"trm" trm54321at hotmail.com wrote in message
...
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

"trm" trm54321at hotmail.com wrote in message
...
Unfortunately, I don't have a scope....
Should I replace these parts by default?
The P/N of this supply is 22-8230.
Cheers


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

trm wrote:


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


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

Dave D wrote:
If you don't have a scope there's a great way to check for ripple on
supply rails.

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