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46 Scan Power Supply Most ground connections on the MM101 series chassis are cold, ( ), indicating they are isolated from the AC line. However, there are many "Hot" connections, ( ), meaning direct connection to the AC line. The AC input and primary side of the main and standby supply circuitry are examples. Always use an isolation transformer and consult service data when performing service on this chassis and other chassis in this family! Scan Supply Overview In order to control raster width at different scan frequencies in the MM101, a separate power supply system is used to power the horizontal scan system. The system may be divided into several blocks (Refer to Figure 4-1). The first is the ZVS Scan Supply, used to supply B+ for horizontal scan frequencies 2H and above. It is turned off when the scan supply is set to 1H. Next is the Series Pass Scan Supply generating B+ used by horizontal scan for 1H frequencies. Both supplies are modulated to provide E-W pincushion, E-W corner correction and E-W trapezoidal correction. All three correction waveforms are generated by the scan control IC and adjusted via the IIC bus. The final part of the scan supplies is the control signal used to turn the ZVS (2H) supply on and off. The ZVS scan supply regulates the scan width assuming the high voltage is held constant. This is because scan width is directly proportional to yoke current if secondary effects such as "S Shaping", pincushion, trapezoid, etc. are ignored. Also if the retrace time of the flyback pulse is held constant, the peak voltage amplitude of the flyback pulse is proportional to the peak-to-peak value of yoke current. Since at least two distinct scan modes must be accommodated, 1H and >2H, two Scan B+ supplies are required. They are switched in and out of the horizontal output transformer circuit to gain more or less current through the horizontal yoke.
+76V Filtered
SERVICING PRECAUTION!
Variable IsoTap
Monitor ADD
AC Voltage Out with 120VAC In
1H VCC FROM DEFLECTION SIP
1H: High 2.xH: Low
2H SCAN OFF
1H: High 2.xH: Low
2.xH ZVS SCAN SUPPLY
1H SERIES PASS SCAN SUPPLY
+160V
+67V CR14809
SCAN B+
Figure 4-1, Scan Power Supply Block Diagram
Scan Power Supply 47
+15Vr
R14802 62K
C14800 1UF 100V R14801 68 +76VrFIL L14801 180UH 10
T14800 SCAN_B+
11 EY14805 14
+76Vr C14813 100UF 100V
1
EY14800
Q14800
EY14802 7 8
EY14801
R14809 13K 2W 2%
R14803 470 +12Vr +12Vr [R14805] 100 [R14750] 470 [Q14733] R14835 10K [R14749] 1500 R14765 1000 FROM REGULATION CIRCUITS [Q14805] Q14801 R14804 15
C14802 8.2NF 600V
To Scan Loss & Regulation
CR14804
R14815 5.1K 1W 2%
[R14751] 10K 1H Vcc From Deflection
SCAN ZVS OFF: HIGH SCAN ZVS ON: LOW
[R14827] 4700
Q14802
R14807 100
R14808 0.22 3W
Figure 4-2, ZVS Scan Supply ZVS Scan Power Operation The scan power supply works on the same principles as the main ZVS supply. A power MOSFET, Q14800 controls the transformer primary winding current, which in turn induces current into the secondary windings. That current is rectified and becomes the Scan B+ for 2.xH. When the output, Q14800 is turned on by a positive signal on the gate, it begins to conduct from ground through the primary of T14800 (pins 8-14) to a +76V supply filtered from the +76V run supply. Meanwhile, C14800 is charging to +15V through a path from the +15Vr supply, R14802, R14801 and T14800 pins 10-11 to ground, developing a bias supply for Q14800-G. When the junction of C14800 & R14803 reaches a low enough voltage to turn Q14800 off, the stored magnetic field in T14800 begins to collapse (flyback portion) turning on CR14804 to provide Scan B+. C14802 is also charged with energy from T14800. Normal Scan B+ operating voltage is between 120 and 165 volts. When the latch circuit calls for current, Q14800 again turns on, beginning the cycle again. The gate voltage winding is such that heavy current slows the charging of C14800, allowing Q14800 to conduct longer, generating more secondary current. If the load is light, less current is drawn by Q14800 and C14800 charges quickly, shutting off primary current faster, keeping secondary current low.
