Rohde&Schwarz URE RMS-Voltmeter: analog digital communication

Recently, I got more R&S instruments for repair than ever before. This time, an URE RMS-Voltmeter. It can measure AC and DC voltage, in true RMS values, from 50 µV to 300 Volts. 0.5% basic error. So it is about 1% accurate, with a frequency response from 10 Hz to 20 MHz.

It showed a failure message, “error d” which is a combination of errors that aren’t a good sign. So I was not sure how to approach it first. Checking the manual, I decided to proceed first with some basic test to see if the analog circuit is reacting to commands. A simple test involves the check of the reference voltages: there are 1 V, 10 V, 0.1 V and other voltages derived from a main reference on the analog board, and by digital command you can switch either of these to a test point. However, I could not get all of these voltages switched correctly.

Doing some analysis of the logic chips and circuit, the analog board didn’t get a valid command – there was no signal on the A0 address bus. Tracing back the signal path, this bus originates from an optically isolated driver circuit on the CPU board.

There are 12 optocouplers, fortunately, socketed. So I changed the B12 (A0) and B11 couplers, and indeed, the reference voltage issue showed a different behavior. Obviously, something in wrong with these couplers. I tested a little, and replaced the B12 coupler by a 4N28, and this brought the unit back to life, without and failures. Run the self-calibration (necessary to remove a wire bridge to execute calibration command).

Then, after checking more carefully, decided to replace the 4N28 by a 4N35, because this is a part of virtually identical performance to the (obsolete and old type) of optocouplers installed in the unit. Also checked the signal integrity and slopes with a scope, can’t see any difference of the new B12 coupler to the other couplers.

One major advantage was that I got a whole file with all big schematic copies, easy to work with it. I like to put these on the floor for easy reference without damaging the paper.

A final test with a precision level generator. All seems good, even after a few hours there is barely any drift.

L33 Thermal fuse: inner workings

Recently, I had a project that required a reliable thermal fuse. There was little space to accommodate the classical axial versions, so I did some investigations and settled for the L33 type fuses. These come in various temperature versions, here, the 130°C limit, rated 2 Amps, 250 Volts.

The main components are the 2 wires, a plastic case, and some resin. Having never studied one open, I disassembled a few good ones.

Clearly, the resin is filled to certain level. At the top, there is a bridge between the wires, made by low-temperature melting alloy (having tin, bismuth, indium and related metals).

The alloy is quite substantial, likely to be able to handle 2 Amps of current.

Triggering it with heat gun, the alloy melts, and there is enough space in plastic case that it forms a drop, effectively interrupting the circuit.

While the devices studied showed very good consistency of construction, a little overfilling with resin may result in the thermal fuse not opening, I hope the manufacturer has this parameter under strict control. For the 10 pieces I have, the weights measured on a precision balance where quite uniform at least. But if you get such critical parts from some doubtful sources, better you do some tests first and be sure they are reliable.

Rohde&Schwarz NRT Power Reflection Meter: tantalum issues

Recently, I got this very nice instrument – a Rohde&Schwarz NRT “Power Reflection Meter”. It is designed to measure transmitted and reflected power, a quite handy instrument used for the installation of cell phone antennas, etc.

This unit was received with a label “completely dead and burned”, but it turns out that the damage was not all that bad.

Some quick survey showed that there is a 12 VDC power supply installed, which is connected to the main board by a cable. On the main board, there are three capacitors in parallel for the 12 V bus. One of these had burned out, only pieces and black smoke remained. The board looked damaged, but after careful removal, there was no problem with the board found. Used some methylated spirits to clean the mess thoroughly. There one thin trace underneath one of these capacitors, so better don’t work on it with coarse tools or in a rush. I dissected it under the microscope. The ground connection pad of these capacitors has no heat relieve arrangement, but the caps are soldered to the heavy ground plane – a strong soldering tool will be needed to melt the solder in adequate time.

I desoldered not only the bad guy, but also the adjacent capacitor, to test its performance and value (there is no schematic). These are 100 µm low ESR caps. I found it appropriate to replace two of them by one 470 µF electrolytic capacitor, plus, a 10 µF multilayer ceramic capacitor in parallel. Saving money to buy 100 µm small-size tantalum that are difficult to hand solder without risking damage.

