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106 Hi-Voltage Power Supply Overview In previous discussions, it was indicated high voltage must be maintained within more strict tolerances in the MM101 chassis. Independant High Voltage generation is utilized to assure high performance over the broad range of scan rates and beam currents for standard television, computer monitor modes and elevated-drive (video) VGA modes. The high voltage circuit is divided into several sections. The driver generates signal drive for the high voltage output device in the HV Generator. Protection for the generator and IHVT is provided by monitoring secondary current and removing gate drive to the output devices when necessary. The HV Generator supplies the CRT anode voltage and several supplies for CRT operation. The HVR B+ is the main supply for the high voltage generator. The HVR B+ circuit uses the +76Vr and +24Vr supplies to generate the higher voltages required for the HV generator. Varying Reg B+ regulates the output of the HV generator. By sampling the high voltage supply, then varying Reg B+, the high voltage CRT anode supply may also be regulated. Reg B+ also generates a +210Vr supply for CRT grid bias. X-Ray Protection is provided by sampling the IHVT output pulse and, in the case of exessive high voltage, removing gate drive to the output devices. Beam limiting (ABL) is accomplished by sampling the IHVT current and reducing video drive during high CRT beam current periods. Hi Voltage is not affected by AKB (Automatic Kine Bias) or SVM (Scan Velocity Modulation). AKB works to set CRT cutoff bias to compensate for CRT grid cutoff voltage drift.
Run Supply
+210Vr HV Sample
+24Vr
+76Vr
HV B+
Scan H_A H Pulse Gate Drive
Deflection
XRP
SYNC HV B+ Dynamic Focus
Hi Voltage Divider
HV Generator (IHVT)
Beam Limiter
1/3 Tap
Focus / Screen
Filament
Focus 1 Focus 2 Screen
+210Vr
CRT
Figure 8-1, Hi Voltage Block Diagram
Hi-Voltage Power Supply 107 HV Power Supply (Reg B+) The high voltage power supply generates two voltages. One, +210Vr, supplies the CRT drive voltage. CRT operation will be covered later. The second is the regulated B+ for the high voltage generator. Both are critical for proper CRT operation. The high voltage power supply is a traditional scan derived supply with a few variances. The +210Vr supply is generated from the secondary of the high voltage supply transformer, T14751. The AC waveform is rectified by CR14701 and filtered by C14706. CR14701 conducts when Q14751 is on, causing current flow in the primary winding of T14751, and subsequent current flow in the secondary. Regulated B+ for the high voltage generator is produced from the primary winding of the same transformer. When Q14751 is on, current is flowing from ground through the primary, L14760, to the +76Vr supply. At the same time C14751 is charging towards the +76Vr supply. When Q14751 shuts off, the primary of T14751 reverses polarity and begins providing current flow for the HVR B+ circuits. HVR B+ varies according to the scan rate and CRT beam current. The table below shows approximate HVR B+ voltages at low beam current.
HV Reg B+ Voltage ~100 ~100 ~140
Scan 1H 2H
+76Vr
2.4H
L14760 180UH
Figure 8-2, HVR B+
14
C14751 470uF 80V
+
EY14712
12
1
EY14713 EY14714 CR14701 +210Vr + C14706 47uF
EY14711
9 6
T14751
JW14759
JW14760 CR14751 C14765 560 2KV
JW14772 HV REG B+ +24Vr
Drive Signal from U14751-6 Overcurrent Feedback to U14751-3 Q14751
R14757 0.1 2W
R14698 1000
R14697 1000
R14752 47K 1W
Q14750 Overvoltage Feedback to U14751-2
3 1 U14750 2 R14758 845
Figure 8-3, High Voltage Power Supply Circuit
108 Hi-Voltage Power Supply Over Voltage Protection Over voltage protection is provided by regulator U14750. This regulator is a low voltage three terminal adjustable precision voltage reference device. When a reference voltage is reached on pin 1, the device conducts, providing a regulated +2.5V between the cathode and anode (pin 2 to pin3). In this case, the cathode, pin 2, is connected to ground. When U14750 is regulating, pin 3 will be +2.5V referenced to ground. HVR B+ is directly monitored by resistor network R14752 & R14758. If HVR B+ rises, the voltage on pin 1 of U14750 also increases. When the voltage increases and pin 1 of U14750 reaches +2.5V, it conducts, supplying a regulated +2.5V to the base of Q14750, turning the device on. Q14750 saturates and collector voltage reaches about +3.0 volts. This means the bias voltage on U14751-2 is increased. U14752 gain is large with normal input voltage riding around +2.5 volts. When Q14750 raises the input on U14751-2 to greater than +3 volts, the amplifier goes out of range and the output gate drive to Q14751 stops. As current through the output transformer stops, HVR B+ begins to rapidly decrease. When HVR B+ drops far enough, U14750 turns off, U14752-2 input returns to normal and proper circuit operation returns. There is no provision to report the shutdown to system control unless the overvoltage persists. At some point as HVR B+ rises, high voltage also rises. If high voltage increases enough, XRP will trip causing shutdown of high voltage. Scan remains active. Although XRP is also not reported directly to system control, it is indirectly reported when deflection is shut down. More information is included with the XRP circuit description.
