Re: Tektronix 2215, pulsing/chirping noise, no trace.

Samuel Rey

I am aware the faulty component is in the prereg, as I had previously ran
the board with its output disconnected and it still did the same thing.
Tekwiki has a doc on troubleshooting faulty psu for this model, and it
mentioned that if ticking is heard, the rectifier diodes are the first
thing to check. This is talking about actual diodes and not the full wave
rectifier, correct?

El El mié, 10 jul 2019 a las 16:13, <tekscopegroup@...> escribió:

I doubt that you have a problem at the output of the pre-regulator that is
causing the problem and it shutting down due to an abnormal condition on
the load side. As you pointed out the scope works fine with an external +43
volts being fed into the secondary power supply. So that would point
towards a faulty component that is not allowing the prereg to start up. In
my experience repairing this section in a 2213A some years ago was that it
is absolutely necessary to check everything in that stage specially
transistors and diodes, even if in-circuit measurements do not point at the
part being faulty before you even apply power at all. You need to pull
every single transistor, diode, etc in the preregulator and test it, and
all the lower value and/or higher power resistors as well for open circuit.
Of course it goes without saying also check the power input components,
like the 75uF cap and the associated rectifier, etc. Double check the
Mosfet and driver transistor. Even if it was fine last time you checked,
you need to double check everything in one batch before ever applying
power. Check the regulator IC as well, I seem to remember it was a TL494
that sometimes just quits. Pull it out and place a socket then put in a new
one. The socket will also allow for easier future testing & replacement.
You might also have a faulty T906 switching transformer, which will be
difficult to verify, so leave that possibility aside for last when you are
sure everything else is ok and the preregulator is still not working. As a
matter of fact my scope happened to have a bad T948 switching transformer,
and the only ay to make sure it was the problem was to replace it. But I
digress as this particular trasformer is located downstream from the
preregulator circuit.

Unfortunately dealing with a switching power supply you will usually be
able to follow a trail until you hit the fault because it works in a closed
loop where everything either needs to be right, or all (or most) will be
wrong. You will have to test every active component and make sure it is not
open or shorted, leaky, etc. The fact that the fault happened while you
where using the scope would suggest that it is not related to some
component slowly going out of tolerance, but rather of something popping,
usually at start up which is the (very short) time interval of highest
stress for a switching regulator until after a few milliseconds when things
quickly settle down.

This prereg stage, as with usually most other switching power supplies is
very temperamental and will absolutely not work unless all is good, and
will also tend to blow freshly replaced components during the intermediary
"let me check if that was all that was bad" situation, and then you might
be quickly sent back to square one (or even further back) with one or more
of the higher power parts blown and/or smoking.

Again good luck with the hunt, and don't be discouraged. You will soon be
rewarded with that green power LED on the front panel coming back on a
fully working scope. And please don't even consider using some sort of
permanent external power supply as a hack to operate the scope instead of
fixing the problem.

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