Various

Mettler AE240-S Analytical Balance: investigations and restorations

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Recently my workshop has become a repair shop for historic balances (more about these later), and triggered by this activity I got a used Mettler AE240 balance. This AE series is probably one of the best balances you can buy, if you are looking of a analytical balance with 0.0001 g (0.1 mg) or even semi-micro 0.01 mg resolution. Having used these balances before, the AE is certainly my favorite, because of its mechanical quality and long-term stability. See also some earlier posts:

Mettler AE 163 Dual Range Analytical Balance: Swiss Made equipment, in Japan

Force compensating precision balance: a few very interesting, very rare schematics

Unlike modern equivalents, there is no software linearization or fancy load cell, but all is based on a solid mechanical design, careful study of the thermal effects, etc., the pinnacle of Mettler engineering. Furthermore, it is fairly easy to repair it, there are schematics available, and the components that fail are mostly the tantalum capacitors, but not the two proprietary Mettler ASICs.

There is a 100 g calibration weight built-in, therefore the balance can be easily checked without any need to handle precision weights.

There is a marking on the transformer, year 1988 – nearly 40 years old.

One key element of these balances are the hinges, machined (maybe by EDM or similar?) from sheet metal. Several of these – of identical design – are used in the AE balances.

Another key assembly is the light gate position sensor. Using an IR emitter, left (SFH401-II), and a differential photodiode BPX48, right.

Following the explanations in earlier posts, there is a force compensation system working such that the load added is compensated by the electromagnetic force of a coil in a magnet. The current through the coil is proportional to the load. Firstly, I checked the current through the relevant resistor R49, a 390 Ohms value resistor, and the current is roughly 20 mA at full load (200 g).

Rather than measuring the current by a voltage drop, Mettler design a fairly ingenious way to sum currents of the coil, of the temperature compensation circuit and from a precision current source in on node (which is kept at GND), feeding it to an integrator capacitor. The integrator voltage is compare to a rather fast ramp of ~1.638 ms period. Eventually, the precision current source on-time is proportional to the weight applied. There is a comparator, LM311, and the output has a convenient test point at the pull-up resistor.

I connected a highly precise counter to it, to measure the pulse width with various weights applied.

The pulse width test proceeding….

With 50 g applied, the on time is about 1/4.

At 200 g applied, the precision current source needs to be on most of the time, to keep the time-averaged voltage of the integrator stable (keep the capacitor, on average, at constant charge).

Some measured data – note that these were not taken with outmost precision, but just single cycle measurement. We would have to measure averages over several seconds for ultimate precision.

In any case, the tests showed the strict linearity of on-time of the comparator output with the load applied.

Having now understood the inner workings of the marvel to a larger degree, a small issue is the dim display. Over 20 years back, when working in the US, I had balance serviced, and remember that at that time, the service center also replaced the display. The AE series uses a vacuum-fluorescent display, called VFD in short. Surely we could replace the VFD by a LED display or similar, the signals would be easy to process, but better to keep all original and search for a replacement.

Checking the part, it is a Futaba 9-BT series 26ZA display. Futaba is no longer manufacturing displays, so the only chance are new-old-stock parts.

Looking around I found some really nice parts, 9-BT-28ZA. Comparing the pinouts precisely, the 28ZA display resembles the 26ZA. There appear to be some differences in the filament voltage, but these can be easily compensated by either a regulator or a resistor (will check once received).

To have some stock, ordered two pieces from the seller, who resides in Bulgaria (!) – let’s hope the package doesn’t get lost.

Various

Measuring the Speed of Light: Fast light pulses, just about 10 m long

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Revisiting some circuits built 15(!) years back – feels more like 10 years back, high speed light pulse transmitters and receivers. These come in handy to measure the speed of light. Nowadays, I would rather send out some train of pulses with applied digital modulation, say, a pseudorandom sequence, and then calculate the time difference digitally, from the received signal. This would surely be be very much more sensitive, but also much less instructive than receiving light pulses discretely.

