Mits CS35701 PS trouble, strange indications
 

Hi;

I'm back. This time it's a pesky STR-M6511 SMPS.

Tshot it after other tech replaced the 6511, it also had a shorted HOT
and the usual open fusible. SMPS is pulsing. Pin 4 (internal IC power)
is indeed dropping to 10V when it starts, it quits then the voltage
ramps up again repeating the process. I found the rectifier that is
supposed to supply this voltage to pin 4 and it checks good. It has
ocurred to me that this high speed diode could be bad in the same way
as a 164589 15V rect in a CTC169, but scoping the anode shows that
there is not enough to rectify. Thinking of many things I changed the
filter connected to pin 4 even though it checked good.

I looked at the output pin of the 6511 and sure enough, the amplitude
was not what I expected. When stopped I of course saw the normal hot
side DC, but when running, the lower portion of the waveform is
dropping near zero, but I expected the peaks to be twice the DC. Is
this not correct ?

The peaks only rise about 50V above the DC level. I looked for an
overload using the scope trick, when it shuts down see how fast the
voltage drops, all supplies held long enough except the 37V. It dropped
like a rock, but when I disconnected it the pulsing SMPS persisted.

One thing bothering me is the frequency is too high, but this is a
subjective judgement, it might be normal without the load, and there is
no H drive. Or is there another cause ?

The other thing bothering me is that the set indeed did need the HOT
and fusible AND the 6511. Is this a clue to a destructive failure in
the SMPS that will not recur because it was already running when it
happened ? Of course I checked the opto with an ohmmeter at least.

Seen anything like this ?, any insight would be appreciated, thanks.

JURB