[SOLVED] Pyramid PS-21KX Repair - Voltage Stability Problem - New Regulator

[Steve Wayne]

I have a Pyramid PS-21KX DC regulated power supply that drops in voltage output as the load increases. This 20A supply drops from 13.8V to 12.7V under a 7A load and continues dropping (I’m sure) under increased load.  The initial problem with this supply was a direct short under loads greater than 150mA, while otherwise appearing to power-up normally (except the no load voltage was low, at 12.4V.)  I replaced all 4 of the pass transistors as they were all found to be shorted at base to collector.  The mains rectifier tested ok.  I replaced the LM723CM regulator IC as well.  The associated TPI41C transistor (typically) tested ok.  The 5 filter capacitors “look” ok, and the control circuit board components have no obvious signs.

I’d certainly appreciate opinions from anyone who has knowledge of similar symptoms and diagnosis reported in this or similar Pyramid supplies.   Thanks!

[Vgkid]

Not sure about this model. Have you checked any power resistors in the unit(ones needed for parallel transistors), that are in series with the load.

[TheMG]

As Vgkid suggested, I would check resistors and make sure they are within tolerance. Chances are one or more of them could be open or have gone out of tolerance due to the excessive current through the resistors when the transistors failed short.

[Steve Wayne]

Thanks Vgkid,,  yes I had checked those 4 base resistors and came up with the requisite 2.2M Ohms.

[Steve Wayne]

Thanks for your assistance.  I obtained another PS-21KX and found that it behaved the same as the one I repaired.  The voltage drops as described above when loaded.  I certainly didn’t expect this to be the case. So much for “linear” as a power supply.  S~