The first motivation years back was a request from a school to build several sets of light transmitters and receivers, so that students could measure the speed of light by determining the time of travel of light pules. It follows closely this wonderful article of Mr. Ehret, unfortunately, I only have it in German:

Bernhard EHRET: Messung der Lichtgeschwindigkeit mit Lichtimpulsen.
Journal: Praxis der Naturwissenschaften. Physik, Volume 41 (1992) 4, pages 17-35

I built several versions of the light transmitter, the first sending pulses of small power, at a current pulse amplitude that doesn’t hurt the LED for a long time. Good enough for some shorter distance measurements. As a reflector, a back-reflector used for light gates is recommended (cat’s eye reflector), approx. 30×30 cm will do fine.

The schematics all are similar: there is an input capacitor, a current limiting resistor to charge a pulse capacitor, and a transistor used as avalanche device to generate the pulse. The pulse duration is very much limited by the LED rather than the transistor.

Mr. EHRET recommended a BC546 resistor, and I have also tried this first. The collector-emitter breakdown of the BC546B which I used in the circuit was above 200 Volts, and with the energy stored at such voltages in a 4n7 capacitor (a FKP1 Wima pulse rated capacitor), the LEDs used were quickly destroyed.

These are the two test circuits, the received and the sender, similar to the suggested circuits from the journal article, but with voltage stabilization of the receiver (LM317T added to regulated the voltage to 18 V).

The receiver is a wide-band FET-input amplifier, the resistors at the source of the FET are quite important: together with the PNP transistor, these provide negative feedback to counteract the Miller capacitance effect of the FET. The amplifier circuit could be replaced by any modern fast transimpedance amplifier, but hard to beat the cost of a BF256 FET and a BF979 transistor. Both parts can be replaced by similar devices, eventually with some minor adjustment of the resistor values to adjust the currents appropriately.
The photodiode, rather than the BPW34 I used a fast Hamamatsu S5972, because I had several in stock from other projects. The S5972 operates at low voltages, wide spectral response also for visible light and has about 500 MHz bandwidth.

The transistor conductive breakdown will happen in less than 1 ns, but the pulse is about 43 ns long, because of the intrinsic properties of the LED, like, its inductance.

With the high breakdown voltage, it seems the light output is not much larger compared to the pulses generated by a 2N2369A at ~90 Volts, and there is considerably more ripple and noise at high breakdown voltages (white trance below is the BC546B circuit compared with the 2N2369A – yellow trace). Definitely, the classic 2N2369A (I have a stock of “JANTX” mil-spec tested devices and don’t recommend to use some copies or fake 2N2369 but rather metal TO-18 case old stock to be sure about the breakdown characteristics – modern versions and copies/fake 2N2369 may work well for common uses, but could have a completely different die inside, with random breakdown voltages).

The 2N2369A gives reliable breakdown performance, and the rate of the pulses can be controlled by the voltage applied to the circuit, adjust to about 20 kHz, which requires roughly 90 Volts.

This is the board, with the BC546B soldered, it is a rather modern version of that transistor. Maybe I should have tried some older stock BC546B, BC238B and so on, but whatever you use, the breakdown voltage should not exceed 150 Volts, otherwise, you are at risk of damaging the LED. With high efficiency LEDs and 90 Volts breakdown, there is no practically relevant aging of this circuit, but at 4n7 capacity and >100 Volts, some aging will show up eventually. The lifetime of the LED will likely also depend on the pulse current, but the test circuit was build with rather low inductance traces (short wires, barely 1 cm), so the currently will be close to the highest obtainable. The LED could be soldered in with shorter leads.

This is really the best choice, the 2n2369A!

The high voltage the drive the transmitter is conveniently generated by a YH11068A DC-DC converter, available from Aliexpress for just a few dollars. Even though this module has considerable noise, it doesn’t disturb the receiver, and because of the current limiting resistor in the pulse, has also no effect on the pulse performance.

It is such a nice experiment and easy to build, some small danger with the DC voltage, but most of that circuit is severely current limited. If you have a scope that is capable of ~50 MHz bandwidth, it is a nice experiment for nights outdoors, and even kids will be interested to set up the mirror and see how light travels at limited speed.

