Pyramid Power supply too much voltage

[LightFoot]

Hi Again,
My friend has a power supply made by Pyramid Gold Series. Model Number is PS-8K. Output voltage is supposed to be at 13.8 volts. The problem is that its reading about 23.5 Volts : . We changed the the 2N3055 silicon NPN power transistor. No Difference. The circuit board inside is small about the size of a cigarette pack. Is there anything else that controls the voltage ? There is a variable pod inside but it doesnt do anything. Thanks Guys...............

[linx]

I'm not familiar with THAT particular model of power supply, but have owned a few Pyramids in the past (not now...thankfully).

Sounds like a current pass transistor (or two) went out. As you said, there's not much circuitry involved and there's not a voltage regulator on the board.

Is there a NPN pass transistor on the back heat sink? If so you need to test that transistor to see if it's still good.

This is a COPY/PASTE from a link I had stored on my PC. I didn't write it, but it's good information.

How do you TEST an NPN transistor with a VOM or DVM??

You have to pull the pass transistors off one at a time, being careful not to bend the pins and to watch where the placement of the pins are in relation to the top or bottom of the TO-3 transistor and socket.
Also keep an eye on the placement of the thin mica insulation material. it is easy to break it.
If you do, your local electronics store will have them.

If you are lucky enough to have a power supply that has been built with transistor sockets, it will be easy to take them off one at a time to test them. If the wires are soldered directly to the emitter and base pins of the transistor, we have to take care as not to heat up the solder connection too much. It doesn't take a lot of heat, if we have the proper equipment.

Don't worry about the white sticky substance. It is a heat conducting silicone material. When re-assembling the transistors to the heat sink, you might want to have some of the silicone grease to put on the surface of the transistor and some on the heat sink where the transistor sets.

Some of the larger units have the same type of transistor as a driver from the regulator circuit to the pass transistors.
It will also need to be tested.

Test with a (Analog) VOM

I like to use an old D'varsnal (analog) type meter. You know. The one with the old meter movement in it.
I just never took the time to get comfortable testing with a digital meter. I use a digital for checking voltages, of course.

Once you get the transistor off of the heat sink, set it on the work bench with the pins facing up. Position it so that the pins are nearer to one end of the transistor. We will refer to this as the top of the transistor.

The following test is done for the NPN type transistors, which most of the commercial manufacturers use.

Ok, the pin on the left, with the pins facing up and toward the top end, away from you, is the BASE.
The pin on the right, is the EMITTER.
The case of the pass transistor is the COLLECTOR.
Using your meter, put the black lead on the BASE pin and the red lead on the EMITTER pin.

The meter should give you about a half scale reading on the 1K scale, plus or minus.
I cannot give you an exact reading because it seems that none of the transistors will read exactly the same. This means that the transistor is fine. If there is no reading at all, the transistor is junk and it is open. If the meter slams against the peg with movement, the transistor is shorted.
Either one of the later situations, pitch the transistor in the trash.

Next, leave the black lead on the pin on the left, the BASE, and drop the red lead down onto the case of the transistor, which is the COLLECTOR.
Once again the readings of the meter should be the same as the readings in the previous paragraph.

[LightFoot]

Thank you for replying. Yes We changed the Transistor on the Heat sink. It was only $2.00. Still voltage stayed High. Could it also be the transformer also? Thank you.............

[linx]

Was there just 1 transistor?

Another very crude way I have found to be somewhat successful. Take your voltage meter, and start where the power comes in and test every single component in line even on the board. When you find a part who's voltage is more than 12.8 then that will identify the bad part.

I doubt it would be the transformer, but anything is possible.

[LightFoot]

Thanks Link. Im going to do this know. Will keep you posted......................John.

[LightFoot]

yes there was only one on the heat sink. I tried it your way and 5 big diodes read at 24 volts. I tried doing a resistancs test. Posite wire on banded side of diode showed high reading . Then switched it around and it showed low resistance reading. with power off Someone once told me thats how you tell if the diodes are good.

[153]

yes there was only one on the heat sink. I tried it your way and 5 big diodes read at 24 volts. I tried doing a resistancs test. Posite wire on banded side of diode showed high reading . Then switched it around and it showed low resistance reading. with power off Someone once told me thats how you tell if the diodes are good.