TECH TIP
HV REG B+
+24Vr
R14752 47K 1W
R14699 18K
R14697 1000
R14698 1000 Q14750
3 Norm: +2.5V R14758 845 1 2 U14750 Overvoltage Feedback to U14751-2 Normal: <2.5V Over Voltage: >2.5V
Figure 8-4, High Voltage Over Voltage Circuit
Hi-Voltage Power Supply 109
R14748 4700 C14759 0.015UF 4
OSC IN
+24Vr R14755 360 1W
SYNC FROM IHVT 14777 2870
5
HVR Drive U14751
FROM HVPS OUTPUT Q14751-S 3
CURRENT SENSE OUT
6 Drive To Q14751-G
2 R14780 3000 1%
HV SAMPLE
Vcc +18V
7
R14776 681 1%
1 [R14760] 47K FROM HVR B+
ERROR AMP OUTPUT
+5V REF
8
+ C14754 0.1UF
Figure 8-5, HVR B+ Waveform Generator HVR B+ Over Current Protection The HVR B+ supply output is also protected against exessive current. The bulk of current through the output transformer is provided by Q14751. All current through Q14751 must also pass through R14757 (see Figure 8-3). As current through R14757 increases, voltage across it also increases. This voltage is monitored directly by pin 3 of the HVR B+ drive generator, U14751. As monitored voltage increases, the output waveform duty cycle is decreased, reducing on-time and subsequently current flow in Q14751 and the output transformer T14751. As current flow returns to normal, the output duty cycle is also allowed to return to normal operating parameters. It should be noted that anytime HVR B+ circuit protection reduces the output of Q14751, the +210Vr supply will also be reduced.
+76Vr L14760 180UH
14
TECH TIP
C14751 470uF 80V
+
EY14712
12
1
EY14713 EY14714 CR14701 +210Vr + C14706 47uF
EY14711
9 6
T14751
JW14759
JW14760 CR14751 C14765 560 2KV
JW14772 HV REG B+ +24Vr
Drive Signal from U14751-6 Overcurrent Feedback to U14751-3 Q14751
R14757 0.1 2W
R14698 1000
R14697 1000
R14752 47K 1W
Q14750 Overvoltage Feedback to U14751-2
3 1 U14750 2 R14758 845
Figure 8-3, HVR B+ Circuit (Repeated)
110 Hi-Voltage Power Supply Reg B+ Drive Output Waveform Generator The HVR B+ circuitry contains its own drive generator. The generator, U14751, supplies a drive waveform for the HVR B+ output device, Q14751. The output waveform is locked to an incoming sync signal from the IHVT, but is not dependant upon the sync signal to generate an output waveform. The generator runs at 2.xH. If scan is between 2H and 2.4H, it syncs directly. At 1H the generator runs at 2H. U14751 has an RC network consisting of R14748 & C14759 connected to pin 4 to set a free running mode. The free run frequency may be locked to incoming horizontal sync from the IHVT pulse coupled to pin 4. U14751 power is supplied by the +24Vr supply. The output waveform duty cycle is set by pins 1 & 2. When the output voltage is correct, both pins will be at about +2.5 volts. A sample of high voltage from U14775 is on pin 2. As pin 2 input increases, indicating an increase in high voltage, the output duty cycle decreases, reducing "ontime" of the output device, Q14751. When the output of Q14751 decreases, HVR B+ and high voltage are also reduced. If high voltage lags, (for example, during heavy beam current), the voltage at pin 2 drops. The output duty cycle now increases, increasing on-time of the output, Q14751. As the duty cycle increases, more current is supplied to the HVR B+ transformer, raising HVR B+. This increases high voltage. There is a reference voltage coupled from Q14751 current limiter, R14757 to pin 3. If pin 3 is over voltage (too much output device current), the duty cycle of the output is reduced, lowering output current. When the output device current falls back into an acceptable range, output returns to normal control. An undervoltage lockout is also used to prevent driving the output device too hard during turn on.