At the time, I did also experiment with laser diodes, red and orange diodes ranging up to 35 mW, but there is no particular advantage in using these. Focusing a laser beam over 10-20 meters of travel it also not an easy task. Better just to use a LED, and large lens of appropriate focal length (say, 200-250 mm).

In the archives I also found some of the earlier measurements (note the datecode: September 2011), using a 54720a scope with a 54721a plug-in (4 GSa/s with 1.1 GHz bandwidth). Some pulses are barely 25 ns wide when measures correctly, which is less than 10 meters of light. I.e., the light pulse is so short that it is just about 10 meters “long”. The light source was a HL6323MG laser diode, a 639nm, 30mW AlGaInP laser diode of Ushio corporation.

Various

Steep Edges: Revisiting some old tunnel diode circuits

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A phone call recently reminded me about some experiments a long time back, using some tunnel diodes to generate fast edges. Various circuits were built at the time, this is one of the final working versions. All soldered to just a piece of double-sided circuit board with two SMA connectors fitted.

The trigger signal, a reasonably fast square signal is provided through the BNC connector. The bias circuit is separated from the tunnel diode by a few nanoseconds of delay line, which helps to make the transition stable.

From these “old days” I still keep many tunnel diodes that were at that time ubiquitous from the left-overs of even older soviet times (made in the late 1970). There must have been several boxes of tunnel diodes that made it to the Western market. Maybe from some surplus military sale or give-away, because many of the diodes are “military grade”.

The diodes are for various currents, like, 5 mA and 10 mA, and there are both GaAs and Ge (germanium) types. For this circuit, I have the 1i305B mounted, a 10 mA fast-switching optimized Ge tunnel diode.

With a bias of about 8 volts, roughly 8 mA of current, I get the circuit to trigger nicely. At the BNC input, there is a protection circuit (a DC block and a 1N4148 diode to limit the amplitude). Tunnel diodes are susceptible to soldering heat, so be careful and use pliers to conduct away the heat from the terminals while soldering. I solder them “from the side of the board”, melting the solder on the board or connecting part, and then barely dipping in the pre-soldered terminal of the tunnel diode for merely a fraction of second, then cooling it with my finger. Slight burns may result but the diode will be safe.

The rise and fall times are nice, the triggering is stable and precise. I have sold all my super-fast oscilloscopes (used to own a HP 54750a 20 GHz scope and a even a 54720a 1 GHz real-time scope, but sold these several years ago after completion of certain projects that required these – I don’t want to start a museum of test equipment so I regularly sell equipment that is no longer needed, especially equipment that is expensive and prone to degradation).

For the tunnel diode, there is even a datasheet available, albeit in Russian.

Easy enough to read with Google Translate, but even without translation, the key properties are obvious: peak and valley current, current ratio, capacitance. Over the years I have used some of these tunnel diodes to repair old oscilloscopes and trigger circuits. Typically, to substitute diodes that are no longer available (certain types of General Electrics) or otherwise prohibitively expensive. So far, never any trouble finding a suitable Russian diode.

A quick schematic from the archives, sorry it is not written very clearly, but already more than 10 years old. Left SMA is the output – there is a 47 Ohms series resistor, and a small network at the outlet to suppress ringing. The delay line is just a piece of regular RG 174 (50 Ohm) cable.

There are also datasheets for a variety of other tunnel diodes available in the Manuals’ archive, contact me in case you can’t find them.

Ancestry

The Family Chronicles: Linoprint cover page

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With the chronicles related to my father’s parents completed, and the book decorated by hot embossing (see: The SCHRÖDLE Family Chronicles: decorative hot embossing) it is time to consider the decoration of a similar book about the parents of my mothers. Fortunately, the contents are nearly complete, but I didn’t want to do just another hot embossed front page. After some brainstorming, woodcut print seemed like a nice idea. Surely, I have no time and patience and skill to cut a complex plate from wood with knifes, but intended to use a digital design and than LASER-cut it into the material to general a relief suitable for printing.

First I tried to use actual wood, but the results were not that nice with ordinary plywood. The edges break off easily with thin elements.