I had a 52 amper that did the same thing. It ended up being one of the transistors that it sounds you have already replaced. I believe the transformer is OK because that is the correct voltage for it. The transistors help reduce. As far as checking diodes they shouldn't have much resistance. They are just a electical 1 way valve so to speak. 1 of their uses is to convert ac to dc by allowing current to flow in 1 direction. Just keep workin with it.

[LightFoot]

Thanks 153. I use to do electronics about 30 years ago. I'm in the health field now. Alot of my skills are way way in the back burner. So Im limited. And my eyes are not what they use to be. Hmmmm I must be getting younger,.....

[Darkskies]

It's sounds like the voltage regulation circuit that feeds into the power transistors went bad. I just finished building my own 60 amp variable regulated power supply using the same (3055) transistors. I used an old 100 amp 24 volt unregulated power supply, LM317T voltage regulator and (4) 2N3055 transistors. There are several other parts in the circuit but these were the major players in my design. I connected a Texas Star Killer Bee to the variable regulated circuit, adjusted the voltage to about 15 volts, keyed up radio with a big loud audioooooooo, and checked amperage draw and voltage drop wattage output. Amperage draw was about 30 amps, voltage drop was minimal and Amp swung out to about 425 watts. My next project is adding (4) more 2N3055's to get everything I can out of the 100 amp power supply. I'm not sure if the 317 will drive all (8) 3055's so I'll probably use some other NPN power transistor after the 317 to drive the (8) 3055's.

Good luck,

[LightFoot]

Ive already checked all the simpler things, changed out the 3055,
checked the zener as OK, checked the thermal switch as OK.

The problem is that the base of the 3055 has 24 volts on it.
I wasn't sure at first, but hooked up as a follower, it seems to be doing like
intended for it, to follow the voltage (at a higher current) of the voltage on its
base (less it's internal voltage drop)

I found a pinout of Q3, which is a 2sc1384 in a T0-92 case. it's got 12V on its base while center and outer pin shows 24 v
I think it should have 12V on its emitter regardless of the voltage on it's collector.

My best guess now is that Q3 is bad, although I hate to say I'm not entirely certain.So I will change her out and keep my fingers crossed.....................................Thanks Guys for being with me on this journey of electronics............

[Darkskies]

If there is 24 volts at the Base of the 3055 theres the problem. The Base is what controls the Emitter output. Look at the 3055 as is was a valve. As you increase the voltage to the Base the voltage from the Emitter will increase as well. The input to the voltage regulator circuit and the Collector (case) should have 24 volts going to it. Check the voltage going into and out of the Zener diode. The diode may be OK, but the resistor that connects to the cathode of the Zener may be open or shorted. To figure the resistor value use this formula.

R = V in - Zv / I or Voltage to Zener - Zener voltage / output current.
For example 12 volts in - 9 volt Zener = 3 / 50ma (0.05) = 60 ohm.

or 24 volts in - 12 volt Zener = 12 / 50ma (0.05) = 240 ohm.

Check the specs on max current on Zener to get output current for resistor value.

I'm not famillar with the insides of that power supply so the above maybe of no use for your problem.

Once again, good luck.

[LightFoot]

Thank you for the input there Darkskies. I will surely look into what you said...................................John.

[LightFoot]

Quick question I'm getting 12 volts at the anode of the Zener Diode and 24 volts at the cathode are you saying the resistor at the cathode end is responsible? Thanks..........

[Darkskies]

I'm not exactly sure where your testing the zener voltage at but if you test across the zener while under power you should be reading around 12 volts. The series resistor thats connected to the cathode bleeds (disappates) the excess voltage/current that the zener is shunting. Google zener diode (if you haven't already) at take a look at a zener in a simple voltage regulation circuit. Input voltage connected to the series resistor should be the 24 volts, the other side of the resistor (cathode end) should be your regulated 12 volts. Like before I'm not famillar with that power supply so there maybe another circuit after the zener that might be back feeding a causing the 24 volt signal your seeing. If anything, replace the zener and see what happens to your output voltage.
If anyone notes errors in my explanation please let me know, I'm still learning as well.

Hope this helps, good day...........