R14748 4700 C14759 0.015UF 4
OSC IN
+24Vr R14755 360 1W
SYNC FROM IHVT R14777 2870
5
HVR Drive U14751
FROM HVPS OUTPUT Q14751-S 3
CURRENT SENSE OUT
6 Drive To Q14751-G
2 R14780 3000 1%
HV SAMPLE
Vcc +18V
7
R14776 681 1%
1 [R14760] 47K FROM HVR B+
ERROR AMP OUTPUT
+5V REF
8
+ C14754 0.1UF
Figure 8-5, HVR B+ Waveform Generator (Repeated)
Hi-Voltage Power Supply 111 High Voltage Generator Once the high voltage power supply is operational, the high voltage generator may begin to generate high voltage for the anode supply of the CRT. High voltage generation is accomplished using a pair of MOSFET switches, a resonant capacitor and an IHVT (Integrated High Voltage Transformer). Remember, scan current is not provided by this circuit! The high voltage section also has its own frequency generator, independant of scan and the HVR B+ supply. Low level signal generation is done by U14711. This IC was designed to be a phase detector and oscillator in a low-level horizontal frequency section. The PLL IC, U14711 has a free run frequency set by the oscillator input pin 7 and the RC network, R14714 & C14712. This pin may also be "trimmed" to provide different output frequencies. Initial free run frequency is set by adjusting R14710. Bias from the Horizontal DAC, U24800, shifts the free run frequency of the 2H PLL, U14711, for optimum locking between 2H and 2.4H modes. Pin 8 sets the duty cycle of the output, normally about 50%. R14712 & R14713 set a voltage ratio that determines the internal trigger of the pre-drivers in the IC. Pin 1 is the square wave output of the IC. Pin 4 provides a feedback sawtooth waveform from the IHVT that is compared to an incoming horizontal sync pulse on pin 3 from deflection. If the two signals are in phase, there is no change on the output of pin 5. If there is a phase difference, a correction output from pin 5 is coupled to pin 7, shifting the oscillator input to control timing.
+12Vr
R14721 10
1 R14710 25K 3 2
R14711 130K
R14712 2740 1%
R14714 15K 1%
R14716 20K C14712 0.002 [R14773] 200K R14713 2740 1%
8
7
OSC IN
6 Vcc
5
PHASE DETECT OUT IHVT SAMPLE
C14714 1.5UF 100V
RATIO
U14711 2H PLL OUT 1 2
H SYNC IN
FROM FREQ OFFSET DAC
[R14722] 2000
3
4 C14715 .1UF 100V
To High Voltage Drivers
HORZ SCAN A FROM IHVT
Figure 8-6, High Voltage 2H PLL
112 Hi-Voltage Power Supply The output from U14711-1 is direct coupled to driver transistors, Q14703 & Q14704. These drivers supply an 8V p-p signal to the gates of the high voltage generator output devices, Q14700 & Q14701. From here, the generator acts as a traditional high voltage output circuit. Q14700 & Q14701 are parallel devices required to provide the current needed by the CRT. Anode voltage is regulated by varying the HVR B+ supply on pin 1 of the IHVT, T14700. CRT filament current is also provided by T14700. The filament windings (T14700-3&4) supply about 8 volts p-p which is rectified by CR14703 and regulated down to about +6.2 volts DC by U14701. Additional voltages supplied by the IHVT include a 1/3 Tap for focus, a connection to the ABL (Automatic Beam Limiting) circuitry, and a pulse used by the XRP circuitry. The XRP pulse is coupled to the 2H PLL, U14711-4, (as a sawtooth), the HVR B+ waveform generator, U14751-4, for regulator control and the XRP circuit.