Cutting is easy, I am using a Sculpfun S9 cutter with a blue diode LASER. The material is held to a metal plate with some magnets, there is no further workholding needed. Compressed air is used to blow away the dust and to protect the laser optics. All the fumes are extracted by a fan to an exhaust – the whole LASER cutter is in a protective enclosure..

After seeing the mixed results with wood, quickly ordered some “art supplies”, good old natural linoleum (don’t get the PVC type, this cannot be LASER cut and will emit toxic fumes!). Linoleum plate is made from linseed oil, wood powder, limestone filler and some additives. Just a few days later, two plates arrived. There is a very nice and particular smell of fresh linoleum in the workshop.

Before going to the endeavor of large format prints, good to start with some smaller test piece. Because of the thin beam, the engraving works best here at 10 lines per mm, 1300 mm/min speed. After a few minutes of engraving, a small cat relief was ready. The raw cut needs some treatment to remove the burned residues. First tried with an eraser, works well, but eventually a good method is to bead blast it with compressed air.

There are two kinds of paints, water and oil based. Water based paint has the advantage of fast drying and easy cleaning, but for the traditional look and ever-lasting persistence, oil based paint is preferred. After doing some research, the DALER-ROWNEY ADIGRAF seemed to be a good choice. A 250 mL can is just about 10 EUR and will last for many prints.

Next, we need a rubber roller. Using some piece of an old typewriter, cut to size and turned down to a little smaller diameter on the lathe, and with a simple wood handle attached, a sturdy roller was fabricated quickly.

The cut linoleum relief is stuck down to some old chipboard, and the printing setup is ready.

To my greatest delight, the first print turned out well. A lovely cat print. Just used my fingers to transfer the ink by circular motion.

The cat motive is good for experimenting: the quantity of ink, the uniformity of ink application, attaching the paper, transfering the ink, all needs some practise.

With many cats printed, it is time to engrave the big relief for the book cover page. The artwork went through quite some iterations, because I wanted to incorporate motives from my ancestors’ past, without it looking to fancy. Giving it a little worn and historic look, with simple elements. A certain variability of the ink application will give a nice individual touch.

The engraving took about 3 hours. Not fast, but for a very affordable 5.5 Watts LASER, fair enough.

Cleaned it out by bead blasting (soda glass beads of ~300 µm at 8 bars), and some manual work to cut it out with some tapered edges.

After the cat exercise, inking the relief worked pretty well and the first print gave a good result. Switched form 150 g/m2 paper to hard 250 g/m2 board to give the book some stiffness.

In no time several prints were made.

While the deep red color is a nice, it is lacking a little contrast. From very old books, like this copy from 1692 (which is one of the oldest in my library), a black-red color scheme is known, which has been also used for woodcut prints and similar, for example, for “ex libris” cards and other more art-oriented printing.

For inking, a small roller was needed. Rather than buying one, made it myself, this time from some paper pushing roller of an old printer (good that I keep a large assortment of parts in my basement…).

With a piece of steel wire, the small roller (also called a brayer) is ready for use.

To roll-out the ink to a thin layer, some pieces of old glass sheet attached to a wooden board are hand, because of their smooth surface and resistance to solvent cleaning (methylated spirits are good for cleaning of the oil based ink).

The inking is about three times the effort compared to a single-color print, but at least we don’t need to worry about the alignment of the paper when all two colors are printed at one, rather than consecutively.

The result turned out well, such a nice contrast of red and black!

Now, many prints have been made, and these need to dry thoroughly, which will take at least 5 days, better 2 weeks, for the oil based ink.

Various

Broncolor Primo: the photo flash is flashing again

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A very unusual and dangerous repair, a defective photo flash. This is a professional unit, for use in photo studios. Generally, these units work by charging a big capacitor bank to 300-400 VDC, and then discharging this energy in an instant through flash bulbs. This unit has 10 pcs. of 2450 µF capacitors, charged to 360 VDC. About 1600 Joule, which makes this unit very dangerous, I would strongly discourage anyone not familiar with power electronics to even open the case.

For its size, it is quite heavy unit, and has a 16 Amp fuse, it will recharge quickly, drawing substantial power for a short time.

