bench power supply repair

BENCH DC POWER SUPPLY 0-40volts DC 0-5 Amp Repaired- Eramtronic Model 405

After many years of good service by my digital dc power supply now it was not working. How do I know it was not working?

Recently when I decided to test a 40 volt 10 watt light bulb to check if it good or bad, after I hooked up the wires to the bulb I noticed that the light was dim. It supposed to produce 40 volt DC but now only 32 volt. The display shown was 32 volt and this means I have to troubleshoot it.

For your information there was no schematic diagram for this set.

After opening the cover there was no SMPS found. It was using a linear transformer as seen from the photo below.

benchpowersupplyrepairing

It was a very big linear transformer with three PCBs. One PCB for display panel and the other two for regulation of voltages.

I saw two big electrolytic capacitors with 4700 micro farad 80 volt, a big bridge rectifier and two big transistors 2n3055 at the back panel that was attached to a big heatsink.

heatsink in bench power supply

I started to check the dc power supply and I found the two big capacitors could be the main suspect. The reason for it was because the dc power supply already more than 10 years old and it could be the time the capacitors to breakdown.

After discharged the big capacitors I checked the value and found both caps had decreased in capacitance value and need to be changed. The two big transistors 2N3055 were found to be good. There were also many dry joints in the circuit board and I resoldered all of the joints. Now the DC power supply is working fine and producing 40 volt dc.