48 Scan Power Supply
+15Vr
R14802 62K
C14800 1UF 100V R14801 68 +76VrFIL L14801 180UH 10
T14800 SCAN_B+
11 EY14805 14
+76Vr C14813 100UF 100V
1
EY14800
Q14800
EY14802 7 8
EY14801
R14809 13K 2W 2%
R14803 470 +12Vr +12Vr [R14805] 100 [R14750] 470 [Q14733] R14835 10K [R14749] 1500 R14765 1000 FROM REGULATION CIRCUITS [Q14805] Q14801 R14804 15
C14802 8.2NF 600V
To Scan Loss & Regulation
CR14804
R14815 5.1K 1W 2%
[R14751] 10K 1H Vcc From Deflection
SCAN ZVS OFF: HIGH SCAN ZVS ON: LOW
[R14827] 4700
Q14802
R14807 100
R14808 0.22 3W
Figure 4-2, ZVS Scan Supply (Repeated)
If the chassis is placed in 1H mode, the presence of the 1H Vcc signal turns on Q14733, turning on Q14805. Q14805 then removes voltage from the gate of Q14800, turning it off and shutting off current flow in the output transformer primary. This turns off the ZVS supply. Note that Q14801 and Q14802 form a latch circuit which is used to control the gate drive of Q14800. If the latch is "set" or active, Q14801-E voltage is low removing gate drive from the output. In other words, the supply runs unless the latch removes gate bias. If 1H VCC were not present, indicating the set is in 2.xH mode, Q14733 and Q14805 would be off. Now the +15Vr supply would be available for output device gate drive. Once the supply is up and running, the bias supply developed on C14800 would supply gate drive to the output device, Q14800. Q14802 and Q14801 form a familiar latch circuit used to control the on/off time of Q14800. When Q14802-B is low, the latch is off. Q14801 is off allowing gate drive to Q14800. As output device, Q14800, current increases, the IR drop across R14808 also increases. When it develops enough voltage to turn on Q14802, Q14801 also turns on forming a current path from ground, Q14802-E/C, Q14801-B/E and the bias supply. Q14801-E is near ground and Q14800-G drive is removed. When output current has decreased far enough, Q14802 turns off, shutting off Q14801. The bias supply again provides drive to Q14800-G.
Scan Power Supply 49 OverVoltage Protection The Scan ZVS Supply is protected against overvoltage conditions by monitoring Scan B+ directly and also by indirectly monitoring yoke current. This is done by watching the flyback pulse (Scan H_A) from the horizontal output transformer supplying horizontal yoke current. A resistor divider network consisting of R14809 and R14815 establishes a voltage at their junction of about +45V. This is coupled to Q14811-B by a 51 volt Zener diode, CR14803. If Scan B+ rises such that this junction increases past the combined voltage drop of the zener and the emitter-base junction of Q14811, the transistor turns on. The output of Q14811 then drops low, placing a low on the latch. This shuts off gate drive to Q14800, shutting down output current and the supply. At the same time, Q14813 turns on, sending a SCAN LOSS detect signal to tell the microprocessor there has been an overvoltage or unlock condition in the scan. The microprocessor will immediately (<1 msec) blank the screen, disable sync, and recalculate the new scan mode of the chassis, if possible. It will then re-apply sync and wait for up to 100 msec for the interrupt condition (overload or unlock) to be corrected after which the microprocessor will re-enable the SCAN_LOSS and unblank the screen. This is not a "Batten" routine.
10
7 11 14
EY14801
FB14802
CR14804 SCAN B+ EY14800 8 T14800 C14801 2u 250V 1 FB14801
R14809 13K 3.0W 2% +12VR
To Q14801-B
BC14801
+15VR
+15VR
R14815 5.1K 1.0W 2%
CR14803 51V R14845 100
SCAN LOSS
R14821 680 Q14804
[R14811] 4K [Q14811] [R14842] 680 2% JW14805
[R14836] 1K
[Q14813]
CR14805 36V R14820 330 SCAN H A
[R14814] 2K
To Q14802-B
+15Vr [R14819] 680 2% Q14803 R14824 1K [R14817] 680
R14810 510 [R14825] 4.7K [R14822] 7.5K
CR14807
C14807 0.1uF
R14816 13K
R14812 6.8K
CR14802 5.1V
TO Q14808-B [CR14812] [CR14811] [CR14810]
Figure 4-3, Scan ZVS Overvoltage Protection
50 Scan Power Supply Regulation and Overvoltage Further overvoltage protection and regulation is provided by monitoring the flyback pulse (SCAN H_A) of the horizontal output transformer. The pulse is rectified by CR14807. If the rectified voltage increases enough to bias 36V zener diode CR14805 on, Q14813 and Q14811 also turn on. This has the same effect as the overvoltage shutdown circuit previously described. However, to regulate Scan B+, the same voltage supply is used. If the SCAN H_A rectified voltage at CR14807 is high enough to bias Q14803 on yet not high enough to cause overvoltage shutdown via Q14811, Q14803 turns on. This turns on Q14804, turning on Q14802-B, setting the latch controlling Q14800 gate drive. When Q14802 is on, gate drive is removed and output current stops. Scan B+ now begins to drop. As Scan B+ drops, horizontal yoke current decreases and the rectified SCAN H_A voltage at the cathode of CR14807 also decreases. When it decreases enough, there is not enough bias to hold Q14803 on. It then shuts off, shutting off Q14804 and the latch returns to its normal "off" state, allowing gate drive to return to the output device.
