How Bright is Your Light Part II: Repairing More Modern Illumination Systems

Zeiss Laboval 4, base open to show illumination power supply
Closeup of power supply circuit board – four transistors, one integrated circuit, two fuses, three trim pots, and four round gizmos that I cannot identify (left side – bridge rectifier??)
..and all to light one little bulb

This article continues from Part I, which discussed repairing the illumination control circuit of the Nikon S, as well as similar very  early electronic microscope power supplies.  Here, we discuss the principles behind more modern microscope illumination systems and suggest fixes for blown lighting circuits in scopes of this vintage.

The simple fix described in Part I worked well for the uncomplicated, 1970s-era intensity control circuit on the Nikon S, and will probably be applicable to similar early microscope illumination systems. Unfortunately,  the greatest number of quality used microscopes are of more recent vintage, use more complex electronic control systems, and are more of a challenge to repair.

Removing the base plate from a newer generation microscope will likely reveal a printed circuit board bearing a number of small capacitors, resistors, coils, transistors, and integrated circuits.  The Laboval 4 power supply shown above is complex, with some very Russian-looking components.  Built in East Germany in the late 1980s to early 1990s, the Laboval 4 is built like a Russian tank and has excellent optics – but buying replacement electronic parts would be almost impossible.  You can see why a scope of this type would be scrapped if the power supply blew.

Besides, what do all of these electronic gizmos have to do with running a light bulb?  I must admit to having had almost complete ignorance about electronic microscope power supplies, and yearned for a good old autotransformer that could break a toe if dropped.  However, if your power supply dies and you are faced with repairing it or spending bucks for a new scope, don’t give up.  That small, mysterious printed circuit board in the base uses very standard circuits, and the supply can be rebuilt or replaced more easily than you might think.


In the decade after the Nikon S-Kt, microscope manufacturers worked hard to cram a smaller but equally powerful illumination system into the microscope base and eliminate the external supply.  Since the transformer is the largest and heaviest element in the illumination power supply, this meant reducing transformer size while maintaining capacity.

The solution was the so-called “electronic transformer” – actually a misnomer for a “switch mode power supply” or SMPS.  Don’t let the fancy name scare you; this type of power supply arises from an ingenious but simple concept and is made from basic circuit components.  The essential idea is that the size of a transformer, for a given wattage, is inversely proportional to the frequency at which it operates.  So a transformer operating at 300 Hz (or 300 cycles/second) can theoretically be 1/5 the size and weight of a similar, old-fashioned unit working at 60 Hz, or AC line frequency.

220 to 12 Volt “Electronic” Transformer.  This is only a few inches long and weighs a few ounces.

This type of power supply operates by transforming 110V or 220V line AC into direct current, and then transforming it back into alternating current via a simple transistorized switching circuit – but at a higher frequency.  This “re-manufactured” higher-frequency alternating current can then go through a smaller and very efficient transformer to be stepped down to a lower voltage suitable for powering an incandescent or halogen lamp. Many of the small supplies work at quite high frequencies of 20,000-50,000 Hz; they are thus quite efficient, and any interference signals produced are well above the threshold of human hearing.

The circuitry for rectifying the AC current to DC and then”inverting” it back to higher frequency AC  that can be re-converted into whatever you need (much like the 12V to 110V inverter one buys at the hardware store) is light and compact, and the final transformer is a fraction of the size and weight of an old 60 Hz unit.  The AC ==> DC ==> high freq AC ==> low voltage AC transitions for lamps are handled on a board a few inches long and weighing only a few ounces.  For incandescent or halogen lamps, the low voltage output can be either alternating or direct current – lamps with filaments aren’t fussy about what they eat for dinner.  For a direct current-only device, a final step is added in the form of a second bridge rectifier to convert the low voltage AC into smoothed DC:

Simplified circuit for a DC output switch mode power supply, with second bridge rectifier converting low voltage AC output to low voltage DC

The filter capacitors store energy as the rectified direct current pulsates, taking in energy during the peaks and releasing it in the valleys, much as an old-time millpond provided a constant flow of water to the mill’s water wheel.  The result is a smooth direct current output voltage.