+12Vr R14721 10 T14700 HVR B+ [R14745] 750 EY14401
1
CRT ANODE
[R14723] 220 Q14703
1/3 TAP to FOCUS EY14411
10 5 7
ABL XRP Pulse To U14711-4 PLL R14702 100K 1/2W
3
R14709 47 R14724 1k CR14702
Q14700
EY14706
4 8
R14704 1
R14706 47
C14702 .015UF 1600V
CR14703 R14774 1.5 2W C14701 33UF + U14701 7805
R14701 0.18 2W FILAMENT SUPPLY TO CRT
FROM HORIZONTAL PLL, U14711-1
Q14704
R14705 1 Q14701
EY14705
CR14707 CR14708
Figure 8-7, High Voltage Generator
Hi-Voltage Power Supply 113
+24Vr R14748 4700 C14759 0.015UF 4
OSC IN
ANODE
L14775 1MH
+24Vr R14755 360 1W
SYNC FROM IHVT R14777 2870
5
R14767 EY14702 R14779 22K 2 + 3
U14775 7 6 4 R14778
5000 20%
HVR Drive U14751
FROM HVPS OUTPUT Q14751-S 3
CURRENT SENSE OUT
6 Drive To Q14751-G
2 R14780 3000 1%
HV SAMPLE
Vcc +18V
7
EY14709
R14776 681 1%
1 [R14760] 47K FROM HVR B+
ERROR AMP OUTPUT
+5V REF
8
+ C14754 0.1UF
Figure 8-8, High Voltage Regulation High Voltage Regulator The high voltage regulator monitors anode voltage directly, then compensates by varying the drive to the HVR B+ supply. If HVR B+ rises, high voltage also rises. If HVR B+ decreases, high voltage also decreases. R14767 is a 1 gigohm resistor divider network connected between the CRT anode and ground, with a tap for monitoring a portion of high voltage. The HV sample is fed to an op amp, U14775, whose output varies directly with changes in the high voltage. As high voltage increases, U14775 output increases. This voltage is directly coupled to the HVR B+ frequency generator, U14751-2. As voltage on pin 2 increases, the output duty cycle decreases, lowering HVR B+. When HVR B+ decreases, high voltage is also decreased.
U14775 is a normal high impedance op amp. With a single-ended supply, and both inputs balanced, output pin 6 (and pin 2) will normally be about +10 volts. DO NOT MEASURE the high voltage sample at R14767 or anywhere from that point to U14775-3. Due to the extremely high resistance in this leg of the circuit, it could easily be loaded down by a DVM, giving false readings and increasing high voltage!
TECH TIP
114 Hi-Voltage Power Supply Troubleshooting Troubleshooting the high voltage section may be broken down into two areas. First, is high voltage present? Second, if high voltage is present, is it correct? Remember that unlike previous chassis, high voltage may exist without the presence of scan. Confirming the presence of high voltage indirectly is not easy. Listen for the familiar high voltage "snap" shortly after startup. If that sound is present, chances are the high voltage system is working. High voltage is also not dependant upon the scan voltages being correct for high voltage to be correct. If scan and all associated supplies are working, but high voltage is not correct, brightness and focus may be the only visual indications of problems. High voltage supplies only CRT beam current, CRT bias voltages and filament current.
TECH TIP
If the filament is glowing, the high voltage generator is probably working! If no catastrophic indications are apparent, begin by confirming the proper supply voltages. Associated high voltage test points and normal values are shown in figure 8-9.
Test For HVR B+ HV Retrace HVR B+ Switch Drive Regulated Supply HV Generator Gate Drive HV PS Switch HV PLL +210V Supply Description Varies Slightly with Pix Content IHVT Retrace Pulse HVR B+ Drive on Q14751-G +76 Vr Supply Drive for HV Generator PS Transformer Drive PLL output waveform CRT Grid Supply Location Choke Board L24601-7 Choke Board L24601-3 jct R14762 & CC6 R14764 L14760 Q14700-G Q14751-D CR14702-C CR14707-C LL7 EE7 NN6 HH4 Zone Area Voltages
1H - ~100V HV Supply 2H - ~100V 2.4H - ~140V HV Generator HVR Supply HVR Supply HV Gen HVR Supply HV Gen HV Reg B+ 800 Vpp 12-18V PWM +76V DC 10-12V Square Wave 100Vpp PWM 10V Square Wave +210V DC Voltage varies with beam current ~11V with no beam current
Beam Limiter
ABL
C14700+
RR5
HV Gen
Figure 8-9, HV Supply Test Points
Hi-Voltage Power Supply 115
Voltages should be accepted as nominal. PWM means the signal is Pulse Width Modulated. Viewing on an oscilloscope may not result in a nice usable waveform, but rather a systematic waveform recognizable as a true signal and not high level static or interference. However, the waveform voltage should always be in the acceptable range. The locator guide is current as of the printing of this manual and should only be used as a general guideline. Consult the TCE Electronic Service Information for the chassis under test to confirm the exact location.