10
7 11 14
EY14801
FB14802
CR14804 SCAN B+ EY14800 8 T14800 C14801 2u 250V 1 FB14801
R14809 13K 3.0W 2% +12VR
To Q14801-B
BC14801
+15VR
+15VR
R14815 5.1K 1.0W 2%
CR14803 51V R14845 100
SCAN LOSS
R14821 680 Q14804
[R14811] 4K [Q14811] [R14842] 680 2% JW14805
[R14836] 1K
[Q14813]
CR14805 36V R14820 330 SCAN H A
[R14814] 2K
To Q14802-B
+15Vr [R14819] 680 2% Q14803 R14824 1K [R14817] 680
R14810 510 [R14825] 4.7K [R14822] 7.5K
CR14807
C14807 0.1uF
R14816 13K
R14812 6.8K
CR14802 5.1V
TO Q14808-B [CR14812] [CR14811] [CR14810]
Figure 4-3, Scan ZVS Overvoltage Protection (Repeated)
Scan Power Supply 51
If Q14800 is replaced, it is a good idea to also replace Q14801 & Q14802 at the same time. They carry much of the gate current of Q14800. The normal failure mode of Q14800 is a gatedrain or drainsource short. This may cause excessive current to flow in Q14801 & Q14802. This will overstress them. All other components in the area should be checked.
TECH TIP
52 Scan Power Supply Scan Supply Series Pass When the MM101 is operating in 1H mode, a lower voltage scan supply output is required. This supply is provided by the circuit in Figure 4-4. It is more a regulator than a supply and is active at all times. It is kept from supplying scan B+ during those times when the chassis is operating in >2H mode by CR14809. The output voltage is 6070 volts during normal operation. If the 2H ZVS supply is on, the cathode of CR14809 rises to over 120 volts, reverse biasing it and isolating the series pass supply. The regulator reference voltage is provided by CR14808, a 5.1V zener. It is biased by R14830 and as long as the +76VrFIL supply is operating the reference voltage is coupled to Q14806-B by R14828. Q14806 is biased on providing a regulating voltage on Q14807-B of about +74 volts through a current path from ground through R14832, Q14806-E/C, R14829, Q14807-B/E and R14837 to the +76VrFIL supply. Th series pass scan supply is modulated to provide fine-tuning of the Scan B+ supply for E/W pincushion correction, trapezoidal correction and E/W corner correction. Q14808 modulates the Scan B+ supply using a width reference waveform from the deflection DAC, U24800.
+76VrFIL R14837 0.3 2W R14841 200 R14834 22K 1/2W
Q14807
CR14809 SCAN B+ (1H) C14805 39uF 100V
Q14809 R14830 15K 1W R14828 1000 JW14452 CR14808 5.1V 2%
R14829 22K 1/2W Q14806
R14839 10K 1W
R14831 33K 1/2W
Q14808 R14832 1500
[R14840] 3300 2%
[R14838] 20K
WIDTH REFERENCE (FROM DEFLECTION DAC U24800)
Figure 4-4, Scan Supply, Series Regulator
Scan Power Supply 53 Overcurrent Protection Overcurrent protection of the output device is provided by Q14809. As the voltage across R14837 increases, indicating excess current through the output, Q14809 will be biased on when the voltage difference between its emitter and base is greater than 0.7 volts. When Q14809 is on, the forward bias current of output device Q14807 is removed, shutting it off. During normal operation Q14809-B is around +75.8 volts, or about 0.2 volts less than the power supply. This is due to bias conditions set up by the resistor network, R14837, R14841 and R14834. If excess current is drawn, Scan B+ begins to drop and the junction of R14841 and R14834 also decreases (moving towards the output voltage). The +76VrFIL supply will continue to remain close to its specified voltage. This creates a voltage drop between the emitter and base of Q14809, eventually turning it on. The E-C voltage drop of Q14809 during conduction is very low, placing a voltage on Q14807-B less than 0.2 volts of Q14807-E. This shuts Q14807 off causing Scan B+ to shut off. Once on, the resistor divider network of R14837, R14841, R14837, R14834 and R14840 between the +76VrFIL supply and ground assure that Q14809 stays latched on. The +76VrFIL supply must be removed to reset the circuit. This is done by turning the chassis off, then on again.
+76VrFIL R14837 0.3 2W R14841 200 R14834 22K 1/2W
Q14807
CR14809 SCAN B+ (1H) C14805 39uF 100V
Q14809 R14830 15K 1W R14828 1000 JW14452 CR14808 5.1V 2%
R14829 22K 1/2W Q14806
R14839 10K 1W
R14831 33K 1/2W
Q14808 R14832 1500
[R14840] 3300 2%
[R14838] 20K
WIDTH REFERENCE (FROM DEFLECTION DAC U24800)
Figure 4-5, Scan Supply, Series Regulator Overcurrent Protection