Hi, I own a TDS694C which has a similar power supply. Worked fine but after
sitting on the shelf for a long time died on the first power up.It produced
modulated high pitch sound on standby power, and died completely on
attempt to turn main power on by the front button.
The first thing I found was a dried and blown 1000uf EC in the power
supply, s main line, C124 in TDS520B schematics. After replacement nothing
good - no standby power. Measured conductivity of integrated PWM (U12)
output transistor - open in both source-drain directions, not good. It was
TOP200YAI Replaced with a newer and more powerful TOP204YAI. Still nothing,
no input voltage on standby PWM. High voltage Zeners were ok (tested with
curve tracer, you can also do that with auto transformer and rectifier on a
breadboard, but be careful with high voltage!).
Walking along the supply line I have found a non-conducting current
limiting 1W resistor, R176. It was 2.4Ohm in my PS and *fused* 27Ohm in
TDS520B. It just cracked when the integrated PWM was blown by the voltage
spikes. Very thin crack, visible under microscope only. So it is fused in
TDS520 for a reason. Replaced with a new one and the problem was solved - I
immediately got standby power and good main power after.
Best,
-Andrei
