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Saturday, September 10, 2016

Reconditioning a 50+ year old microscope - part 3 - LED conversion

The microscope came with what I believe it must have been the original illumination system:

  • a 30 Watt / 110 Volts tungsten bulb (in this case a GE branded one):

  • a light dimmer circuit apparently made up of just a Triac, some capacitors and the potentiometer to control at which threshold the waveform is chopped (therefore causing the light to dim proportionally to how much of the wave is chopped):

  • and interestingly, a Hitachi branded mains plug:

However for current use, this system would be impractical several fold:

  • the specific type of tungsten bulb it requires is very difficult to obtain today, and those available (mostly new old stock) are not cheap;
  • as listed in the specs of these light bulbs, the lifespan is stupidly small, in the range of 50 to 60 hours.

  • incandescent illumination is not ideal because of reddish spectrum of light in lower power levels. A blue filter is normally required to adjust to a more even spectrum:

  • given the low efficiency of tungsten bulbs, heat dissipation and IR radiation are high, affecting some types of specimen;
  • in my location the mains voltage is 220 Volts instead of the 110 Volts required. This would imply adding a transformer to convert the voltage;
As such I studied the conversion to an LED light source, which I knew it would have multiple advantages:
  • LEDs are widely available and cheap;
  • lifespan is high, in the order of tens of thousands of hours;
  • light spectrum is consistent across all levels of intensity. Also (depending on the selected LED), a nearly pure white source is possible without having to add any filter;
Nevertheless I had to take into consideration a few aspects:
  • make sure a good thermal dissipation would be allowed. LEDs don't like too much heat;
  • ensure the maximum current delivered to the LED is actively limited, ideally by some regulator circuit;
  • Like in the original system, provide a means for the LED intensity to be varied through a potentiometer installed in the same location of the original one.
As such I put my hands to work. Started by removing the original illumination gear:

Ordered from banggood:
  • a bunch of 3 Watt white LEDs (rated at about 210 lumens each);
  • a power LED tailored heatsink:
  • a step-down (buck) converter rated at a maximum of 3 Amps, with controls for constant voltage and constant current applications:

The work started with the preparation of the heatsink to fit the area originally occuped by the bulb. Some cutting had to be done in order to make it a good fit. Tested the LED mounted on the heatsink for proper operation:

Made some tests with the step-down converter, limiting the current to 500 mA (below the rated 700 mA but more than enough for the lighting requirements), and replacing the on-board multiturn pot by a 500 Ohm panel potentiometer and a 220 Ohm resistor in series (and as such obtaining only the safe voltage range to be used for controlling the light):

  • machined an aluminium disc to provide the necessary width for fitting a DC connector in the hole where originally the thick mains cable would pass through:

Well these specs won't remain true though (in particular the 20 Watts of consumption) :) 

While possibly not making much difference, preserved the original black mask:

Put it all together, inside the microscope base (mounted the LED + heatsink, the step-down converter PCB, the DC connector, and the potentiometer, in the place where the original one would go).

Fired it up and..Bob's your uncle:

See also:

Reconditioning a 50+ year old microscope - part 2 - XY specimen stage mechanism lubrication

Like pretty much every moving part in this microscope, the XY specimen stage mechanism also suffered from dried/sticky grease, preventing the X axis from moving at all. As such like in the previous case, the only solution was to tear it down, clean it and apply new grease and oil.

The separation of the mechanism from the stage is simple: in the bottom of the geared side (where the knobs are) this forelock is moved as shown in the picture, and by moving the Y axis to the end of its travel range, the mechanism is removed:

Then it is possible to separate the parts that compose the mechanism:

For cleaning of the stage it is also interesting to be able to remove this part entirely:

The first thing to remove are the knobs. An Alan and a Philips key are sufficient to remove the two knobs:

As seen above I have also removed the slide retainer tweezer to also clean and lubricate with grease.

The X movement is simply driven by a long worm gear turned by the inner knob. To better clean the worm gear and have access to the sliding rail underneath it, I  dislodged it from the casing by removing the two screws of the holder part (left side of the worm gear):

The Y movement is simply a pinion that rolls against the geared rail in the stage. I cleaned and lubricated both parts with standard mineral oil:

After all the cleaning and lubrication was done (gears with oil and contact rails with grease), it was time to put it all back together and start doing some observations.

Next part will be about how I did the conversion of the specimen illumination to LED.