It seems my 0 ~ 24 bench supply has lost some of it's punch and only puts out about 19.5 max now. I've also noticed the meter itself is not linear going up in voltage as tested with my TEK VM connected on the out puts. I'm assuming the 4 ~ 5 internal caps have gone bad and should be replaced.
Regulation (bad regulator IC) Have you placed a scope on the output? Loss of pass transistor (2N3055 ?)
gb
No, you need to do some basic troubleshooting to find the fault. Linear power supplies (assuming that's what this is) are one of the easiest pieces of equipment to troubleshoot as they all tend to follow the same basic set of building blocks.
There'll be a transformer, bridge and smoothing capacitor arrangement. The regulator circuit will usually consist of either a dedicated regulator IC, op-amps or discrete transistors, or a combination of these. These will usually feed the base of the driver transistor with 1.4-ish volts above the final output voltage to compensate for the 0.7-ish Volt drop across the driver and pass transistor(s) B-E junction.
Basically, you need to start tracing back through the chain from the pass transistor, driver transistor and into the regulator to find where the problem is. Make sure there's adequate voltage on the collector of the pass and driver transistors, I'd expect at least 28V. Check any zener diodes for the correct voltage drop, and check resistors for open or changed resistance.
The most valuable tool for fixing linear supplies is a good old multimeter, as most problems will be fairly easy to spot and narrow down with a few basic checks.
If you don't have a scope there's a great way to check for ripple on supply rails. All you need is a reasonably sensitive audio amp, a set of cheap PC amplified speakers might do, attach a croc clip to the ground and a probe to the audio input (preferably coupled via a capacitor, though there should be one in the amp itself) and you have a way of listening for any hum! Attached across the smoothing cap of a power supply it should give a good indication whether the cap is doing its job. This should be done with a reasonable load on the power supply.
It's crude but it worked well enough for me before I could afford a scope. Obviously it should not be used on high voltage sections for safety reasons.
Dave
Much appreciated!
Cheers
One of my most useful items of test gear has resided in my toolbox for over 30 years, I made it myself back in the 70`s. It`s a crystal earpiece with a croc clip on one lead and on the other lead a probe made from an old Biro with a capacitor in series with a sharp point. You can use it to listen for audio signals from phono levels right up to loudspeaker levels, trace for distortion and check rails for hum and fizzle etc, or as a simple input device for testing audio stages (you blow into it)
Ron (UK)
-- Lune Valley Audio Public address system Hire, Sales, Repairs www.lunevalleyaudio.com
If the ripple is bad enough, you can set your multimeter to AC volts and measure the rails. You'll get a slight bump when you first touch the probes to the rails, but then it should stay at zero. If not, you've got AC on the rails. I found a very dry filter cap in a Tek scope this way.
Matt Roberds
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