One intriguing part came up during repair, a 78ST105S regulator, which is there to provide a 5 V rail from the 12 V input. Quite a nice part, now obsolete, but first time a ever saw such design. It is essentially a high efficiency voltage regulator, easy to use, but it has a buck circuit installed to get higher efficiencies compared the customar 7805 regulators.

Some quick check of isolation resistance and the power supply (without anything connected) – it provided stable 12 VDC to a test load, so the shorted cap didn’t cause any damage to the supply. The moment of truth: switching on the NRT instrument. Working fine.

To complete the unit, I will need to see where to get an affordable NRT sensor. These don’t come cheap. List price is 3800 EUR each, used units go anywhere from 500-2000 EUR. Definitely, expensive.

Christmas Bakers: Dominosteine – dominostones

This year a good friend has been asking me to prepare “Dominostones” together. It is a kind of German special Christmas cookie, which is normally purchased at supermarkets because fast machines can make these things much more accurately and faster than human had. However, always up for a challenge. Handmade goods of such rare nature will also be a handy gift.

First, we have to make a thin dough base.
120 g of honey (cheap honey is OK)
30 g of sugar
1 tablespoon of mixed spices (cinnamon, etc., use gingerbread spice)
100 g of wheat flour
100 g of heavy rye flour
1 egg
3 g of ammonium bicarbonate (“Hirschhornsalz”) dissolved in some warm water (10 ml)
3 g of potassium carbonate (“Pottasche”) (dissolved in a little it of warm water)

Warm up the honey a little in the microwave to make it liquid, dissolve the sugar in it. Some little remaining sugar is no problem. Add a mixture of the flour and spices (best, pre-mix as dry powder), add the egg. Add at one side the dissolved ammonium bicarbonate, mix a little, add the potassium carbonate solution at the opposite site (don’t mix these two things together).
Let the dough rest for a few hours (wrap it tightly with cling wrap; not too cold, say, 15-20°C).

Use a good pan with some anti-stick paper, be sure to roll it to a uniform layer. The size of my pan is 43×32 cm. Similar size will also work.

Bake approximate 8-10 minutes at 200°C (upper/lower heat; air convection oven you may try 180°C).

Let it cool down.

Take about 600 g of apricot jam (screen it through a mesh to remove any solids), add 150 g of water with about 9 g of agar-agar (a pale powder). Boil for 2 minutes. Let it cool down with occasional stirring, but don’t let it get firm. Apply to the dough base in one go. First leave a little distance (say, 0.5 cm) to the edge, then apply more and more ensuring that it doesn’t flow down. If to liquid, wait a little, however, it is best to apply all in one operation, to avoid forming layers of gel that can later separate.

After some cooling, apply a marzipan layer.

400 g of marzipan (baking marzipan)
150 g of powdered sugar (not icing sugar or similar mixed products, must be 100% pure powdered sugar)
Kneed the marzipan and sugar to a uniform mass by hand. This takes some effort. First work in 1/3 portions, then combine all together. First, it looks as if it cannot mix, but after a while, it will. Roll this material to a thin layer large enough to cover the full pan. Use small quantities of flour to prevent sticking.

Cut the combined layers. I like to double-stack them: dough-gelly-marzipan-dough-gelly-marzipan. It is recommended to make so nice bite size pieces, not too small. It is an item served in single-piece quantities.

Cover with chocolate, by dipping them first, piece by piece, in chocolate and removing excess chocolate with a wood stick or similar tool. It is important to liquify the chocolate glaze in water bath, don’t overheat it.
About 400 g of chocolate (dark is preferred) will be needed.

Finally, put them in a box for safe storage. Otherwise, they may get eaten faster than you believe. Better to store at about 15-18°C, constant temperature. Don’t put in the fridge.

