Thank you Geri,
Some places call the horizantal output transistor "chopper".
I have checked everything, have tried 3 driver ICs (upc1377c). signal is
OK, but only frequency is unacceptable to me. 5 KHz is too low, D1878
base is driven in flyback mode, when frequency at startup is so low, the
magnetic energy can't supply enough current to hold the transistor in ON
state, and as soon as transistor gets out of saturation, power
disspation in collecor rises and destroys the transsitor.
However the frequency of the driver is determined by some feedbacks from
the FBT ... as long as D1878 doesn't work there is no feedback ... so
there is a real vicious circle (loop) here !!!
I can see the H. frequency responds to AFC (tried manually) ... So the
question is how much should be the free running frequency of the driver
when the feedback is not in action.
by the way the monitor is almost never used so clean inside as hard to
believe it has ever been used. My guess is there is a fabrication
problem, but what ?
Thanks,
Dario.


Geri wrote:
I can't find info on that model. The 2SD 1878 isn't the chopper, it's
the horizontal output. Check for bad high voltage caps, bad connections
in the horizontal drive, & around the flyback. Also try to apply power
with the horizontal output unsioldered. You MAY be able
to check the drive waveforms, & B + to the flyback, & output. Geri.



Dario wrote:

Hello There,
I have a SONY MONITOR (PVM-20M2MDE)
First fuse was blown and chopper(2SD1878) was destroyed. I replaced the
chopper and it destroyed again (anything else around was fixed,
checked OK).
I had to open the chopper and see the horizantal drive signal, it is
only 5 KHz which certainly is too low for FBT to tolerate. I replaced
the horzantal IC (upc1377c) and the signal is still the same.
Here comes the headache ... there are a lot of feedback paths from FBT
to this IC, which affect the frequency, I can't connect the chopper (to
have feedback)as certainly it destroys again.
Has anybody experience with such a problem ?
I should add that the destroyed choppers had all pins shorted (this
usually means an error in drive circuit).
Any help will be appreciated.
Dario.