power supply problems

[maverick79]

I have a pyramid model ps 52 k and it is putting out 20 volts i am woundering what would cause this and what i can do to fix it please help

 

[Robb]

Sounds like it lost the regulator

 

[711]

second that. when pass transistors go, they will read 20 volts on the ouside shell of transistor. if it is 2n3501 most rebuilders I see substitute 2n3771 as a better replacement.

 

[Captain Kilowatt]

Sounds like it lost the regulator

Usually when a regulator goes the voltage drops to almost nothing as the pass transistors have no drive.

second that. when pass transistors go, they will read 20 volts on the ouside shell of transistor. if it is 2n3501 most rebuilders I see substitute 2n3771 as a better replacement.

Agreed. Most likely a bad pass transistor allowing the full preregulator voltage to appear at the output. Not a good thing. I am surprised the power supply does not have over voltage protection. those pass transistors may be 2N3055's BTW.

 

[unit_399]

MAVERICK 79 -

Captain KW is right. It sounds like a pass transistor is shorted. But the overvoltage protection circuit evidentally isn't working because when the output voltage exceeds 16 volts the OV protect circuit should cause the fuse to blow. This unit comes with 2N3771 transistors from the factory. You need to have a good tech check the entire supply out.

- 399

 

[Captain Kilowatt]

MAVERICK 79 -

This unit comes with 2N3771 transistors from the factory. You need to have a good tech check the entire supply out.

- 399

I guess I'm showing my age a bit by mentioning the 2N3055's. They were the defacto standard back in my day.

 

[Skyrider]

Mouser should have the replacement transistors. Check the board to see if any other components look to have been messed with or bypassed, usually, as already mentioned, power supplies will cut out when a certain voltage is exceeded.