Since solid state components (integrated circuits, transistors, and diodes) can now be mass-produced for a few cents, tiny SMPS power supplies are so cheap and convenient that they are used for supplying most small electronic devices – battery chargers, cell phones, tablets, clocks, lamps, etc. – and most households have at least a dozen hiding in wall sockets and behind bureaus.  A home with many electronic gizmos may have a hundred of these small devices.  In electronic slang, the small, socket-mounted variety is called a “wall wart“. This derisive moniker arose from the tendency of the older, larger chargers to eat up space on wall sockets and power strips, and to be heavy enough to fall out onto the floor at intervals, crashing the instrument they were supplying. Today’s socket-mounted  SMPS chargers are much smaller and lighter than the “linear” designs of twenty years ago, which, using a bulky transformer followed by a rectifier and filter, often drooped from the socket.

“Wall Warts”: Old linear transformer-based power supply (bottom) drooping from the socket, versus contemporary tiny iPhone SMPS (top)

One should not, however, disparage the older “linear” power supply.  The term “linear” refers to a supply that, unlike the SMPS, does not convert and reconvert electricity from AC to DC to AC, changing the type of current AND the frequency AND the voltage.  SMPS supplies have taken over in most consumer applications because of their small size, light weight, low cost, and high efficiency.  However, their components are constantly working as tiny switches; they consequently have a higher noise level and poorer voltage regulation.  The older “linear” (transformer ==> rectifier ==> filter) supply, named because its components are operating in the linear phase of their operating curves, is inherently heavier and produces more heat, but has less noise superimposed on the output current, better regulation, and faster recovery from transient spikes in the line voltage.  They are still used where very pure, low noise, very tightly regulated current output is required.  A chart comparing the features of the two systems can be found on the Acopian site.

From a microscopist’s standpoint, the tiny but powerful SMPS supplies and halogen bulbs resulted in leaner, lighter scopes and brighter lighting without bulky power transformers.   HOWEVER, the downside of this innovation was that those clunky but durable transformers were replaced by electronic components with a much more uncertain lifespan.  These may be difficult to replace after a few years have passed (one service specialist struggles to find parts for some twelve-year-old models), and the first generation of these electronic systems is now approaching the 50-year mark.


One has three alternatives to repair a blown electronic power supply: move to a simple external supply, buy a power supply that will fit, or custom-build a small supply and control unit that fits into the same space as the old unit.  Building an external supply was discussed in Part I – a cheap dimmer driving a 6V or 12V transformer, possibly of type used for a doorbell and available from a local home supply store.  Buying a new power supply is easiest, and these are cheap and standardized – this small SMPS works on 85-265V AC input, puts out 12V DC at one ampere, and costs $8.40 on Amazon Prime:85-265V to DC 12v "Electronic Transformer"One may have to figure out how to tie the microscope’s light intensity control potentiometer into the circuit, but this is a cheap, prepackaged fix if the supply fits the space available in the base.  These small power supplies are available from online stores in a wide variety of combinations of wattage and output voltage.  A few have the dimming function already built in, and are intended as replacement power supplies for single halogen bulbs:

6 Volt, 30 Watt Dimmable Halogen Lamp Replacement Power Supply, $22.99 on eBay

If you do buy a generic power supply, try to pay a bit extra and get it from a reliable manufacturer.  There are many cheap electronic devices available in the online market, but many of these are of overseas origin, poorly-constructed with substandard components, badly designed, and pose a shock hazard (see Elliot).

One very cheap solution is to look for an old computer or printer power supply in an appropriate voltage.  This may take considerable ingenuity and searching, but these can be found readily at thrift shops or Salvation Army stores.  Depending on the supply’s circuitry, its output may be dimmed either with a low-resistance rheostat on the low voltage side, or an appropriate 110 or 220 volt dimmer on the line side.  The supply can either be mounted in its plastic case, or opened and the circuits removed.  An older, linear-type supply may be easier to dim.  It may be necessary to try two or three thrift shop power supplies until you find the one that works.  You might even consider buying an old, variable-intensity halogen desk lamp at the thrift store and transplanting the power supply.


