Sharp 76GF-63 (serv.man2) Service Manual ▷ View online
DA100 (50Hz) Chassis – Article for Television Magazine
Scan Velocity Modulator
The purpose of this circuit is to maintain the luminance level as the electron spot moves across the
CRT face.
The distance that the electron beam travels from the electron gun to the CRT face at the outer edges
of the CRT is greater than the distance from the electron gun to the centre of the CRT face. This
distance is emphasised more on 16:9 CRT’s than 4:3 CRT’s with deflection angles of 110°
This result in the intensity of the electron spot varying as it travels across the CRT face. To overcome
this a coil is placed around the CRT neck, close to the gun assembly. The purpose of this coil is to
increase/decrease the speed, the spot travels across the CRT face, therefore, maintaining luminance
transitions.
CRT face.
The distance that the electron beam travels from the electron gun to the CRT face at the outer edges
of the CRT is greater than the distance from the electron gun to the centre of the CRT face. This
distance is emphasised more on 16:9 CRT’s than 4:3 CRT’s with deflection angles of 110°
This result in the intensity of the electron spot varying as it travels across the CRT face. To overcome
this a coil is placed around the CRT neck, close to the gun assembly. The purpose of this coil is to
increase/decrease the speed, the spot travels across the CRT face, therefore, maintaining luminance
transitions.
Figure 59: Scan Velocity Modulator Circuit
Circuit Description
The signal that is used to control this circuit is derived from IC801 pin 34 and the amount of
correction can be adjusted by the end user via the picture menu. The drive signal is based upon the
luminance signal and controls the conduction of Q5407 and Q5408 depending upon the intensity of the
signal at that point of the scan/screen location. If there is a bright area at the edge of the picture,
the current in the SVM coil will increase and accelerate the electron beam. This results in a brighter
picture at that instant. If the scene is dark, the coil is not energised. As picture information is
changing all the time, so does the SVM signal. If viewed on an oscilloscope, the output signal looks like
an inverted luminance signal.
correction can be adjusted by the end user via the picture menu. The drive signal is based upon the
luminance signal and controls the conduction of Q5407 and Q5408 depending upon the intensity of the
signal at that point of the scan/screen location. If there is a bright area at the edge of the picture,
the current in the SVM coil will increase and accelerate the electron beam. This results in a brighter
picture at that instant. If the scene is dark, the coil is not energised. As picture information is
changing all the time, so does the SVM signal. If viewed on an oscilloscope, the output signal looks like
an inverted luminance signal.
Page 61 of 80
Sharp Electronics (UK) Limited - March 2003
Revision 2
DA100 (50Hz) Chassis – Article for Television Magazine
Faults in the Picture Rotation and Scan Velocity Modulator Circuit
One of the most common problems in this area occurs after a receiver has been serviced. The plugs
for the scan velocity modulator and the rotation coils are the same, and it is easy to mix the two up.
If no adjustments are made to either of these circuits, then there may not be any noticeable change
to the picture. If left in this condition for a number of hours, the rotation output transistors will fail
prematurely.
for the scan velocity modulator and the rotation coils are the same, and it is easy to mix the two up.
If no adjustments are made to either of these circuits, then there may not be any noticeable change
to the picture. If left in this condition for a number of hours, the rotation output transistors will fail
prematurely.
Figure 60: SVM and Tilt Coil Connectors
Page 62 of 80
Sharp Electronics (UK) Limited - March 2003
Revision 2
DA100 (50Hz) Chassis – Article for Television Magazine
Preventative Maintenance
Ensure that:
1. The 170V avalanche diode is fitted across C720.
2. C604 is rated at 105C.
3. R713 and R714 are metal film types.
4. R1721 and R1722 (Pro-Logic power supply) are metal film types.
5. On the 66FW53/4H and 76FW53/4H that the two extra capacitors are fitted to the CRT base,
1. The 170V avalanche diode is fitted across C720.
2. C604 is rated at 105C.
3. R713 and R714 are metal film types.
4. R1721 and R1722 (Pro-Logic power supply) are metal film types.
5. On the 66FW53/4H and 76FW53/4H that the two extra capacitors are fitted to the CRT base,
as shown in the diagram below.
6. There are no dry joints in the horizontal output circuitry, especially on C613 and R613.
7. The EPROM fitted is correct for the CRT and circuit combination (see IC information at the start
7. The EPROM fitted is correct for the CRT and circuit combination (see IC information at the start
of this article for more information on this).
