On 19.02.2016 20:20, Cursitor Doom wrote:
Hi all,

I've completed my tests of the main transformer and am now 99%
certain that it is the cause of all the problems I've been
experiencing with this old analogue scope. It's clear there's
something very wrong with the large, multi-tapped output winding.
Here's the schematic again:

https://www.flickr.com/photos/128859...in/dateposted-


I removed ALL connections from the transformer. ALL the other
output windings are giving exactly the outputs I would expect from a
given input; it's just the long winding on the lower right hand side
that's giving nonsense outputs. As you can see, the centre tap is
grounded and there are 3 tapping points either side of it. When
injected with a 20kHz sine wave of 50V p-p to the primary winding,
the peak-to-peak outputs from the problem secondary at each tap are
as follows (from top to bottom)

13V 13V 3V 0V (gnd) 3V 1.8V 1.8V

I would have expected these voltages to be symmetrical either side
of the 0V centre tap, but as you can see, this isn't the case at all.
I can only conclude from this, to use a technical term, that this
tranny is ****ed. If there's something obvious I've overlooked
(which I doubt) please feel free to point it out. Otherwise I'll be
opening it up to perform an autopsy over the weekend. Thanks again
to everyone who tried to help.

Hi

If you had a reliable connection to the primary and all secondaries were
in fact free (either the tranny completely removed from the circuit
board or at least no diodes anywhere remaining), then that would mean
the voltages are likely screwed up... But wait:

Are you sure that you have not mixed up the windings? Maybe the two 1.8V
windings are actually the 2 symmetrical "innermost" ones, the 3V ones
are the "medium" ones and the 15V are the "outermost" windings? Your
measured winding voltage ratios "1.8:3.0:13.0 volts" and the schematic
output voltage ratios "6.7:13.4:60.7 volts" (I've added a little
compensation for 0.7V silicon diodes) are (from a purely ratiometric
point of view) not very far away from each other. In fact they are so
close that the differences between the smaller ones can be easily
explained by your measurement errors (how accurate was that 0.8V
measurement anyway?) and the possibly intended uneven loading of the
power rails in the scope.

So, considering the winding connections slightly rearranged, the
transformer looks just fine to me.

But once you have it out and disconnected, please make another test:
apply ca. 15V RMS to the 12.7V winding (to the one where you measured
3V) instead of to the primary. And check if any isolation looks like
breaking down. Note that the 15V value contains some compensation for
the fact that the power supply uses inductors after the rectifiers (and
therefore the normal winding voltage is higher than the normal output
voltage). That would load the transformer close to its normal condition
and any breakdown should become apparent.

Regards
Dimitrij