General

HXX GCS900-3D Digital Readout: new addition to my small lathe

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A nice offer from the famous “Shenzhen Hengxingxing Precision Instruments Co., Ltd.”, or, HXX in short, lured me into buying a “retro-style” DRO for my lathe. There are all kinds of touch-screen DROs available, and I have the lathe connected to LinuxCNC including a DRO display, but nothing beats the charm of 7 segment LED displays and some buttons in terms of productivity and handling convenience in a workshop with oily and dirty hands.

The offer, just a little over 50 EUR, shipping from Spain – so there is no risk of customs duties or value added taxes and similar, all is included.

The device came well packaged, in a strong box, with foam padding, in a plastic cover. A paper manual is included.

Surely, I needed to open it and check it out before use. There are three 8 digit displays (XYZ) and an 8 digital alphanumeric display on top. Many function keys and a build-in calculator, so we don’t have to scroll through some menus but can access each function by pushing a button the old school way.

The connectors are wired for a typical Chinese DRO pinout: Pin1 5 Volts, Pin 2 Ground, followed by A B and Reference. Sino and other common brands use different pinouts, but you could easily solder the wires as you like. There is a metal plate that holds all the connectors, so in very special cases, it is even possible to fit some completely different connectors without harming the original case of the DRO.

The seller was very concerned and helpful, because I ordered the display without glass scales (I have these already installed) and contacted me proactively to inform about the pinout and connectors. Very nice customer service. There can be various comments and thoughts about buying such equipment from China, but so far I have only the best experience – try buying from a German company: they will charge you an arm and a leg, you can’t expect to reach anyone with any know-how except the list price by phone, messenger or email, and they may eventually respond that all is proprietary and they won’t help you for your specific case. Eventually, shipping and handling would cost more than the total cost of the AliExpress purchase.

The signal first passes through some low pass filter and protection circuit: a 10 k resistor, and inductor, a R-C filter and current limiting resistors.

The filtered signal is further conditioned by a 74HCT14 Schmitt trigger to clean up the edges. I use the same chip for my own DRO signal conditioning, a good and cheap choice. Furthermore, these are just common SOIC parts, easy to fix (rather than some trigger input of a programmed microcontroller).

The processor is an ARM32 type, ARM Cortex-M0+ core 32-bit microcontroller, quite powerful for the application.

The display is driven by AiP1629A drivers, these are 16-segment 8-grid LED drivers (and keyboard scanner, but this function is not used for this DRO application).

All is powered by a small switch-mode power supply, which appears to be of decent quality considering the isolation features and capacitor rating and quality level of the Y capacitors.

Now, for the connection to the lathe, we have do solder some wires to the junction box that currently links the DROs to a MESA Anything IO 7I92TF card.

All a bit messy inside the box, but it is dust tight and has provided more than 10 years of good service in my workshop.

Finally, I can compare the readout the my LinuxCNC powered display (which interfaces to the MESA card via ethernet).

A few difficulties came up with the configuration of the DRO: the configuration menu cannot be accessed by pushing the A/I key or any other of the key “6 times” as often mentioned and also by instruction manual. Rather, you have to keep the “Enter” key pressed during start-up, and then scroll through the options with the arrow keys until you find the mode selection “MILL-3D” or similar, then push Enter until the desired mode shows up. By pushing the “.” key the settings are saved. For the resolution setting of the cross-slide, which uses a finer DRO, I had to use the 1 um resolution and a 250 mm/m linear compensation to get the correct reading. There are probably ways to set this differently, but it is working just fine this way. If you set up your lathe with glass scales, I highly recommend using a high-resolution scale for the cross slide, because working with diameters will half the resolution you get (10 mm travel corresponds to 20 mm diameter change!), and on a lathe, 10 um is a long distance when it comes to diameters of holes for bearings and similar.

There are also a few manuals about these DROs in my collection (the paper version seems to be a little outdated, anyway, good enought). If you need any of these and can’t get them from the supplier in time, just contact by email.

HXX GCS900 Instruction Manual

Sino DRO Manual (different brand from HXX)

Vevor DRO manual

A little study of the parts cost: probably hard to have less than 30 EUR in material, even considering mass production parts cost. If you buy the parts retail, easily already 50 EUR just in parts.
There is still the holder, wall bracket, two large screws and nuts, and related hardware that may cost another 3-4 EUR. Hard to believe that such device can be made, marketed, sold, shipped and supported (by an estimated 5-10 min of customer support time) at a profit to the HXX company.