The SCHRÖDLE Family Chronicles: decorative hot embossing

The major project besides all the work in the electronics lab and mechanical workshop here has been the writing of the family chronicles and ancestry research, starting with my father’s side: the grandparents SCHRÖDLE and REITSAM. Now as the draft has been completed, I have put further thought to the publishing. The specialized contents won’t justify a large printing operation with many copies, so the book will be done by digital print rather than professional offset printing. Also, there may be some corrections necessary after the first print, therefore I don’t like to print to many pieces that then need to be corrected, but will only print the copies needed. Binding will be done by a spiral binding machine that I have recently purchased used, a heavy-duty version of a spiral binder that I have owned for years to make small “user manual” booklets and similar.
For the cover page, I thought it is a good idea to not only print it black and white, but to give it some nice appearance by gold print.
Golden letters can be printed by various means, but the best result is obtained by hot embossing. A heater metal (or special polymer stamp) is pushed into the paper, with a hot printing foil in between. The foil has multiple layers: a PET carrier, a layer of gold-colored aluminum, and a heat activated glue. Where heated the the embossing stamp, the foil will stick to the paper and the gold layer will transfer to the paper when the hot print foil is peeled off.

While the theory is easy, the challenge is to do the hot embossing in the home shop without a specialized machine. I have some experience with milling brass stamps that I have made for some friend years back, CNC milled. But with current case, I would need a 90×50 mm brass block, which I didn’t have in stock, so I took some hard aluminum alloy instead. Durability is no issue anyway, because I will need less than 100 copies anyway.

After preparing the stamp block (it will be directly heated by two 180 Watts 230 Volt heater cartridges, 10 mm diameter, 80 mm long), I set it up on my CNC mill to cut the letters. Don’t forget that all must be mirrored!

The program has many lines, but all no problem with LinuxCNC, which can handle machine programs of essentially any size.

First, roughing out the main parts, with a 3.8 mm endmill, 1.5 mm deep. You could also use a 2.5 mm endmill, but from experience, it is ok to just mill away the major gaps and cut the other areas by am engraver tool.

The fine engraving is done with an approx. 60 degrees engraving single-lip cutter. Surely, my CNC’s spindly is not fast enough for this, using about 2800 RPM where I should run at 10000, but the feeds were adjusted to roughly 140 mm/min, to get away with the “slow” rotation. The single-lip cutter (made from K30 tungsten carbide) is holding up well in hard aluminum and leaves a shiny cut even without any lubricant.

For a test print, I heated the metal stamp on a hot plate. It has some crude temperature regulation, so it is not difficult to control the temperature.

The supplies prepared: some pieces of paper (always use the actual paper you want to print on), some cut strips of heat printing foil (this foil I have purchased more than 20 years back, a mid-sized coil, many meters because it is petty thin), and a thermometer to sense the temperature of the block while heating (the green wire is the thermocouple).

The printing itself, I did on the print press. It is recommendable to print on a piece of thick paper or cardboard, to have a certain depth of the impression. You can also print on a flat and hard surface, but the result is then less impressive. Temperature, pressure and time need to be established by trial and error, say, 180-200 degrees centigrade, intermediate force, and just a few seconds are a good starting point. Too hot printing will damage the foil and paper, too cold printing will give bad transfer and low durability, too high pressure will give unclean corners and edges, and too long printing will result in transfer to areas that are not letters, leaving a “dirty” print. You can check with an eraser: a good hot embossing will not be affected by using a soft eraser on it. If the gold comes off, either the paper is not good, the temperature is too low, or you are not pressing hard enough.

At an angle, there is a nice reflection. This is printed with a semi-matte gold foil, I will also try a shiny reflective gold foil. But I guess my grandparents would have been happy with any style of gold print.

LH0021CK Power Opamp: an analog autopsy

From an earlier test equipment repair, I had two broken LH0021CK power amplifiers, both died because of a design issue with an YIG driver board (lacking distance of a heat sink from a board trace leading to an occasional short circuit).

The LH0021CK are +-12V output, +-18V input, 1 Amp capable devices, and if treated nicely, they will last 40+ years of service. Still available today, despite being obsolete, at prices ranging from 10 to 300 EUR a piece. These were quite common parts in high-end controllable power supplies, servo drivers for scientific instruments, and similar apparatus.

Finally, I cracked these open – not anticipating the fragile ceramic substrate inside.

I pieces together a picture from multiple images, because this part is a little too large for my microscope, and too small for other cameras to see it clearly.

The datasheet provides an internal circuit diagram, but is there really a 741 opamp inside? What about all these resistors?

Checking the parts, indeed, there are 4 discrete transistors, laser-tuned resistors (you can see the burn lines of the laser to adjust the value of the resistor in processing of the circuit, some have even two tuning lines), and an opamp die.

