence, there may be limits to the allowable power of SRD units, make sure you know the rules!

Various MMIC gain blocks exist to provide amplification and output power, but why not go for a discrete transistor solution, using a BFG541 (or BFG591) device. These 9 GHz/7 GHz transistors are SOT223 devices, very robust and easy to work with, and they are pretty low cost, less than 0.5 USD a piece. All the other parts needed are just sub-1-cent capacitors and inductors, except, maybe, for the electrolytic cap.
The small inductors and capacitors at the input/output improve the input/output match (to 50 Ohm impedance), and provide some low-pass filtering (about 800 MHz).

The test circuit on a piece of perf board (adhesive copper tape used for the back plane). Note that this test circuit still has variable capacitors that were replaced by fixed caps in the final design. Also the bias voltage trimmer can be replaced by fixed resistors, it was just added here for convenience of bias current adjustement during test.

440 mhz amp test circuit

440 mhz amp bfg541

The gain measurements were done at +7 dBm input power. To get accurate results and to avoid overload of the VNA input, a 6 dB attenuator was attached to the output (with 2 Watts load capability), followed by a test cable, and with a 20 dB attenuator, directly at the VNA input. This gave about 26 dB (plus minor cable losses) of attenuation, or about -5 to 0 dBm at the VNA input, which is good. The gain offset introduced by the attenuators was removed by recording a reference trace and subtraction from the measured gain trace.

440 mhz gain

As you can see, well above 15 dB gain, and all reasonably flat (note that this is not the true gain, but the limiting characteristics; gain, at lower input power, is larger). We don’t want too much gain above 800 MHz, otherwise, amplification of harmonics and spurious signals comming from the SRD output (which typically is not filtered very well) could interfere with other communications.

Here a few plots of the output power, at various input levels.

440 mhz amp output pwr

440 mhz amp output pwr vs input pwr

To get about 200 mW output power, about 5 mW (7 dBm) are enough, at 440 MHz, even less, at lower frequencies (in case you need to amplify other signals). 200 mW should be plenty for all practical applications related to SRD or ISM personal devices.

Vintage IC Stock: listing

For all folks that are into repair of vintage gear, here is another list (earlier list: Vintage Transistors) of circuits that I have in stock.


There are many more in stock, but these below have been listed and are stored in a way that I can find them easily… Primarily, these are for my private shop&repairs, but if you are in desperate need for one of these goodies, just shoot me a line (I may ask for a fee to cover my expenses&time).
Note that I don’t keep exact stock lists – some of the parts may become unavailable over time. Listings of more outdated ICs – will be added soon.

Location Part Count

K-T01 LA7212 – 1 Pcs
K-T01 M58653P – 1 Pcs
K-T01 LM3915N – 2 Pcs
K-T01 TD62103P – 1 Pcs
K-T01 NE5534AN – 1 Pcs
K-T01 LC24085P – 1 Pcs
K-T01 LC74084P – 1 Pcs
K-T01 UAA170 – 1 Pcs
K-T01 TBA120 – 1 Pcs
K-T01 TBA120S – 1 Pcs
K-T01 SAB2022P – 1 Pcs
K-T01 SAB1046P – 1 Pcs
K-T01 SAJ110 – 1 Pcs
K-T01 TDA2721 – 1 Pcs
K-T01 WD1100V-12 – 1 Pcs
K-T01 TDA2710/1 – 1 Pcs
K-T01 TDA2730 – 1 Pcs
K-T01 TDA3780 – 1 Pcs
K-T01 TDA2560/3 – 1 Pcs
K-T01 TDA1054M – 1 Pcs
K-T01 SAB1009B – 1 Pcs
K-T01 TAA611 – 3 Pcs
K-T01 CA3081 – 4 Pcs

