Adrian Glaubitz wrote in
:

Hi there,

Are you sure it's *completely* dead? I'd probe it's outputs with another
scope to be sure. You may be able to see it pulse it's outputs in an
attempt to power up before shutting down. If you do see pulses on the
outputs, it could just be a matter of high ESR capacitors.

Also, try disconnecting other boards/modules in the unit. One of them
could be overloading the power supply.

Yes, it is completely dead. I followed the troubleshooting procedure
step by step from the "service-manual" (they don't have schematics) from
Tek and from the flowchart it said "replace PSU".

I did some more testing today. It appears, that after the blown resistor
there is an PWM-circuit with a TOP200YAI PWM-Switch. After replacing the
resistor I can read the 300V at the input of the PWM-circuit, but there
are *no* 5V DC at the output. I did a web-search for the PWM-Switch
datasheet and found it. It actually contains a schematic for a typical
application and the PWM-circuit in the PSU appears to be completely
identical.

I tested all diodes, resistors and the windings of the transformer in
the circuit and all parts indicate ok. So I presume the PWM-Switch
itself to be bad. The thingy costs about 8 US-Dollars and I only can
find a TOP201YAI and TOP204YAI at my local dealer. Can I use those or
does it have to be the 200 ?

check their datasheets.I suspect the TOP201 will work fine.

Anyone knows where I could rip one out ?

I'd be interesting if anyone could comment on this :-).


Regards,

Adrian


high ESR of the electrolytics is a big problem area in switchers.

--
Jim Yanik
jyanik
at
kua.net