2N3055 Transistor help

[Chipperi]

I have a power supply that is putting out 17.8 VDC when it is supposed to be putting out 13.8 VDC. When I cracked the case open I noticed the 24v 0.18a fan is broken, that's not a big deal to replace. But it was suggested to me since it is an old used power supply to go ahead and replace the 4 2N3055 pass transistors. I took one out and want to test it to see if it is any good or not. Now from what I understand if you lay the transistor on its back with the pins facing up, and the pins above the center line than the left pin should be base, the right pin should be emmiter and the case itself is the collector. With my digital multimeter (Fluke 16) set on ohms test... + lead attached to collector and - lead attached to base I get 1.2ohms and the same if I reverse leads. Now E to C C to E E to B and B to E I get O.L Nothing. I also connected E to C while the meter was hooked up to B and C still nothing.

Am I doing something wrong? Or is this Transistor Kaput

I was told this should remedy my over voltage problem and if it did not I may have to replace the KA723 IC, Which hopefully I wont have to.

 

[creakndale]

Put your meter in "Diode Test" and retake those same measurements.

The instruction sheet for the Fluke 16 can be downloaded from here:
Fluke 16 Multimeter with Thermometer

creakndale

 

[JimB]

Charles? is this the PSU mentioned on qrz.com?

since it is an old used power supply to go ahead and replace the 4 2N3055 pass transistors.

Faulty pass transistors are the best bet for a fault of this type.

Now from what I understand if you lay the transistor on its back with the pins facing up, and the pins above the center line than the left pin should be base, the right pin should be emmiter and the case itself is the collector.

That is correct.

With my digital multimeter (Fluke 16) set on ohms test... + lead attached to collector and - lead attached to base I get 1.2ohms and the same if I reverse leads.

Then that transistor is dead.

Now E to C C to E E to B and B to E I get O.L Nothing. I also connected E to C while the meter was hooked up to B and C still nothing.
Am I doing something wrong?

As mentioned already, to correctly test transistors etc, you must use the "diode" range on the meter. The ohms ranges do not have enough voltage to turn on the diode junctions in the transistor.
Using the diode range of the meter you should see something like 0.65 from base to emitter and base to collector (meter +ve lead to base) and OC when the meter -ve lead is to base.
Measuring between collector and emitter should give OC both ways.

I was told this should remedy my over voltage problem and if it did not I may have to replace the KA723 IC, Which hopefully I wont have to.

Replacing the 2N3055s should do it, (hint, replace them all, even if there is only one faulty)
It is not beyond the bounds of possibility that the 723 is also dead, when buying the new 3055s, I would buy a 723 as well at the same time.

A note of caution on 723 voltage regulators, they come in different packages, 10 pin metal can and 14 pin DIL. They are interchangable, within limits.
The metal can type can easily replace the 14pin DIL, it is just a matter of arranging the leads.
However the 14 pin DIL may not replace the metal can without creating a spaghetti mass of bodge wires!

Also, check to see if this PSU has a crowbar circuit, if it has it is also fault if you got 17volts out of the PSU. Fix it.
If there is no crowbar, make one and fit it.
A couple of $$ for the parts to make a crowbar is a lot cheaper than $200 for a smoked radio.

JimB

 

[Chipperi]

Yep this is the same one as QRZ

I put the meter in Diode test and the results were the same except when b and c were connected I got 0.0002.