Tony G0MQG Tips

A Power supply repair....

A friend gave me some miscellaneous equipment amongst which was a 13.8 volt power supply of the type used on cb radios.
On trying it for the first time after a quick look inside to ensure it was safe to plug in i found that there was no output and the LED for output indication was out.
I removed the control board to investigate, the rectifier is 4 off 3 amp diodes (in 2 parallel pairs) in a bi-phase configuration using a transformer with a centre tapped secondary, 2 ofthe diodes were open circuit.
The circuit was quite simple, the dc from the rectifiers and smoothing capacitors feed the collector of a TO220 type transistor, the emitter of which feeds the base of an (unmarked)TO3 type output device the collector of which is also fed from the dc output from the rectifiers, the emitter of which becomes the regulated output. Clearly the transistors were NPN.
The TO220 transistor has a 15 volt zener diode connected to it's base which is fed via a resistor from the rectifier output, the 1.2 volts dropped across the base – emitter junction ofboth transistors (0.6 volts across each) gives 13.8 volts output.
Further investigation showed that both transistors and the zener diode had also failed
I replaced the diodes with 1N5401's, the zener diode with a 1N5352, the TO220 transistor with a TIP 31 and the TO3 output transistor with a 2N3055, all now worked fine.
It should be noted that these supplies are, in my opinion "built to a price" and can fail with the output device going short circuit applying unregulated output (which on this one isabout 25 volts) to an (expensive) radio so i would hesitate to use one on an amateur transceiver but should be ok on auxiliary equipment.
                                                                                      
Tony g0mqg

Christmas 2017

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