CRT TV SMPS

[pfm]

I continue to try and repair a CRT TV. The set's main SMPS is a
discrete, self oscillating design. The set powers up for a few seconds
and then shuts down; I think the Micon controller chip releases the AC
relay. The B+ open circuit voltage is 139V (ie disconnected from the
horizontal output). When a 60W bulb is substituted as a load for the
'horizontal', the voltage is zero, not a glimmer from the bulb. How
can the B+ power up the set, albeit briefly, but not light the bulb?
Is there a problem with a simple resistive load? Is it possible that
the horizontal stage is essential to the operation of the SMPS? Any
thoughts please?

BobH

 

[Arfa Daily]

"pfm" <bobh@pfmbox.com> wrote in message
news:a98c8cf8-c9fd-4dbd-b1cb-fe88c415e9a3@z35g2000yqd.googlegroups.com...

I continue to try and repair a CRT TV. The set's main SMPS is a
discrete, self oscillating design. The set powers up for a few seconds
and then shuts down; I think the Micon controller chip releases the AC
relay. The B+ open circuit voltage is 139V (ie disconnected from the
horizontal output). When a 60W bulb is substituted as a load for the
'horizontal', the voltage is zero, not a glimmer from the bulb. How
can the B+ power up the set, albeit briefly, but not light the bulb?
Is there a problem with a simple resistive load? Is it possible that
the horizontal stage is essential to the operation of the SMPS? Any
thoughts please?

BobH

A light bulb is not really a 'simple resistive load'. When the filament is
cold, its resistance is much lower than when it's up to temperature, so it
might just be that when the power supply tries to drive it from cold, it
just represents a bit too much of a load, causing the PSU to trip back to
standby.

Arfa

 

[Jim Yanik]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in
news:YXQln.119129$h21.75057@newsfe07.ams2:

"pfm" <bobh@pfmbox.com> wrote in message
news:a98c8cf8-c9fd-4dbd-b1cb-fe88c415e9a3@z35g2000yqd.googlegroups.com.
..
I continue to try and repair a CRT TV. The set's main SMPS is a
discrete, self oscillating design. The set powers up for a few
seconds and then shuts down; I think the Micon controller chip
releases the AC relay. The B+ open circuit voltage is 139V (ie
disconnected from the horizontal output). When a 60W bulb is
substituted as a load for the 'horizontal', the voltage is zero, not
a glimmer from the bulb. How can the B+ power up the set, albeit
briefly, but not light the bulb? Is there a problem with a simple
resistive load? Is it possible that the horizontal stage is essential
to the operation of the SMPS? Any thoughts please?

BobH

A light bulb is not really a 'simple resistive load'. When the
filament is cold, its resistance is much lower than when it's up to
temperature, so it might just be that when the power supply tries to
drive it from cold, it just represents a bit too much of a load,
causing the PSU to trip back to standby.

Arfa

problem is probably bad electrolytic caps.

--
Jim Yanik
jyanik
at
localnet
dot com