TO: THE “ALL ABOUT CIRCUITS” WEB SITE MEMBERS


REFER TO THE ATTACHED SCHEMATIC while reading the explanation (a .jpg file)


I have a problem with ½ of a 2 section power supply board that supplies +47 volts to a pulpit microphone via several pre-amplifiers. The board is part of a Electro-Voice B-42 Stereo Mixer Panel.


The voltage at the input to the bridge rectifier reads 63V AC (and looks normal). At the rectifier output, the voltage reads 83V DC.

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The voltage at the output of the circuit should read +47 volts, but reads 0v (no load)


The problem appears to be at the “pass transistor” (TIP28C) Q554. The indications are that it is not conducting. I have replaced it and the transistors (Q551 & Q552) that drive the TIP28C with NTE replacement transistors bought at FRY’s electronics.


The voltage measurements at (TIP28C) Q554, with Q551 removedand Q552 installed, are as follows:

Emitter to ground = 12.7V

Base to ground = 13.7V

Emitter to ground = 82V


The voltage measurements at (TIP28C) Q554, with both Q551 and Q552removed, are as follows:

Emitter to ground = 55.1V

Base to ground = 54.7V

Emitter to ground = 78.6V

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Zener diode 1N5261 was also replaced. The original was a 1/4w component, but only a 1/2w unit was available. The voltage across the 390Ω resistor is 1.83v which is connected

in series with the Zener diode.


Any help would be appreciated to get this circuit functioning again.


SM1WRI

 

Did you check the resistor values.

 

Hello and welcome to AAC!
I have changed the title of your thread to something more descriptive to make it easier for members to help.

 

Sounds like the resistors feeding the base are faulty or the capacitors C553,554, what voltages are across those,remove Q551,552.
Disconnect the Emitter of the Tip28c and measure again B,E, C..

 

You show four different values for 'emitter to ground'. Should two of those be 'collector to ground'? If so, it's suspicious that the collector voltage is different from the diode bridge output voltage. Dodgy connection in that area?