Gary, VK8BN / VK2DBN reports on an unusual fault in his power supply. Keep this in mind if you suddenly find your power supply voltage drops. Over to you Gary.

I have been using A Dick Smith D-3800 Power supply for many years to power my HF Rigs. I purchased two of them at an amazing discount price when Dick Smith Electronics folded.

A couple of weeks ago I turned on my rig for the Friday 14.337 sked, however the dial lights were dim on my SWR meter and I noticed the output voltage had dropped to about 5 Volts. I didn’t make the sked that day!

The Dick Smith power supply is manufactured by Manson Engineering Model EP-925 (pic above). I pulled the power supply apart and started checking some voltages around the LM723 regulator, the output pin of the regulator pin 10 was about 5.8 volts. I did a test of the five 2N3055 output transistors and also the TIP31 driver transistor all were OK, a cursory check of various components around the LM723 revealed nothing of interest, so I changed the LM723 however this resulted in no change to the situation. Adjusting the voltage control pot VR3 gave a change in output from about three to five volts.

I checked all of the resistor values in the feedback R16, R17, R20, R21 as well as checking C7 and found no component faulty. At this time I became puzzled as to what the problem may be. I lifted one leg of R17 to double check its value and by chance I turned the power supply on with no feedback, the output voltage jumped to 28 Volts. Here comes the end to the mystery the only component that I had not checked in the feedback loop was the potentiometer VR3. VR3 measured about 560 ohms while the circuit showed a value of 5K. I placed a 3.9 K resistor in series with R17 and the voltage jumped from 5 to 13.8 volts. The potentiometer was the fault and I must admit I have never before seen a potentiometer go low in value.

With the Potentiometer replaced all is back to normal.