K-T02 Z85C3008PSC – 1 Pcs
K-T02 SN76477N – 1 Pcs
K-T02 4N33 – 3 Pcs
K-T02 TDA2151 – 1 Pcs
K-T02 TDA2160 – 1 Pcs
K-T02 KM6264AL-10 – 1 Pcs
K-T02 MM74C906N – 1 Pcs
K-T02 SN7493AN – 1 Pcs
K-T02 ICM7049AIPA – 2 Pcs
K-T02 SN16913G – 1 Pcs
K-T02 MAX690A – 1 Pcs
K-T02 TL7705ACP – 1 Pcs
K-T02 TDA2320A – 1 Pcs
K-T02 SAJ110 – 1 Pcs
K-T02 TDA1950 – 1 Pcs
K-T02 HM4334P-4 – 1 Pcs
K-T02 TDB0556A – 3 Pcs
K-T02 SN76660N – 3 Pcs
K-T02 TC5508P-1 – 12 Pcs
K-T02 UA733CN – 2 Pcs
K-T02 TAA710 – 3 Pcs
K-T02 TDA3562A – 1 Pcs
K-T02 CA3083 – 1 Pcs
K-T02 ULN2111A – 1 Pcs
K-T02 SAA1027 – 1 Pcs
K-T02 TDA2790 – 1 Pcs
K-T02 TDA2740 – 1 Pcs
K-T02 TDA1170S – 1 Pcs
K-T02 TDA3770 – 1 Pcs
K-T02 TDA4942 – 1 Pcs
K-T02 TBA440C – 1 Pcs
K-T02 TDA1940 – 2 Pcs
K-T02 TDA2591 – 1 Pcs
K-T02 TDA2730 – 1 Pcs
K-T02 TAA630S – 1 Pcs
K-T02 TDA1180P – 1 Pcs
K-T02 TA7630P – 1 Pcs
K-T02 MC14584BCP – 1 Pcs
K-T02 709CJ – 1 Pcs
K-T02 MC1307P – 1 Pcs
K-T02 WIC7015 – 1 Pcs
K-T02 V4001D – 1 Pcs
K-T02 MC14503B – 1 Pcs
K-T02 TDA2140 – 1 Pcs
K-T02 M3-7603-5 – 1 Pcs
K-T02 N82S25 – 1 Pcs
K-T02 N8T10B – 1 Pcs
K-T02 L6506 – 1 Pcs
K-T02 TCA830S – 2 Pcs
K-T02 TAA930 – 1 Pcs
K-T02 TDA2522 – 1 Pcs
K-T02 TDA2532 – 1 Pcs
K-T02 SN76001ANQ – 1 Pcs
K-T02 TDA7250 – 2 Pcs
K-T02 WD1100V-03 – 1 Pcs
K-T02 R5620 – 1 Pcs
K-T02 WIC6020 – 2 Pcs
K-T02 LM339N – 1 Pcs
K-T02 DS8836N – 2 Pcs
K-T02 A10N – 1 Pcs
K-T02 TA7171P – 1 Pcs
K-T02 9602PC – 1 Pcs

IBM Thinkcentre M50 P4 3 GHz: electrolytic capacitor repair

One of my computers, a trusty IBM Thinkcentre Desktop, decided to fail on me. Symptoms – sometime it starts up normal, sometimes it doesn’t. Already hangs at the boot screen. Smells like a hardware issue, and in fact, it is a hardware issue – with some electrolytic caps (note the brown substance leaking out from the top vents; fortunately enough, no damage to the board).

ibm board1

ibm board2

ibm board3

Electrolytic cap failure are a very common feature of modern consumer electronics, the remarkable thing here: only some of the caps failed – 1500 µF, 10 V, and 1000 µF, 10 V. Maybe these see particular load, or they are from a batch that wasn’t all that good. All were high quality Nichicon brand, 105°C, HM series, specially designed for PC motherboards, low impendace, etc.

nichicon hm series electrolytic caps

Board with new caps installed…

ibm board rep caps

All caps were replaced by 1000 µF, 25 V – these were the only caps I had available with the given footprint and capacity range; 100 n X7R multi-layer caps were added (solder side), hopefully, to prolong the lifespan of the electrolytic caps.

ibm board aux caps x7r

These are the culprits… a last look before they go into the bin. Board is working good as new. Let’s see how long the repair will last.

ibm dead caps