Electrical Repair of a Beseler Variable Contrast Head
The variable contrast head on my Beseler 23 suddenly began blowing bulbs; the bulb which I had been using for some time, and then a couple of replacements. The head was unusable in that condition.
This unit, my Model "23C III VC head", takes an ESJ type projector bulb. The ESJ is rated at 82V 85W and is not intended for use directly on household line voltage (120VAC here in the US). Some power conditioning is necessary.
Rather than a transformer or more elaborate power supply, the Beseler engineers used only a diode in series with the bulb to condition the power. A slick design, really, IMHO. The single diode functions as an unfiltered half-wave rectifier, feeding pulse DC to the bulb at a frequency of 30Hz. This is more or less equivalent, for a resistive load, to feeding the bulb 60 VAC.
The diode is to be found under the nameplate at the front of the VC head (six screws to remove the nameplate; take care not to lose the little steel balls below the selector knob - not sure what these guys do). The OEM part nomenclature on my unit was a Micro Semi KP848 diode.
Once I got the thing out of the VC head to examine and take a multimeter to it, I found that the diode had developed a dead short internally. It measured zero ohms resistance in either direction. The cause of my problem was clearly that, by developing an internal short, the (former) diode was now feeding full AC line voltage to the the bulbs, overloading them.
Such diodes can also fail by opening up and manifesting infinite resistance in both directions. In this/that case, the bulb won't light; presenting another set of symptoms to prevent use of the (otherwise really rather practical) VC head.
The KP848 diode evidently hasn't been manufactured for some time (Beseler built my VC head in the 1990's). After some rather tedious searching, I was able to locate some NOS KP848's. I replaced the diode and I'm back in business.
Had I not been so fortunate in finding the needed part, there were/are ready alternatives.
The specifications for the KP848 (as per the web) are 5 amps, 800V. Diodes with similar characteristics weren't too difficult for me to find. Mouser Electronics had 99 of these available for US$7 each; take your beating on the shipping. The substitutes don't seem to have the push-on connectors fitted to the KP848 but that shouldn't present much of a problem to anyone prepare to switch out his component.
It must be noted here that it was the only electronic component on my (otherwise) steel and glass 1950's-era (and perfectly useful) enlarger that chose to fail. It was also this electronic part that became obsolete and difficult to replace before replacement was needed. A parable for our times for those of you toting digital Leicas (or worse)