Eventually, I opened two of the broken parts, an old one, golden case, and a newer one, silver case, both made by National Semiconductors. There are some small differences of these parts, also related to the circuit. The resistance values are rather similar, but the newer part (shown in the picture) has an additional resistor at the COMP input. Both parts have no connection of GND to the negative supply input of the opamp – this is apparently a mistake in the schematic shown in the datasheet, because the circuit wouldn’t work if there is such connection.
The newer part seems to have a small diode chip (marked light blue), but checking it just gave a 0 Ohms value. Maybe just spacer for wire bonding.

The failure mode is clear for both parts – the SC+ bond wire is blown (with molten ends clearly visible), and the Q4 power transistor shot.

Clearly this part could save a lot of space in the old days, replacing it with discrete parts would take about 5 times the space (1 large TO-3 NPN, 1 large TO-3 PNP, 2 transistors, one TO-99 Opamp, and several resistors…. I have been successfully replacing these parts with TDA2030A audio amplifiers, they seem to be be a good substitute, even if they may lack some detail performance characteristics (eventually, the TDA2030A has even higher power and better bandwidth, say, 100 kHz vs. 20 kHz).

Some detail study of the opamp die showed that it is indeed a National Semiconductor part, 741H printed on it, and the shape of the capacitor (the light colored silver area in the middle) is the shape of the typical National 741.

Blaupunkt OSTIA Home Radio: revamping an old beauty

As a family heritage, we ever had an old radio from my great-uncle (brother of my grandfather) named Modestus, and ever since I can remember is had been standing in the bedroom of my parents. Eventually, it was no longer used there and moved into my mechanical workshop where it still serves to play background music while I am operating machines.

It was build some time in the early 70s, and is based on germanium transistors – to be precise, 11 germanium transistors. The sound is not bad given the relatively simple circuit. However, in the last year it must have suffered some degradation of the FM tuner, because this tends to drift, and the reception is not clear always. Especially in winter, switching it on after a “cold” start, it needs some re-tuning after about 30 minutes of operation. A little inconvenient. Rather than spending a long time trying to fix an old FM tuner, I decided to take another approach – adding a new digital (PLL) tuner.

In my stock of old parts I had a no longer used PCI TV card, which incorporates a Philips FM1216MK tuner, a combination TV and FM tuner use a TSA5523 PLL, and can operate from a single +5 V supply (because of an internal DC-DC converter).

The card is a combined ISDN-TV-FM card. The tuner can be easily desoldered. Control is by i2c bus, two wire interface. Some libraries exist, but I didn’t use those. Rather straightforward to send the bytes needed to set the frequency and to do some more configuration needed. The tuner has a stereo decoder, but I operate in mono mode – there is only one speaker in the OSTIA radio.

A quick setup with a i2c LCD added for debugging. Using a Arduino Nano3 board clone with an ATMega168p microcontroller. But any microcontroller will do.

Now, integrating the new tuner to the OSTIA – my objective was to not destroy the old beauty, integrate minimally invasive. A first attempt to use the build-in transformer failed, because it could not provide the roughly 200 mA current needed for the Philips tuner.

To feed the audio signal, I cut a bridge on the board (which carries the FM audio from the old tuner), and injected the audio from the new tuner via a 100 nF foil capacitor.
For control of frequency, there is no an incremental encoder on the back of the radio (I rarely change the station if at all), and when you push on that encoder, the last frequency set is stored in EEPROM. The LCD has been disconnected, not needed during operation.

Finally, the OSTIA back at its accustomed place in the workshop. Reception is good and stable now, all frequency locked to a small quartz crystal.

Certainly this radio now has no longer just 11 transistors – maybe 500 transistors now!

A Garden Pump: a surprising level of complexity

Now, after winter has ended and frosts are no longer severe enough to endanger my outdoor water system, time to restart the pumping system. For garden watering and general outdoor water needs I operate a small well, including an automatic pump, but during winter time, this is shut down and the pipes all emptied to protect the pipework and pump from freezing.

Normally, just a task of a few minutes to start the pump after filling it with water, but this time, no success. No water, no suction. Checked the well – plenty of water there. Also the pipework is all good and sound.

After some searching hear and there, I decided to open to pump and found a very unexpected defect – one of the critical inner parts, the so-called ejector which is needed to prime the pump (by removing air from the system) had a crack. Probably because of fatigue of the plastic material.

