90V out on a protek 6003L bench power supply

[matt6ft9]

A Protek 6003L is a single adjustable output, 0-60V, 0-3A bench power supply.  It also has a 5V fixed output.  When I received it, it would power on, but the Vout was in the 80-90V range!  The voltage set knobs don’t seem to effect the output voltage.

After taking the case off, it was obvious the 4700uF, 160V cap had failed.  (see the photo of the label peeled off, the top of the cap was also domed)  It is interesting to see the main transformer appears to be a toroid.  The fan in the back of the unit was also seized.  So, a new 4700uF cap and DC fan was installed. 

Powering unit back up after the repair, the DC output was still in the 90V range.  The voltage on the 4700uF, 160V cap is 101.6V.  The front panel reads 79.6V and the terminals on the front read 101V)  While powered off, I measured the bridge rectifier with multimeter on the diode setting, the bridge rectifier seems good.  Without a schematic, it’s hard to tell what to look at next.

[SeanB]

Pull out that heatsink and you will find that the power devices on there are either very leaky or shorted. You do not really need a circuit diagram for a simple power supply.

[Kleinstein]

Having the full voltage at the output very much indicates blown power transistors (or whatever power device is used). There may be other parts (driver transistors) blown as well.

60 V and up to 3 A sounds like a lot of power - so there should be way more than just one power device. If there are relays for switching between transformer tabs, these can be also blown (at least don't test at high current before they are checked). A stuck relay may cause the output stage to overheat under load.

[matt6ft9]

Good call!  I removed the heatsink, there are two, Toshiba 2SC5200 attached.  (NPN, 250V, 17A, 150 watts)  The one on the left in the picture appears to have nearly shorted leads.  (less than 12 ohms)  The insulation on one of the wires is cracked, it looks like it might have become too hot.  I’ll have to put a couple of 2SC5200’s on my next order.

[matt6ft9]

Update:
The 2SC5200's (mouser part number 512-2SC52000TU) arrived. 
The transistors were installed and the power supply is now operational.
Thanks for the help.
Matt