Rebuilding the power supply or building it from start is a bit more challenging, but can be done even with modest electronic experience.  You may be able to clean off and use the appropriately-shaped circuit boards from the old illumination system.  Depending on the illumination system of the microscope, there is a multiplicity of adaptable basic circuits on the internet; these may use two transistors, thyristors, or a small integrated circuit for the switching circuit, as there are a number of ways that this simple circuit can be set up.  The circuit below uses a bridge rectifier (left side) to produce DC from 220V mains AC, then employs an IR2153 integrated circuit and two IRF840 MOSFET (Metal Oxide Semiconductor Field Effect Transistor) devices to chop the DC to rapid frequency AC; the latter is then stepped down by the small transformer on the right to light a halogen bulb.

halogen lamp circuit

Given the many variations in space and circuitry used by microscope manufacturers, there is no single fix that is adaptable to every vintage scope. Building a replacement illumination supply will take a bit of ingenuity and perhaps a visit to your local electronics store (NOT the consumer radio store in the mall – you need a REAL electronics store with racks of equipment and perhaps a bit of dust in the corners).  Which circuit you use will depend on available materials and components, what kind of advice you find, where you live, and how much power your light needs.  Finding a truly simple dimmable power supply circuit for a single halogen bulb is not an easy task; the above circuit is one of the least complex.  But actually constructing one of these power supplies is not rocket science and can even be done as a first electronics project.


This is an example of a tiny, home-made halogen lamp SMPS power supply – look closely and you will see that the wiring and soldering are pretty rough, but it works:

OLYMPUS DIGITAL CAMERAHomemade Switch-mode" Halogen Light Power Supply

This small, home-made switch mode lighting circuit supply uses two thyristors (or possibly MOSFETs, they all look similar) and a small integrated circuit.  Note how compact it is – this should be able to be fitted into the limited space in a microscope base and work with the old bulb and lenses.

Sixty years ago, this unit would have been completely impractical, with vacuum tubes and their power supplies occupying the space of a packing crate, the unit weighing 70-100 pounds, consuming more electricity to heat filaments than it produced, and

emitting enormous amounts of heat.  It is a testament to modern microelectronics that it is practical to convert and reconvert power several times in this manner, and that the process can occur in  a packet the size and cost of a box of lozenges.

For those to whom this is all new information, the concepts behind the electronic power supply are very simply and coherently explained at the Williamson Labs’ site:

Note this especially:  If you think that this is all a long way from looking at small wriggly things and batting mosquitoes in the swamp, you are right.  A quick look at the specification sheets above for components like the IC and the MOSFET rapidly convinced me that I’d rather just sit back and applaud than try to understand what’s happening in all those little black thingies.  BUT – if this lets me buy a research-level Zeiss or Olympus with blown electronics for $200, and then have it sitting on my bench to use every day, it’s worth it.  And you don’t really have to understand it, except in a general sense.  You just have to know enough to solder together the wires and close up the case.

If, however,  you are inspired to learn more about these ubiquitous and amazing little devices, read Ken Shirriff’S dissection of the Apple iPhone power supply (the little white cube in the above image).  His description of how an SMPS (including transformer ), with sophisticated voltage regulation and overload protection, is compressed into a one-inch plastic cube is fascinating.  The images below are courtesy Mr. Shirriff:

The images above show how ingeniously every cubic millimeter was used in the iPhone cube, as well as the sophistication of the SMPS circuitry to handle a variety of loads with reasonable regulation in such a tiny space.  Fortunately, microscope illumination systems can be much more basic and much less space-intensive, with simple circuits that do not need to adapt to such a multiplicity of circumstances.

Hopefully, this information will take some of the mystery out of microscope power supplies.  I wonder how many sophisticated microscopes have gone into the junk bin or been broken up for parts because the owners assumed that a busted power supply was irreparable?  I would like to have suggested one simple, all-encompassing solution that would replace the power supply for all of the “semi-modern” microscopes that are new enough to have electronic lighting supplies, yet old enough that parts are no longer available.

Unfortunately, what will work as a replacement power supply is dictated by space, the power and voltage requirements of the bulb, the materials at hand, your experience with tools, and, ultimately, your budget and patience.  There is no single universal fix.  Ingenuity and backwoods improvisation are often the essential elements to salvaging  a sophisticated scope with bad electronics.  The point is that these power supplies from 20-30 years ago can be replaced or rebuilt, and they are not mysterious, irreplaceable devices. If you have an idea of how these circuits work and are willing to dive in, your reward will be a functioning, classic microscope and the satisfaction of having a fine piece of equipment that you have restored.