Figure 61: CRT Base PWB Modification (66cm and 76cm sets only)
Reference Description
Part
Number
C1826 Capacitor,
4.7nF
500V
VCKYPA2HB472K
C1827
Capacitor, 10nF 250V
RC-KZ0029CEZZ
C604 Capacitor,
330µF 10V 105C
VCEAGA1AW337M
C720 Capacitor,
100µF 200V
RC-EZ0258CEZZ
D735
Avalanche Diode, 170V
RH-EX0875BMZZ
R1721 Resistor,
560kΩ 0.5W Metal Film
VRC-MA2HG564J
R1722 Resistor,
560kΩ 0.5W Metal Film
VRC-MA2HG564J
R713 Resistor,
560kΩ 0.5W Metal Film
VRC-MA2HG564J
R714 Resistor,
560kΩ 0.5W Metal Film
VRC-MA2HG564J
Page 63 of 80
Sharp Electronics (UK) Limited - March 2003
Revision 2
DA100 (50Hz) Chassis – Article for Television Magazine
Page 64 of 80
Sharp Electronics (UK) Limited - March 2003
Revision 2
Servicing Tips
When servicing this chassis, it is advisable to remove it from the plastic support frame, by taking out
the eight or ten fixing screws. This only takes about a minute and can save many more when searching
for a component located underneath one of the bracing bars! It is not acceptable to remove any of
this support frame as this may result in the chassis cracking.
Note that the 56cm version of the chassis does not have a support frame because the base of the
cabinet and rear cover supports it. On the 56FW53H the front of the chassis locates into a slot at
the front of the cabinet. If this not correctly aligned then the PWB will crack when the rear is cover
is put on.
When pushing the chassis back into the cabinet on the 56FW53H only, take care not to damage IC705
on the bottom of the PWB. This device can catch one of the strengthening ribs on the bottom of the
cabinet if the chassis sags when being pushed back into place.
For general fault finding, the flow chart below is invaluable for tracking down the most probable fault
area. As with all fault finding flow charts it is not infallible.
It is possible to work on the chassis away from the cabinet and CRT. When power is connected to the
unit, it will start up and respond to the remote control just as if it is connected to the CRT and other
peripheral components. However, it is important to ensure that the chassis is not run in this condition
for too long due to incorrect loading of the horizontal and vertical output stages. This results in
excessive heat being generated in the driving semiconductors, which can lead to their premature
failure.
In the case of some power supply failure this is one of the most convenient ways of fault finding, and
to ensure that the chassis can be worked on for a reasonable time, disconnect the horizontal feed
resistor, R623.
If the speakers are disconnected, the protection circuit may operate intermittently. This will cause
the set to shut down. It is therefore important to leave the speakers connected. This is not a
problem as the leads are fairly long and allow the chassis to be moved without impediment.
the eight or ten fixing screws. This only takes about a minute and can save many more when searching
for a component located underneath one of the bracing bars! It is not acceptable to remove any of
this support frame as this may result in the chassis cracking.
Note that the 56cm version of the chassis does not have a support frame because the base of the
cabinet and rear cover supports it. On the 56FW53H the front of the chassis locates into a slot at
the front of the cabinet. If this not correctly aligned then the PWB will crack when the rear is cover
is put on.
When pushing the chassis back into the cabinet on the 56FW53H only, take care not to damage IC705
on the bottom of the PWB. This device can catch one of the strengthening ribs on the bottom of the
cabinet if the chassis sags when being pushed back into place.
For general fault finding, the flow chart below is invaluable for tracking down the most probable fault
area. As with all fault finding flow charts it is not infallible.
It is possible to work on the chassis away from the cabinet and CRT. When power is connected to the
unit, it will start up and respond to the remote control just as if it is connected to the CRT and other
peripheral components. However, it is important to ensure that the chassis is not run in this condition
for too long due to incorrect loading of the horizontal and vertical output stages. This results in
excessive heat being generated in the driving semiconductors, which can lead to their premature
failure.
In the case of some power supply failure this is one of the most convenient ways of fault finding, and
to ensure that the chassis can be worked on for a reasonable time, disconnect the horizontal feed
resistor, R623.
If the speakers are disconnected, the protection circuit may operate intermittently. This will cause
the set to shut down. It is therefore important to leave the speakers connected. This is not a
problem as the leads are fairly long and allow the chassis to be moved without impediment.
Click on the first or last page to see other 76GF-63 (serv.man2) service manuals if exist.