Smoking VEB Statron 3224

During full load some resistors on my VEB Statron 3224 started to smoke, so let’s investigate. The power supply is a fairly simple construction using a LM723 and a large pass transistor.

The first two resistors are for clipping the maximum current on the 8.5V (4A) range. It’s the two large resistors at the top left of the ciruit board.

The two resistors are 2R7 and 0R82 in parallel, giving an equivalent resistance of 0R62. My electronics supplier had no resistors with >5W in those two sizes, but I can arbitrarily chose an alternative as long as the equivalent resistance is close to the 0R62. My first thought was 1R2 resistors but my supplier also did not had these in stock. I replaced them with two 1R5 which nominally give 0R75 but in this case actually measures 0R68 (with kelvin leads). The current now gets clipped to 3.92A which is fine by me.

The second resistor that started smoking on full load was the 0R2 current sense resistor for the front panel readout. I replaced it with a 0R2 from ARCOL.

I had to recalibrate the current sense using potentiometer R22 a little such that the front panel matched up with the actual current again, as it was off by 200mA with the new resistor.

Update 15.08.2023

My electronics supplier finally had the proper resistors. I now get around 4.5A, which is 200mA more than with the original ones installed, so it’s a little bit on the high-side but should be fine.

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