Re: Modifications to 577 Curve Tracer Main Power Supply
Chuck Harris <cfharris@...>
Hi Gary,toggle quoted messageShow quoted text
I was tired when I read your post, and somehow took
"main power supply" to be "main transformer"... 12 hours
in planes, and airports might have had something to do
Anyway, one of the least reliable of semiconductor parts
I find are Darlington power transistors. They are hard
to test... suggesting a curve tracer, and often they will
have the output transistor develop a B-E short, and expose
the input transistor to the output terminals... This makes
them appear as a good transistor when tested with an ohmmeter,
or simple transistor tester, but a transistor having way too
low gain to do the job.
Otherwise, this circuit is trivially simple. You should
be able to troubleshoot it by making a couple of simple
Note, the two 6.8V tantalum capacitors are most likely
underrated. They are being used in a way that commonly fails
dead short circuit. You said they were good, so maybe not
Before you do anything, measure the +40V and -40V unregulated
supplies to ground. Don't get hung up on the voltages, they
must be at least 35 or so volts, and could be much more. They
should have around a volt of ripple...
Next, we must get the -30V supply to work. It is absolutely
needed for the +30V supply to work.
The grayed out components form a snubber network that is
there to slow Q772 down. If it is missing, but is now
necessary, you will find Q772 is producing AC.... your scope
is your friend, measure the collector and look for a several
volt oscillation. I truly doubt this is the problem... unless
you have substituted Q772.
Because the voltage is way too low, the entire regulator should
be working maximally hard to pull the -30V output all the way
to the -40V unregulated supply.
Start with Q788. It must be fully turned on. That means its
VCE should be less than 2V, and its VBE should be more than
1.5V. If not, it is either bad, or being driven incorrectly.
Next, look at Q786. It must be fully turned off. That means
its VCE should be the same as the -40V unregulated supply. If
not, it is either bad, or being driven incorrectly.
Next, look at Q772. It must be fully turned off. That means
its VCE should be about at the -40V unregulated supply...
probably somewhere between zero less, and the zener voltage
less... If not, it is either bad, or being driven incorrectly.
My money is on the Darlington power transistor. I don't like
them, they are mean.
That should get you started.
Gary Robert Bosworth wrote:
Chuck: I did not find anything wrong with my transformers, thank God.