Arista Power Supply

I don't believe that I'm actually quoting Larry King, but here goes... "WHAT'S THE QUESTION???!!!"

The "short" answer to your question is YES, I am familiar with Aritsta Power Supplies. You stated that you have a faulty unit..... so what are the symptoms (other than dead) and what prelimary troubleshooting, voltage readings and component testing have you done? Post the information and troubleshooting results and some repair suggestions might be forthecoming. electricitym

Arista web site (Fremont, CA)

Most linear power supplies use a regulator IC (such as LM723) and pass transistors (NPN: 2N3055, 2N3771) for current handling.

A VOM or DVM can quickly take voltage readings, diagnose components to resolve. Need a DIY "How to guide" ? - Check the KBT DC Supplies web site Menu of knowledge database, reference data sheets, "How TO" on left column (gray)

g. beat

OK thanks The supply delivers 15 volts on the output (should be 13.8) but when I connect a load, the voltage drops to 0, so there is no regulation. The supply uses an IC numbered C324C which I'm assuming is a quad op amp - LM 324 - configuration seems to confirm this. Driver transistor is an SD880 driving a 2SC3281 Across the output on the board there was an open circuit 220mf electrolytic and a burnt resistor which I thought was colored brown, brown, brown (110 ohms)not a preferred value - it measure 331 ohms. heat has perhaps made it change value and alter the color coding? What I'd like is a schematic.

Peter

Peter

Correct, the C324C is a LOW POWER QUAD OPERATIONAL AMPLIFIER (NEC part number) Spec Sheet

SD880 might be a NPN EPITAXIAL PLANAR TRANSISTOR

Pull the 2SC3281 and check with VOM or DVM

It was probably orange-orange-brown and 330 Ohms to start with, so it's likely almost spot on in value.

A schematic? If you have even a basic grounding in electronics (enough to interpret a schematic) you should have no trouble at all diagnosing a simple linear power supply like this!

There should usually be around 15.2V on the base of the driver transistor, and around 14.5V on the base of the output pass transistor to give an output of 13.8V (0.7V B-E drop on each transistor). All you need to do is check this off load and on load and see where the fault is. If the base of the driver drops to 0V or thereabouts under load then you have a problem in the control circuit or current limiter.

Check both the driver and output transistor have unregulated voltage on their collectors as well- I'd expect 18V or more likely higher. Check if there's a current sense resistor in series with the output (usually ground side) and that it isn't open circuit. It will be a fraction of an ohm so a basic continuity check will suffice.

The supply has quite a beefy output transistor so maybe the driver failed? IME linear supplies often take out the driver when the outputs are shorted. Either way it should take about 5 minutes with a multimeter to at least track down the region of the fault to either control, driver, pass or current regulation sections.

Dave

The transistors tested out as OK - voltage readings were very close to what you advised. I replaced the (open circuit)220 MF electro and the 330 Ohm 1 watt resistor as well as 100nf ceramic which were across the output. All three looked heat stressed. After that the supply performed faultlessly - delivered 5 amps into a dummy load fine. Wondering how the o/c electro could have caused the voltage to sag. Also thinking that the 330 ohm resistor runs a bit hot - one can see that the board is heat stressed around this component. It would be passing 40 ma.

Many thanks. Peter

It was probaby there to stabilise the current sense, and without it the current limiter was kicking in when a load was connected due to the sudden output voltage dip.

Just a guess mind, it's hard to say without seeing it.

Dave