  When things were simple: antique microscope light sources – Spencer on left, AO on right

 This article is continued in Part III, which deals with LED conversion.  We will review the construction and function of light emitting diodes, discuss how to power and regulate them, learn why they are NOT light bulbs, and include some of the few articles describing how classic scopes have been adapted to use LEDs.

How Bright is Your Light Part II: Repairing More Modern Illumination Systems

How Bright is Your Light? Part I: Rebuilding the Nikon S Light Intensity Control with a $4.95 Dimmer Switch

Nikon S-Kt illumination system, base removed.  Note old-style “toroidal” transformer at bottom and voltage adjusting potentiometer at upper left.  The toroidal design is actually highly efficient and shows less power loss than square designs.  The thyristor (Triac) is the pillbox-sized rectangular structure at upper right.  Someone has improperly rewired the potentiometer at upper left – note bare wires!


Restoring my beautiful old Nikon S proved to be a continuing saga – and a remarkable learning experience.  In my last article, I described the procedure for replacing a broken nylon fine focus spur gear, a problem that has sidelined many of these finely-crafted old scopes.

I originally bought the Model S-Kt with lenses (in lovely shape from 20 years on a shelf) for only $75.oo because the light would not turn on.  Getting the light working took only ten minutes with a soldering iron to re-secure a loose flange on the light bulb, after which the bulb connected properly.  However, deeper problems lurked within.  After repairing the focus gear and bulb, using the newly-restored scope revealed that the bulb voltage could not be raised over two volts and the transformer was overheating and smelling hot. It was unclear whether the transformer was shorted or the electronics were not working.

This question was solved when, after half an hour of operation, the light suddenly flared to its full brightness and the intensity control stopped working.  Clearly, the transformer was producing full voltage, but the 45 year old electronics had blown.  Removing the baseplate revealed homemade circuits on the intensity control potentiometer; these may have contributed to the overheating and failure of the old circuits.

I knew nothing of light control circuits, but online research informed me that the core of a light dimmer system was something called a thyristor.  So what is a thyristor, and what are those mysterious little thingamagummies on the printed circuits inside the base of any but the oldest microscope power supplies?  I had to educate myself in the history of microscope illumination, and it turned out to be a fascinating journey.

(With thanks to Mark Morris for suggesting this fix)

In the 1950s and 1960s, microscopes typically had external power supplies for the illumination system. These usually had no electronic components, and relied on an autotransformer, or variable transformer, to control light intensity.  The latter is a simple device, with a single winding on a circular core, and a moving contact to take off a variable secondary voltage for the bulb.  Autotransformers are primitive, heavy,

Old Nikon Autotransformer Illumination Control

clunky, and much too large to fit into a microscope base – but they are also simple, sturdy, and almost indestructible under normal conditions.  And when one did (rarely) short out, you tossed it into the trash, bought a new one, plugged it into the scope, and went on with your day.  The insides of the microscope contained a bulb, a mirror and sometimes a few lenses, but little else.  With the exception of dirty contacts or broken cables, these simple systems could be expected to function reliably fifty years after they were first turned on.

As microscopes became more streamlined, power supplies moved into the base of the scope and incorporated newer, more compact electronic control systems made possible by the advent of solid-state electronic devices.

Controlling electric current electronically is not a new idea.  In 1906, Lee De Forest invented the “Audion”, the first functional vacuum tube; this design and its basic theory was later improved by Fleming.  Electrons boiled off a hot filament flowed to a positively-charged second electrode (the “plate”).  A small negative voltage applied to a thin mesh (the “grid”) inserted between the filament and the plate could control this relatively large current, thus allowing the tube to be used as an amplifier and heralding the beginning of electronic communications and controls (see De Forest’s original Scientific American article:  However, vacuum tubes were fragile and wasted large amounts of power to heat filaments.