This part uses the water circulating inside the pump to essentially pump air out of the suction pipe going to the well. With the crack, it can’t effectively evacuate the suction pipe, thus, no water can be pumped.

The plastic material is good, a special form-stable and durable hard plastic, reinforced with glass fiber. Maybe a low-quality material batch, or some design flaw?

The pump, purchased in late-2020, is of the brand “Stahlwerk”. It was not expensive at the time, just about 100 EUR at the time of purchase, stainless version, including a small buffer tank and pressure control system. Anyway, the price is good, but the quality of the “Stahlwerk” goods, you judge yourself! Shouldn’t a garden pump last more than just three summers?

To check if the crack really is the root cause, I sealed it with some hot glue, and despite the deviating geometry of the ejector nozzle, the pump worked right away.

But such temporary repair is no good solution for my garden water system, which requires automatic, unattended and reliable operation all through summer. So I checked “kleinanzeigen”=classifieds and found an almost new, barely used pump close-by, for a very reasonable price.

It is called “Neptun”, after the Roman god of the rivers and seas, hope he will be able to ensure the water flow to my garden for years to come.

The pump, easily capable of 4.5 bars of pressure, stainless construction, and a solid 1 kW motor delivers very nice quantities of water, good pressure, and was easily installed on the existing buffer tank.

For the “Stahlwerk” pump, at least spare parts are available and the customer service was responsive. The spare part was reasonably prices, so I decided on a permanent repair – and after a few days got a new ejector insert delivered in a package.

Still made of the same polymer, but looks a bit heavier and the color has changed from black to white.

It matched nicely the dimensions of the old part. Cleaned all the old O-rings, and re-used these with no problem at all. Just take care when installing not to twist and of the O-rings, keep all nicely aligned and avoid undue force.

Now, while the Neptun will do the job, there is a spare “Stahlwerk” pump in the basement, just in case!

May the well never run dry!

The Schrödle Ancestry: a little time-consuming effort and investigation into local history

Recently, much of my time was occupied with serious work at the my chemical professional job, business travel and, during free time in cold winter, research into the old ancestors and forefathers. The objective being, to publish a little booklet about the “Schrödle” family (my father line) for this year’s Christmas, and one about the “Pabst” family (my mother line) for next year, say, 2026.

Already may years back, in about 1993, I did some investigation simply by interviewing and recording the recollection of relatives that were still alive at that time. Archive records were hard to come by at that earlier time. Still I was able to collect quite a bit of information up to the level of great and great-great grandparents including great-uncles and great-aunts.

I recorded this earlier work in simple diagrams. This time I wanted to set up a proper database in GEDCOM format well familiar in the field of ancestry work. Further intention was to go back to about the time of the “30-years’-war”, a critical time in German history, 1618-1648, where a lot of devastation and movements happened at my homeplace.

The homeplace, it is a fertile area near an ancient meteorite crater, the Nördlinger Ries. At the high rim of this crater, the Schrödle ancestors have ever been working the land as farmers, millers and craftsman. Surely busy folks familiar with the hardship of essentially self-sufficient life. Places like Fünfstetten, Gosheim, Huisheim, Mündling were the villages of the dwellings, these are very old villages, with records dating back 1200 and more years, but certainly the are in and around the crater has settled since the stone age, many famous old bones have been found.

Two reasons facilitated the new wave of ancestry research: firstly, my father’s family has ever been from this famous spot in Northern Swabia area of nowadays Bavaria, Southern Germany, the old Sualafeldgau dating back to the 8th century AD. Secondly, there are now large numbers of records online available, in particular, the old church records about birth, marriage and death. And, fortunately, these have been well-preserved over the centuries in this area.

Surely, it is all a matter of patience to find the right records, assign them to the family tree and record the information in a digital format, say, a GEDCOM file. Some, like the marriage record of a son of one of the oldest Schrödle ancestors, a certain Balthasar Schrödle born in the 17th century Fünfstetten, and which is relatively easily read after some practice, and with my training of ancient latin received at school – final I can put that to some use.

Other records are difficult to read – even relatively recent records from the 19th century, because of bad handwriting of some priests, and there lack of attention to readable handwriting.

But after all, I have been able to reconstruct the family roots back many generations, here, for my grandfather Georg Schrödle, born in 1901. He was a farmer with patience, a kind man duly respected, still in good memory.

