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Sharp TV dead

G

Golf

Jan 1, 1970
0
I have a model #27F-S120 TV that is dead. Raw B+ good to power
transformer. No low voltage on primary of power up relay. I have
studied the board for over an hour and simply can't see where the stand
by voltage is supplied, to further troubleshoot. Hoping someone may
have a schematic, or knowledge of this particular model/problem.
Thankyou group as always, still a beginner here.
 
T

Tech Data

Jan 1, 1970
0
Golf said:
I have a model #27F-S120 TV that is dead. Raw B+ good to power
transformer. No low voltage on primary of power up relay. I have
studied the board for over an hour and simply can't see where the stand
by voltage is supplied, to further troubleshoot. Hoping someone may
have a schematic, or knowledge of this particular model/problem.
Thankyou group as always, still a beginner here.

On the other side of the 'power transformer' , actually the standby
transformer, you'll see 4 diodes. That's the bridge for the standby 24
volts. If that voltage isn't there, then you have an open winding in
that transformer. Really common fail. If it's still available, runs
around 25bucks. If not, there's other options.

www.techdata-kicksass.net
 
G

Golf

Jan 1, 1970
0
Tech said:
On the other side of the 'power transformer' , actually the standby
transformer, you'll see 4 diodes. That's the bridge for the standby 24
volts. If that voltage isn't there, then you have an open winding in
that transformer. Really common fail. If it's still available, runs
around 25bucks. If not, there's other options.

www.techdata-kicksass.net

Thanks asskicker, I have 120V on the primary, zero on the secondary.
The way it looks though, the power on relay must energize to supply
current to the transformer primary, and induce a voltage in the
secondary. How is this standby transformer made/wired? Their is an IC
(full bridge rectifier), on the secondary that is connected to the 24V
of the relay. I trust your info, just don't fully understand. Thanks
again techdata.
 
T

Tech Data

Jan 1, 1970
0
Golf said:
Thanks asskicker, I have 120V on the primary, zero on the secondary.
The way it looks though, the power on relay must energize to supply
current to the transformer primary, and induce a voltage in the
secondary. How is this standby transformer made/wired? Their is an IC
(full bridge rectifier), on the secondary that is connected to the 24V
of the relay. I trust your info, just don't fully understand. Thanks
again techdata.

Ok, to cut it short, the ac line in goes straight to the primary of
that transformer. That alone energizes the windings to induce the 24
volt standby source which also goes to the relay, as well as the 12,9
and 5 volt lines. When the relay latches, THEN ac is routed to the
primary of the swithcing power supply that provides the B+ for the
horizontal circuit. Without the smaller standby transformer working,
NOTHING else is going to work. To prove or disprove, use an ohm meter
and measure resistance on the two secondary pins of that transformer. I
got a buck says they're open. Should read shorted or near shorted.
There may or may not be a thermal fuse inside the transformer but, the
way that's designed, you'll destroy it before you get to it. Best to
replace.

www.techdata-kicksass.net
 
G

Golf

Jan 1, 1970
0
Tech said:
Ok, to cut it short, the ac line in goes straight to the primary of
that transformer. That alone energizes the windings to induce the 24
volt standby source which also goes to the relay, as well as the 12,9
and 5 volt lines. When the relay latches, THEN ac is routed to the
primary of the swithcing power supply that provides the B+ for the
horizontal circuit. Without the smaller standby transformer working,
NOTHING else is going to work. To prove or disprove, use an ohm meter
and measure resistance on the two secondary pins of that transformer. I
got a buck says they're open. Should read shorted or near shorted.
There may or may not be a thermal fuse inside the transformer but, the
way that's designed, you'll destroy it before you get to it. Best to
replace.

Now your talking something I can understand. I am still learing of different transformer designs. If it is designed to induce a voltage into the secondary (on standby power), does the primary have a interal closed loop, that induces this 24 volts into the secondary? The circuit reveals an open on the primary from the outside. Thanks a ton techdata for the response.
 
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