While the cheap parts and microprocessor and such cause no harm, there are two items that are a little “cheap”. Firstly, there is no filter foil for the display, and while it is bright green, the contrast in workshop lighting is not that great. Would be nice to have a green filter to improve readability. Secondly, the case is made of reasonably solid plastic, probably ABS, but some high-end DROs have cast aluminum cases that will survive in rough workshop environments better than a plastic display. For hobby use and for workshops that take care of their equipment, plastic will be good enough for a long time, certainly more durable than some of the recent touch screen DROs or DROs with fancy LCD screens that will fail after a few years of use.

All in all, very happy with the purchase, and speeding up my work. Many Thanks to the HXX company to provide such a nice instrument for an affordable price.

Note: to reset all options to factory conditions, there is password required. My unit had password “2000” when received.

Miscellaneous

Antique Weights: return to scale

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Recently there is lots of work around antique balances and weights to support a collector of these items, and recently a very unusual package showed up at my workshop, a set of old weights for restoration. Basic intention was to remove all grime and dirt by bead blasting, but surely we need to check if the weights are actually usable.

Some were of the traditional knob style, while others have a hexagonal shape and are adjusted with lead at the bottom.

Unfortunately, except one that was heavy (with a lot of dirt on it), all were light. So we have to find a way to add mass to these pieces. Unfortunately, I couldn’t find any instructions how to do this, because all the modern weights cavities closed with screws or a plug of lead that can be hammered in, but these old-fashioned weights are lacking this feature. For the hexagonal weights, it is clear that we have to heat them and add more lead, but for the knob weights, let’s hope there is enough space inside to fit material for adjustment.

All the matters of weights are regulated in the OIML R111 rule: it says that adjustments must be made on this class of weights (“M” is the class for the not-so-accurate weights used for general trade and general use).

The adjustments should be made with heavy metallic material, like lead shot. After drilling open the lead caps of the old weights, and removing the remaining pieces with a screwdriver (it is enough to drill an approx. 6 mm hole and then remove the remaining material by a chisel or screwdriver – avoiding to drill into the cast iron ruining drills and taking time.

What I found inside was indeed some lead shot, but also dirt and sand and other lead pieces. I didn’t clean out all the weights but simply added some old lead fragments to get the weight up to the required range. This has to be done while accounting for the mass of the lead plug. This plug will take about 5 g of lead for the 500 g weights, about 7-8 g for the 1 kg and 2 kg weights.

Various methods may exist to close the lead plug, by I found it the easiest way to first insert a little bit of glass wool and then pouring molten lead from an old spoon, heated over a gas flame.

Best method seems to be to target a final weight a little, say 0.2-0.5 g above the target – closer if you have some practice. The allowable tolerances for M3 class weights – that’s the target I am following can be found in below table. Still my objective was to adjust these to +-0.01 g as much as possible by cutting away some lead from the plug after letting everything cool down.

For the adjustment I used my good Mettler PB3002 balance, 3000 g capacity with 0.01 g resolution. Set up on my surface plate and calibrated with a 2 kg F1 class weights (+-10 mg at most). Also cross-checked the linearity at 500 g and couldn’t see any deviation (measuring as 500.00 g).

It is a little tedious to adjust all the weights but eventually all the pieces were done, and I also fixed some old weights I had for a long time (which were also light because of metal loss from severe rusting). Before the fine adjustment, the pieces were all glass bead blasted and thoroughly cleaned.

Finally, all the weights are done and double-checked. Frequently also put the 2 kg F calibration weight (upper left) to check for any drift, which was found to be absent. FOr the smaller weights, I used my analytical balance – the 100 g weight turned out to be exactly 100.0002 g, lucky!

I can’t really stamp an official mark on these, so I used a letter “M” both to firmly compress the lead plug and to indicate the accuracy class of the weight. Supposedly, these weights will be handled in a kitchen so I made the lead plugs almost flat with the surface, rather than protruding, to avoid hand contact with lead. But by adjusting the lead quantity, it is easily adjustable.

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.