Kenwood PS-30 output questions

[AC5UP]

Then I pulled off one of the large power transistors, and the emitter was shorted to the collector.  I'm guessing that would tend to cause a problem. 

An emitter to collector short is often referred to as a pass transistor that's "welded up".

In a nutshell, the power supply starts out with a voltage higher than what's needed and runs it through a number of pass transistors wired in parallel. As the current travels through the pass transistors it's limited by the voltage on the base of those transistors until the output is exactly what the regulator circuit wants it to be. If any of the pass transistors shorts from C to E there is no regulator action and as you're aware 22.5 vdc will show up at the output.

I agree with the idea of replacing both pass transistors as whatever caused the first one to fail probably stressed the second one as well. It was only a matter of who puked first and the transistors aren't expensive. When you grease up the new parts with thermal compound try to give it enough to cover all mating surfaces with an even but thin coat... Too thick and you might be creating an insulating layer, too thin and there could be dry spots that become hot spots.

[KB3WPN]

I scored two replacement transistors from a friend of mine this afternoon, and tonight I have a working PS-30 power supply!!!  One of the transistors in the supply tested good after I removed it, but I still replaced it to be on the safe side.

Now I just have to get my real antenna put up, and I'll be able to put my TS-130S to use on all the bands it is capable of.

andy b.

[AB8RL]

I just picked up a Kenwood PS-30 at a hamfest.   The DC output is 22 volts or so on the output plug and on the ACCY connector on the rear chassis.   I measured 22 volts on the top of C1 as well.

After reading the posts - would it appear one of the two transistors on the regulator board are bad?   The pass transistors don't appear to be open or shorted in anyway from what I can tell

Looking for help - Thank you

[KB3WPN]

AB8RL,

If you have a decent soldering iron, it isn't too bad to pull off the two large power transistors and test them.  I'll have to dig out my notes, but the testing wasn't that difficult, and it was obvious that one was bad from the test.  I found out how to test the transistors just by doing a search on transistor testing.  All you need is a standard multi-meter with the capability to measure resistance or a diode test.

Andy B.

[AC5UP]

...The DC output is 22 volts or so on the output plug and on the ACCY connector on the rear chassis.   I measured 22 volts on the top of C1 as well.

Two possibilities:

1) At least one of the large pass transistors on the heatsink is "welded up" which is another way of saying "a hard collector to emitter short". If a diode check measures near-zero Ohms from C to E in both directions that's your proof. A working transistor will flow current from C to E in one direction only when there's enough base voltage to turn the junction on. Otherwise, no voltage on the base = no current flow from C to E in either direction.   *

2) The regulator circuit has failed in such a way that the base voltage is being held high enough to drive the pass transistors into full saturation.

Failure mode #1 is far more common than #2. If it was case #2, the output would be approximately half a volt less than the voltage on the big filter cap as there is always some voltage drop across a good bipolar transistor junction.

*   FET's are an exception as a healthy example can remain turned on for several minutes after the base voltage is removed. Very unlikely you have MOSFET or HEXFET pass transistors.