2235 Repair having no power


Hi all,

I am fighting with a 2235 PS.
I previously repaired several 2213 and 2215A's, and it "always" was the primary switcher, but this time....

I removed the Power FET to apply a external 43VDC to see if the scope would work, but no, it looked like a full short, the secondary switcher would not work at all.
I disconnected all secondary diodes from transformer 120-1601-01 , still the same.
I disconnected the 2kV wire and disconnected the two filament wires, still full short, no switching in the secondary switcher.

I checked all components in the secondary switcher circuit, all OK.

I removed the transformer 120-1601-01 (PITA!) and temporarily replaced the two primary windings in the secondary switcher circuit with two 24 Ohm 5W resistors, and behold, the secondary switcher is working!

So, is my transformer dead?

The weird thing is, I applied a 400mV 50kHz sine to one of the primary windings, and all secondary windings give a voltage which looks like OK with respect to what is to be expected??
My thoughts were: if the input is roughly 40V switched, I know roughly what to expect, so if I use a similar, but lower voltage from a generator, I could check the individual outputs?

I still do not know if the transformer is dead or not, but when I connect the two primary windings it will not switch at all.

So to test further, I did the following first: I did a "ringing test" on the transformer.
Basically you apply a fast rise and fall square wave to (one of) the primary winding(s) and measure using a scope the output of one of the secondary windings at the moment of the Rising flank.

More info on this "ringing test" can be found here: https://www.flippers.com/fbt-main.html

Here are some pictures I took: https://groups.io/g/TekScopes/album?id=254000

In the mentioned album you see the result of the TekTronix transformer and the result of an other transformer I got from a switched mode power supply.

When there is (hardly) any "ringing" that means there is a shorted winding, or at least a heavily loaded winding.
But since these transformers are now both out-of-circuit, there should be no load.
However, the TekTronix transformer is showing hardly any ringing, so somewhere in there is a short.... :-(

Now I need to find another transformer for my 2235, preferably in the EU, 120-1601-01 or similar (2213, 2215, 2215A, ...)

Saludos, stay safe,


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