Tektronix TDS744A – Repair LV supply

I owned this Tek scope since 2007 but the scope was burnt fuse in 2009 April when I moved it to a new rack for usage.  Since then I tried to repair the power supply but was not successful without a schematic.  Later in 2010, I was able to find a TDS520B component service manual in which there is a LV supply schematic.  The schematic is not exactly same as my scope’s supply revision but is 85% similar.  Thus I am able to trouble shoot and located the faulty part Q5 – 2SK1018 booster mosfet.  In fact Q5 and Q6 are running in parallel and thus I try to replace Q5 with IRFP460 but it was burnt again after power up.  This inducated that some more parts are faulty!  Thus later I found out that R1 and R2 gate resistor of Q5 is at high resistance and thus I have to replace it with dale 1/2 W resistors 10 ohm.  I ordered some IRFP450 from RS component and is able to delivery in same day.  After replacement, the supply never burnt fuse and works again.  See some internal photo for the TDS744A.

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