Similarly my grandmother, a descendant of the large Reitsam family in Mündling.

Despite all the wars and difficulties of time, some items including books, papers and photographs still remain in possession of the family, but without knowledge of the ancestry, it is hard to assign these to the right people and to understand how these objects relate to family history. Accordingly, one part of the effort is also to identify all the persons on these old pictures and documents, to avoid this information getting lost over time.

One example is a book, which was in possession of the Reitsam family, the family of my great-great grandparents. While the book itself is nothing of particular value, a history of holy people that was similar to a bible probably the only book in the possession of common people, but with hand-written notes about the family members and birthdays. The last entry, written “Simmon”, is my great-grandfather Simon Reitsam, which which I inherited my first name.

Then, with the proper birthdays and other records, I did further research to find sources in archives, also, military archive collections, and many good records could be found.

One sad thing of the more recent history is also to number of people lost to wars, for example, Josef Reitsam, my great-uncle (mother of my grandmother, father side).

After all the years and only unclear recollection by hearsay, at least we now know the exact place and circumstances of his tragic and futile death during World War I.

Many other tragic events also came up during the research, which can serve as a warning to the current day: a Schrödle child, drowned in the river feeding the Schrödle grain mill, a Schrödle ancestor, who got pulled into the gears of the mill and died, a Reitsam ancestor, a carpentor and builder, who got hit by a tree and died, many children of the old families that died of infections diseases, and so on. We can hardly imagine all the hardship of the old days, and maybe get a better appreciation of modern times and the value of peace and a society based on the advancement of science of industry. After doing a fair bit of ancestry research now, there is certainly no desire to go back to the “good old times”.

Another, much more pleasurable aspect is the number of “far” relatives, and newly-found parts of the family. There are many relatives living in the US, emigrated to Chicago and similar places. Some still carry the name Schrödle, and actually pretty close relatives. Even in the local area, there are several families, whose relationship to us has been only vaguely known or even completely unknown. If you contact some of these members to find out more about the relationships and ancestors, so far I have encountered only the most helpful and kind people, so this is really a pleasure and joy. Also there are some local historians doing research into certain places and family groups in certain villages. These, too, are the most helpful and kind people encountered.

Some examples, the Reila family related to the great-grandmother, with roots in the Huisheim Angermühle, a grain mill.

The Sebald aunts, daughters of a postman who left the farm villages to follow job opportunities in Augsburg, the largest city in the closer surroundings.

Very unexpected also the huge number of relatives related to my great-grandmother, Therese Schwendner – a kind and distant relative had a picture of here sister will all her children, dating 100 years back.

Let’s see how things will proceed over summer, with less time and more outdoor work to do.

Sennheiser Momentum 4 Headphones: water and a corroded connector

Normally, I don’t repair consumer electronics, but in this case, a good friend asked me to have a look at these expensive Sennheiser headphones. Apparently, some water had entered the case, and since then, these were not operational. When charging, it blinks the red led 3 times, pause, 3 times, and so on. No other sign of activity and no success to reset it by pushing the button even for a long time.

To have a look inside, you have to first remove the ear protectors, then remove 4 screws, and gently pull-out the speaker. Exercise great care! There is a flat cable to connect the microphone to the intermediary board (the same board that has the battery connector). The flat cable and connector can be easily damaged, better use small tools and a microscope.

The battery connector had signs of visible corrosion. Probably cause by the combination of water and electric power. The battery has about 4 Volts when charged, so it can easily cause electrolysis of water and generate corrosive products if the water contains traces of salt, etc.

Unplugging and re-connecting it, using some contact cleaner, I was able to establish a stable (electrical) connection again.

The battery is contained in a small case, broke it a little when opening, all these internal parts are pretty fragile. But still functional. Inside, the cell is just attached by some double-sided sticky tape.

Here, a close-up of the microphone connector. It needs to be opened with a needle or tiny screwdriver by lifting the black part carefully. Easy to break!

For the current headphones, there battery was still good, but there are spare cells available. The quality of these may be variable so you may better check them before using as a replacement (e.g., by giving them 10 or 20 cycles by an external lithium ion charger). The price is quite OK, but you wouldn’t want to open up the speakers every few months for a battery replacement. The Sennheiser factory battery may last for about 3 years of use.