1915-20 ad for the De Forest Audion Vacuum tube

The transistor, invented at Bell Laboratories in 1947, uses three layers of semiconductor material to accomplish this function much more efficiently, with a small current applied to the “base” layer controlling a much larger current traveling from

Transistor -courtesy Sparkfun

the “emitter” layer to the “collector” layer.  In solid-state devices, current can travel either as electrons or as “holes” – vacancies in atomic outer electron layers that act as

Transistor - courtesy Corollary Theorms
Transistor – courtesy Corollary Theorms


Current amplification by Transistor (Courtesy Sparkfun)
Current Amplification by Transistor – courtesy Sparkfun

mobile positive charges.  In this way, the transistor, employing the base current like the grid of a vacuum tube, can perform the same functions as the vacuum tube, but with less weight, much less power consumption, lower voltages, and much smaller size.  With choice of physical configuration and semiconductor material, as well as “doping” (addition of trace elements to the semiconductor material), an almost infinite variety of functions can be designed into semiconductors.

Model S microscopes were manufactured just as electronic controls were becoming available.  Depending on the amount of space in the base, these microscopes employed either in-base, electronically-controlled power supplies (Model S-Kt) or older-style, transformer-based external supplies (Models S-Ke and L-Ke) if supplementary lenses and levers in the base left insufficient space for transformers and electronics.  The electronic illumination control system in the Model S-Kt was simple – essentially the most basic of light dimmer circuits – but in the 1970s was considered to be very advanced:

“Modern, advanced semi-conductor technology has provided a facility for changing the flow time of electric means. A so-called thyristor of extremely small size has been developed to enable regulation of the brightness of the lamp. The Microscope Model S-Kt has adopted this type of light adjuster built in the microscope base.”  (S-Kt Manual, Page 9)

The heart of the S-Kt system, wall dimmers, motor controllers and many other electric power control devices is the thyristor, a solid state device very similar to the transistor but with one critical difference:  while the current allowed to pass through the transistor is roughly proportional to the tiny current applied to the base, or control electrode, the thyristor (with leads designated anode, cathode, and gate) is completely non-conducting until a small current is applied to the gate, after which the thyristor flips fully to the conducting state. Thyristors therefore function as fast-acting electronic switches and, unlike the transistor, can easily control hundreds of volts and considerable current, even in high-power electrical transmission systems:

Thyristor valve hall at New Zealand power transmission station

Like the transistor, a thyristor is basically a direct current device.  However, if one connects two thyristors in parallel but oriented opposite to each other with the gate connections wired together, one has a very functional, solid-state electronic AC switch:  the Triac, or bidirectional triode thyristor.  Furthermore, many triacs are not fussy about the polarity of the gating voltage, and can be switched on by either direct or alternating current, thus making the control circuitry simple: a small control voltage can be pulled off the main power supply, applied to the gate, and control a much larger current travelling between the cathode and anode.  Although the theory and structure of triacs are complex, if one is not concerned about the niceties of symmetrical AC triggering, harmonics or very fine control (for example, with an incandescent bulb dimmer) the circuitry can be very simple:

Simple Triac Dimmer. Capacitor helps control phase of gating current.
Simple Triac Dimmer. Capacitor helps control phase of gating current.

This circuit also demonstrates the simple concept of the basic incandescent dimmer, with the AC supply, the light bulb, and the triac dimmer wired in a simple series circuit.  For a low voltage bulb, the primary of the bulb transformer can be placed where the bulb is in the circuit.  Essentially, the triac acts as a switch, cutting out increasingly large chunks of the AC supply’s sine wave as the gating current and phase are altered by the resistor/capacitor combination.  Chopping out pieces of the alternating current reduces the effective supplied power:ThyristorwaveformdiagramsThis system works well for incandescent lights, where the filament stays hot between pulses, effectively smoothing out any flicker in the illumination.


So how does one replace a 45-year old thyristor that belongs in a museum???  Excellent older microscopes (and many expensive and not-so-old models) are often sidelined because of failing electronic components and unavailable parts.  This was becoming a thorny problem until Amateur Microscopy friends contributed ideas, and Mark Morris, who has been fixing these microscopes since I was in elementary school, gave me the key idea.