Inside are 12 large capacitors, 2 for the charging voltage stabilization, 10 for the capacitor bank. After duly checking the caps and their safe discharge state, I tested them all, each individually. 3 were bad: 1 completely disconnected at the terminals, 2 with no capacity, worn out.

Flash capacitor need to withstand high discharge current, so we cannot just use any ordinary cap but need to source “flash capacitor” – found reasonably priced one from China, because there were other faults with this unit besides the capacitors, there was no reason to by expensive caps first, without knowing if this unit can be fixed at all. Unfortunately, there are no schematics available.

Studying the electronics, there is a primary thyristor (TXN1012) switch DC stage, a type of coarsely regulated power supply. This had a blown transistor in its control circuit. Failed by arcing. Fortunately, I was able to still read the color rings under a microscope, and replaced the part and an associated Zener diode. Also the thyristor and a MOSFET in the thyristor were replaced (the MOSFET tested good, but I didn’t want to take a risk).

The replacement caps have 2000 µF. 3x 2450=7350, 4x 2000=8000, so I decided to install 4 of the 2000 µF capacitors (2450 µF were not available easily). It results in about 5% higher energy in one bank, good enough.

After some hours of complicated failure search and repair, some very careful tests (checking if the caps load symmetrically, which they did), finally the green light of the “flash read” LED was lighting up.

The flash worked – but only for a short time, then: SMOKE from the flash box. Expecting the worst, opened it up right away. Surely, first removing the cables from the capacitor bank.

Inside of the flash box, 4 Xenon? flash bulbs, with spiral trigger electrodes. The high voltage trigger transformers are right inside the flash box. The smell is awkwardly familiar: an exploded Rifa safety capacitor. 0.1 µF, 1000 VDC.

Fortunately, it failed open, as it is supposed to, lots of bad smell but no damage or fire. There is one capacitor for each bulb, a total of 4 (2 sets).

I cleaned up the mess, ordered 4 original Rifa (now KEMET, but they still print “Rifa” on these), and soldered it all back together. The other 3 were electrically still good (tested for leak resistance several MOhm, and isolation test passed at 1000 VDC), but had many cracks so I replaced all capacitors.

Eventually, all is working again. There is a built-in studio light, quite fancy unit. Hope the repair will last for a while!

Various

ANENG MH15 Isolation Resistance Meter: a remarkable deal

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Typically, I don’t report about test equipment acquisition unless these are associated with repairs, but this time, I will make an exemption. I used to have an old isolation tester, but it has been playing up, and with the analog instrument, difficult to repair. So I checked about more recent instruments, mostly, to check electric installations and motors and similar, for isolation after repair. This requires a 1000 VDC test, because some isolation defects tend not to show up at low voltage.

After a quick search, I found an almost unbelievable offer, just about 25 EUR! Shipping included!

One week later, a badly packaged box arrived, in a plastic bag, but without major damage.

Inside, a handy soft case.

The instrument, the plastic is of good quality. The red plastic is a little hard so it will not absorb too much shock when you drop the device.

Immediately, I got some of the highest value resistors I have in stock, a Remix 1000 MOhm +-5 % resistor. This is good to 10 kV, so there is no problem or leakage when doing a 1000 V test (other resistors may be have voltage rating of 300 V or 500V).

At a first glace, very nice result. -2% within the tolerance of the part.

Checking the voltages, 1000 V is fairly accurate.

500 Volts, even better.

Some more deviation at 250 and 100 Volts.

The specifications are quite good. But eventually, the instrument is not used much to measure resistance, but to check for conduction when 1000 Volts are applied. Normally, it will either show >2000 MOhm, or a spark will fly and the high-voltage isolation test has failed.

For completeness, also checked a 200 Ohm, 0.1% resistor. 1.5% deviation is the specification of the MH15, and deviation found is 0.25%, very good.

All in all, a great instrument for the occasional user, and one more reason to not skip isolation resistance tests.

Various

Rohde&Schwarz URE RMS-Voltmeter: analog digital communication

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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.

Various

L33 Thermal fuse: inner workings

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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.

Various

Rohde&Schwarz NRT Power Reflection Meter: tantalum issues

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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.

Miscellaneous

Christmas Bakers: Dominosteine – dominostones

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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.