Any time either HV generator output device, Q14700 or Q14701, fails, replace both devices. Failure of either will cause overstress on the other, possibly leading to premature failure.
TECH TIP
116 Hi-Voltage Power Supply
Convert Pulse to Sawtooth
U14711-4 Horizontal PLL
Sample IHVT Pulse from T14700-7
Rectifier
Q14901 Switch Off: Normal On: XRP
Q14902/03 Latch Off (Low): Normal On (HI): XRP
Q14904 Switch Off (HI): Normal On (Low): XRP
To Low Level Deflection SIP
Q14703/04: Gate Drive To U14751-4 Sync Pulse for HV Reg B+ Driver
Figure 8-10, XRP Block Diagram
XRP ( X-Ray Protection) The IHVT of the MM101 contains a winding that supplies the XRP circuitry the capability to shut down high voltage in the event it reaches exessive levels. The same winding also supplies two other sync pulses for timing of other high voltage circuits. T14700-7 provides the initial pulse for XRP. The same pulse is also used to provide a signal needed by the 2H PLL, U14711-4. The pulse is rectified, providing a nominal 23-25 volt half-wave DC supply for the XRP circuit. Q14901 is the XRP switch. Bias is trimmed by R14905 to place Q14901 in a normally OFF state. If high voltage begins to increase, the DC voltage developed by T14700-7 and CR14901 also increases. If high voltage increases enough to raise the rectified XRP voltage to approximately 27V (10.7 volts across CR14900, R14900 and Q14901 emitter to base), Q14901 latches on indicating an XRP fault. With Q14901 on, Q14903, Q14902 and Q14904 also turn on. In an XRP detect fault condition, Q14901-C will be about +1.4 volts due the combined emitter-base voltage drop of Q14902 and Q14904. With Q14902 on, Q14903 also turns on pulling down the High Voltage predriver signal and shutting off IHVT current. When Q14903 turns on, it provides a low impedance path to ground for Q14703/Q14704 base drive signal from the Horizontal PLL, U14711-1. With insufficient base drive, output drive current is also reduced shutting off the high voltage generators, Q14700/Q14701, gate drive. High voltage is now stopped.
Hi-Voltage Power Supply 117
+12Vr R14721 10
[R14745] 750
[R14723] 220 Q14703 Part of IHVT T14700 R14709 47 To High Voltage Generator Outputs Q14700/14701 R14706 47 EY14407 CR14702 R14724 1K EY14423 8 R14727 1000 R14903 100
FROM HORIZONTAL PLL, U14711-1
7
R14725 5600 1/2W XRP PULSE
To Horizontal PLL, U14711-4 To HVR B+ Drive, U14751-4
Q14704
CR14901
XRP: >27V
CR14900 10V
Rectified XRP R14901 J14901
H_PULSE to U14350-1
H_PULSE From T14451-14
3
R14905
[R14908] 10K Q14903
[R14909] 100
R14900 1500 2%
1
2
R14904 220K
R14910 10K
XRP TEST
CR14902 Q14902 Q14904 [R14097] 120 R14912 1000 [R14906] 22K Q14901
R14902 39.2K 1%
Figure 8-11, XRP At the same time, H Pulse_A, normally a 20V horizontal flyback signal derived from the horizontal output transformer, T14451-14, is clipped by Q14904. The H_Pulse provides feedback from the horizontal scan system to the Deflection Processor, U14350-1. The clipped signal is interpreted by the Deflection Processor as a scan fault condition and it shuts down all scan drive. Additionally, it signals System Control which logs a "Deflection DAC" fault code. When XRP has tripped, it cannot automatically reset even if the XRP condition corrects itself. Once Q14902 and Q14903 are turned on, they will remain on until the +12Vr supply is removed. Two other faults can turn on the XRP latch. In the case of excessive beam current, a large negative voltage C14700 can turn on zener CR14903 leading to an XRP indication. Similarly, a failure of the +12V supply to the CRT board, which would cause excessive current will turn on Q14706 also giving an XRP fault.