Kepco Model 400 Power Supply
This was donated to me by my friend, John Dahlman. It gets a lot of use.
All the paper/wax capacitors needed replacing. Also, one phase of the negative supply winding is open. I simply created a hybrid bridge off one phase using two diodes together with the existing rectifier, obviously the available current is reduced however, it has worked perfectly since I did this. The other fault was that the high limit pot was open, since it is used as a variable resistor, I simply re-connected it to the other end. The other modification I made was to include a 20 second thermal B+ delay. This provides enough time for the B+ error amplifier to warm up so that if it is turned on with the B+ output switch closed, the full unregulated B+ of 800V won’t appear at the terminals. My Lambda Model 71 has this feature. Here is the underside, the delay relay is located top right:
It is a cheaper design than the Lambda which has a variac that controls the B+ transformer in step with the desired output voltage thereby limiting the power dissipation in the series pass tubes if you require full current at low voltage. This one has four 5881 series pass tubes to deliver 150mA at up to 500V. I haven’t done the math but I imagine that if you want 50V at 150mA, the 5881s will be hard-pressed, not to mention run way beyond their plate voltage rating! I haven’t experienced any problem running at low voltage but I have never drawn much current at low voltage either. One reason why I use it as much as or maybe more than the Lambda 71, is that the Lambda buzzes which I understand is “normal”. Normal or not, it is not peaceful and I value peace in my environment hugely.
Here is my hand-drawn outline schematic, it is does not include every feature of the circuit.