30 Amp 13.8V linear PSU running hot on very light load?

[Chris Wilson]

I cannot access this commercial fixed output voltage linear PSU to get a make right now, but over the last few weeks it has started to run hot and the big muffin fan that's temp controlled started to run intermittently and now runs nearly permanently. It is running a Kenwood amateur band transceiver in receive mode only that draws about 1.5 Amps. The case feels hot and the thermostatically controlled fan only used to cut in when transmitting for a long time, and a 20 Amp or thereabouts load was asked. Any ideas on possible causes? I am thinking a leaky smoothing cap perhaps? I have no schematic but I doubt it's that complicated inside. There's an internal trimmer pot to vary output voltage from about 11.5 to 14V or so, that can be adjusted externally through a slot. Current output  is fixed. No meters. Commercial quality. Very heavy, not ancient. Any hints as to how to diagnose this much appreciated, thanks.

[floobydust]

Take it out of service and apart. It might damage your radio. There's not much that can make extra heat without smoke. If mains voltage is high, the power transformer might be saturating, that's about all I can think of. Maybe a shorted rectifier diode.
Samlex 13.8VDC linear PSU extra cheap and notorious for bad capacitors.
edit: schematics and more: https://www.eevblog.com/forum/repair/jaycar-mp3098-20amp-linear-13-8v-she-dead/

[Chris Wilson]

OK, many thanks, have taken it off line and using a switcher until I can find time to pull it apart. Appreciated!

[coromonadalix]

Seems like a pyramid  psu

[wizard69]

Take it out of service and apart. It might damage your radio. There's not much that can make extra heat without smoke. If mains voltage is high, the power transformer might be saturating, that's about all I can think of. Maybe a shorted rectifier diode.
Samlex 13.8VDC linear PSU extra cheap and notorious for bad capacitors.

That suggestion above is very important.   Ideally you would be using a protected supply to drive such an expensive piece of hardware.   In that regard I mean over voltage and current protected.

As for a power supply there likely isn't a lot to it if it is a linear.    So fixing it could be relatively simple.    Obviously the capacitors should be checked.

[SeanB]

I used to work with some avionics power supplies that used to get failed MC44 diodes, and they would cremate themselves, and only finally fail blowing 2 of the 3 mains 1A fuses after being well cooked. Not repairable, so ordered replacements, which were 6 figures each in price. Reason for not repairable is that the only part not destroyed was the milled out frame and the connector, you would have to replace every other part, from the PCB, the wiring loom (make it up again from the 2 sizes of wire used), the stud rectifiers for the 5V supply and the custom wound 3 phase transformer. None of these were available as spare parts, but the OEM did have some NOS stock on hand for some reason..........