Pyramid power supply PS 14K now 24V!

[W9GB]

It doesn't even have to be a regulator IC failure. A Pass transistor that shorts (for whatever reason) will allow the full unregulated DC Voltage

Larry,

I know that, BUT it may be educational for Jim.

BTW, I am not aware of any "easy" test for the LM723 regulator (failure).

[WA9SVD]

I suppose the Icom V8000 that it was hooked to was tuff...it survived with only minor damage to the speaker wires that melted when they touched the extremely hot component close by. (regulator chip on a heat sink?) It did let out a lot of white smoke!

More than likely a shorted pass transistor. You're lucky there wasn't greater damage!

The LM723 regulator chip isn't mounted on the heat sink; it's a low power device, and that's why the "pass transistors" are needed, and mounted on a heat sink.

The '723 regulator is ubiquitous in use in many linear supplies, from low power (3-4 Amperes) to high power (such as 70 Ampere Astrons,) and even higher current industrial supplies. It's cheap (usually under $1.00) and readily available, even from Rat Shack. So many of us simply replace it whenever a repair is necessary, "just to be safe." I guress the only way to test one would be in an active circuit, to see if it maintains regulation. When a '723 regulator fails, it will often take out one or more of the pass transistors as well. All should be checked before returning the supply to service, to prevent further damage to other equipment. The overvoltage "crowbar" circuit is a wise addition, to prevent damage to delicate equipment.

[WA9SVD]

Larry,

I know that, BUT it may be educational for Jim.

BTW, I am not aware of any "easy" test for the LM723 regulator (failure).

Hello Greg,

We both know that a shorted pass transistor can wreak havoc on equipment; I wasn't contradicting you, but pointing out that point. Just replacing a '723 regulator isn't good enough; the pass transistors must all be checked or replaced as well. (We BOTH know that, but others may not.)

[W9GB]

One of the often forgotten tips in remounting NPN pass transistors .... make sure they are properly insulated from the heat sink.
Proper install and use shoulder washers, TO-3 mica washers and/or Thermo-pads. It is a bad day, when you perform all this work -- and then skip these easy steps.

[WA9SVD]

One of the often forgotten tips in remounting NPN pass transistors .... make sure they are properly insulated from the heat sink.
Proper install and use shoulder washers, TO-3 mica washers and/or Thermo-pads. It is a bad day, when you perform all this work -- and then skip these easy steps.

I'd that with the mica washers, heat sink compound needs to be applied to both sides of the mica insulator. Just don't use too much; just enough to thinly coat the entire insulator.