Created by: gwideman, Sep 27, 2015 12:55 am
Revised by: gwideman, Sep 27, 2015 1:12 pm (6 revisions)


Documentation of tear-down of Hoefer TKO-100 DNA Fluorometer from the mid-90's.
These seem to be readily available on eBay and from surplus dealers. If you are contemplating getting one of these, be aware that these are not general-purpose fluorometers. According to, the unit is intended to be used with a specific fluorescent dye (bis-benzimidazoled, Hoechst 33258) used for measuring DNA (hence "DNA Fluorometer"). The unit presumably excites the sample with the specific wavelength to which the dye is sensitive (352nm), and the photo detector is sensitive to the wavelength that the dye emits (461nm).
Further, the unit is not intended to measure absolute fluorescence. The idea is to first measure a reference sample having known DNA concentration to set the Zero and Scale, and then get a measurement from the unknown sample relative to the reference sample.
None of this is terribly solid info, as we don't have a manual. However, we can at least take one apart!

Some interior photos

In the foreground is the 120 to 1200V transformer that runs the UV lamp.
In this view, the PC board that contains the detector and amplifier electronics can be seen.
On the left side of this picture appears a rectangular black box.
This is the cuvette receptacle and also holds the UV lamp and two photodetectors of some kind.
We suspect that there are optical filters between the cuvette and the photodetectors.
Presumably one photodetector reads the fluorescent light. The other photodetector probably reads the UV lamp.

System design comments

We initially expected that circuitry on the PCB would be configured as a "lock-in" amplifier, a strategy for dealing with very weak signals in the presence of considerable noise.
A lock-in design generally requires two inputs, one from an oscillating "excitation" source, and one from the detector, whose signal also oscillates as a result of the oscillating excitation. The circuitry multiplies these signals together in order to greatly favor the desired detector signal (over noise) which fluctuates in step with the excitation.
In the fluorometer, the lamp fluctuates at the line frequency (60 Hz), and the resulting weak fluorescence signal will also. Loosely speaking, the two signals would be multiplied and filtered to arrive at the desired output. So this seemed like a promising theory for this unit.
However, while inspection of the circuitry does show that a multiplier circuit is present, the components in the lamp input amplifier/filter seem to be for a much lower frequency than 60Hz.
Unfortunately, we can't just test the unit's operation, as we think the lamp is not working, and we don't have the requisite fluorescent dye. No doubt we could spend more time getting this nailed down, but other priorities call.

PC board

Component side
Bottom side


Schematic for the PC board. Does not include chassis wiring, which is relatively trivial:
  • Power switch, upstream from lamp transformer and this PCB
  • Two photodetectors mounted at the cuvette holder, supplying input to the PCB
  • Digital voltmeter, which receives power and analog signals from this PCB
Larger version:
PDF version:
Not guaranteed 100% correct, but probably pretty close, and looks plausible :-).
2015-09-27: The approx half-second time constant of R1 C1 give the "lamp" input stage a very low pass cutoff, suggesting that this is looking at the average lamp light level over seconds rather than the 60Hz fluctuations.
That would suggest that the multiplication (actually division) is just to factor out the intensity of the lamp, which might vary with exact line voltage, or over the lifetime of the lamp. (i.e.: not really a "lock-in" amplifier, except in a very-low-frequency sense!)
Closer inspection of R1 and C1
  • With a meter, in circuit, R1 measures 475k
  • C1 is marked "105" on one side, and "324" on the other. I'm inclined to go with "105" = 10E5 pF = 1uF.