Tektronix 475A +50 Power Supply Regulator Issues


awesker2
 

Well I've replaced all of the power filter caps and replaced the
regulators just so I could have a clean start and powered the unit up.

Upon reading the service manual it specified that I should repair the
+50V supply first since its the reference voltage for the other
supplies. Upon measuring the regulated +50V output I got nothing. I
checked voltage across the filter cap for the +50V and I got 74 volts
so the transformer is doing its part then its getting rectification
and filtering and the +50V hits a brick wall when it comes to the
rectifier circuit.

Upon measuring the base voltage of the regulator switching transistor
Q1426 I read 74 volts (the same voltage as the unregulated +50V
supply). However when I checked both the collector and emitter I got
almost nothing not even a volt on either. So bascially I think that
the switching transistor is conducting however it isn't receiving any
input voltage to regulate so it remains on doing nothing.

Heres a link to a screen shot of the 50 volt section schematic.

http://i15.photobucket.com/albums/a391/krankshaft/50Vsection.jpg

Any suggestions on what I should check?

Thanks

Mark

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