For $4.95, my local Home Depot gave me a cheap Leviton wall dimmer switch.  Filing off the rivets and removing the case revealed the  simple guts of the controller: a potentiometer, a small inductive load, two mica capacitors, and the tiny, all-important chip of a Triac thyristor:


Dimmer switch, front and with back removed. Note barely-visible, 1 cm square, black thyristor behind mica capacitor (small brown object)

Tracing the dimmer’s circuit shows it to be similar to the basic circuit shown above:

Circuit for the Leviton dimmer. The potentiomer, thyristor, and capacitor comprise the basic control circuit.
Circuit for the Leviton dimmer. The potentiomer, thyristor, and capacitor comprise the basic control circuit.

The potentiometer, thyristor, and small mica capacitor on the right comprise the control circuit, while the small coil and capacitor on the left limit interference from the switching process.  As much as possible, these components were unsoldered from the dimmer potentiometer as a unit and reconnected to the intensity control potentiometer on the scope, thus maintaining the physical appearance and controls of this classic scope  – the electrical equivalent of a heart-lung transplant.  The original circuitry of the Nikon light

Components of the Dimmer - thyristor, small ferrite core inductor, and resistor
Some components of the Dimmer – thyristor, small ferrite core inductor (coil), and capacitor.

controller was very similar and equally basic, but the 1970 thyristor’s volume was around 100 times greater than that of the tiny chip removed from the modern dimmer.

Cutting out the old thyristor
Cutting out the old thyristor

After the components of the dimmer were isolated, the wires to the old pillbox-sized Nikon thyristor were cut, and the hay-wired electronic components added to the Nikon light control system (trim potentiometer, capacitor, resistor, and connecting wires) were stripped out, leaving the Nikon potentiometer in place:

Nikon Inensity control potentiometer, all other components removed.
Nikon Intensity control potentiometer, all other components removed.

The tiny thyristor from the dimmer, which had a metal flange riveted to the dimmer’s front plate as a heat sink, was secured to one of the potentiometer mounting screws inside the microscope base, and the dimmer’s circuit was recreated using the Nikon’s potentiometer:

Dimmer components installed on Nikon potentiometer. Heat sink on the thyristor, the barely-visible black chip, is secured under one potentiometer mounting screw.
Dimmer components installed on Nikon potentiometer. Heat sink on the thyristor, the barely-visible black chip, is secured under one potentiometer mounting screw.

One of the problems with this “transplant” was that the Nikon potentiometer had a resistance value of 100K ohms stamped on one side, but the resistance of the Leviton dimmer potentiometer and the trim pot were unknown;  this introduced a degree of guesswork into matching the two systems.  When this configuration was tested, the Nikon bulb would only dim partially.  Clearly, the amount of resistance provided by the Nikon potentiometer was less than the dimmer circuit was designed for.

With a considerable amount of hope and finger-crossing, the small trim potentiometer was soldered in series with the Nikon intensity controlling potentiometer.  When it was turned on, the system worked perfectly!  The trimmer’s resistance set the dimming range of the original built-in Nikon potentiometer; at one extreme, the bulb would not light at all, while at the other end of the trimmer’s range, the Nikon potentiometer could not dim the bulb fully.  Using the trimmer near midrange, it was possible to set the range of the Nikon potentiometer to ideally cover the full dimming range needed for the bulb.  This is the completed illumination control system:

Completed control system. Range-setting trimmer pot soldered to left of main illumination potentiometer
Completed control system. Range-setting trimmer pot soldered to left of main illumination potentiometer.  Tiny modern thyristor barely visible below the trim potentiometer.

The completed Model S illumination control circuit with the new dimmer components is thus:

Final Nikon S illumination control circuit. Dotted line surround components transplanted from the dimmer switch.
Final Nikon S illumination control circuit. Dotted line surrounds components transplanted from the dimmer switch.

This simple fix should work with older, 60Hz illumination control systems based around a simple power control circuit and a transformer.  It could also be used to replace a missing or non-functioning external transformer system:  a dimmer switch and a 6 volt doorbell transformer in a small electronic case, with appropriate cords and plugs, could form a cheap and simple microscope light power supply.  One could add a voltmeter and a little panel light to make it fancy.

Next:  Part II with some thoughts on repairing more modern systems as well as LED conversions, plus references.


How Bright is Your Light? Part I: Rebuilding the Nikon S Light Intensity Control with a $4.95 Dimmer Switch