Theory of Operation for:
Electrohome G-05 Monochrome "Quadrascan" X-Y Monitor
CAUTION!!! LETHAL VOLTAGES ARE PRESENT IN THIS MONITOR, SUITABLE
PRECAUTIONS SHOULD BE TAKEN BEFORE ATTEMPTING TO SERVICE YOUR MONITOR.
IF YOU HAVE ANY DOUBTS LET SOMEONE ELSE DO IT (an enemy?)
PINOUTS
1 Z input .5V blanking 1.0V blacklevel 4.0V full on 220 ohm impedance
2 Y input +/-7.5V 1K impedance
3 X input +/-10V 1K impedance
4 Power GND (center tap)
5 Y signal GND
6 X signal GND
7 30 VAC RMS
8 Z signal GND
9 6.3V AC heater input
10 30 VAC RMS
11 Power GND (center tap)
12 Heater GND
HIGH VOLTAGE MODULE
The high voltage unit produces the following outputs:
12KV at 200ua
400 volts G2 voltage
0-400V focus
90V for Z amplifier
There are two main sections:
REGULATOR: This is a series regulator with Q900 being the control element.
Since any change in high voltage will be reflected back to the 90 volt line,
these variations are sensed, amplified and used to control the series pass
transistor whichthen supplies more or less voltage as may be needed to
maintain a relatively stable CRT anode voltage. The primary function of
R900 is to limit the high voltage generated under a regulator failure
condition. It also serves to limit disipation in Q900. The high voltage
supply is isolated from the main ripple component of the primary filters
by D100 and C900.
HIGH VOLTAGE GENERATOR: The high voltage generator is a free running
hartley oscillator that operates at approximately 30KHz (much nicer than
the annoying 15KHz of most monitors). The oscillator transistor is Q903.
The voltage made available from the pass transistor is stepped up by means
of T900 and rectified to produce a 12KV anode voltage. One other tapped
winding produces operating voltages for G2 and focus electrodes of the CRT,
and Z amp. All of the secondary diodes are fast recovery to operate at
30KHz, using normal diodes are a definite way to fry your flyback!
DEFLECTION PCB
Z AMPLIFIER: Transistor Q504 forms a common emitter amplifier. A TTL
compatable brightness signal is applied by means of P100-5. An amplified
and inverted replica is present at the collector and this is applied to
the CRT cathode. AC gain (contrast) is controlled by R514 and fixed
resistor R513. Transistor Q503 is normally biased on very hard by means
fo R511,R512 and may be treated as a low valus resistor that plays no
significant part in active amplification of the signal.
Brightness is controlled by varying the DC potential at G1 of the
CRT, by means of R517. Diode D506 and C504 isolate and hold the cathode
voltage high during power down to prevent phosphor burn. At the same as
the 90 volt line is decaying, the bias for Q503 is lowered, turning the
transistor off and further retarding discharge of C504.
SPOT KILLER: The deflection signal is sampled for rate of change and
amplitude on both channels, by means of R500,R501,C500,C501 and then
rectified to form a negative holding voltage on C502,C503. This negative
voltage holds Q500 and Q501 off. There is no current flow through Q502
and LED D504 is not lit! When the sampled signal falls below minimum
requirement then the positive voltage applied by R506,R507 turns on Q500,
Q501. This causes Q502 to conduct, allowing the LED to ligh up and apply
sufficient positive voltage to the emmitter of Q503 to cut the transistor
off, thereby blanking the display.
POWER SUPPLY: A 60V CT voltage is applied by means of P100-7,8,9,10.
These lines are fused by F100,F101 and applied to bridge rectifier DB100.
The resulting DC is roughly filtered by means of C100 and C101 and then
applied to the circuitry. R100 and R101 serve to limit inrush current to
the filters and offer some protection to DB100 in case of a fault
condidtion. Typical operating voltages are +/-34V.
the DC voltage to the high voltage supply is taken off before the
current limiting resistors and is seperately fused by F102. The EHT supply
voltage is isolated from the main filter ripple component by D100. With
the EHT supply functional, a normal operating voltage at P500-10 would
be +40V.
DEFLECTION AMPLIFIER: Both X and Y channels are identical. Because of
this only the Y channel will be described. There are minor resistor value
changes in R704,R604 to accomodate the 19" and 15" CRTs of the G05-802 and
G05-805 models. The amplifier is a direct coupled voltage to current
converter. The small signal response of the amplifier is approximately
1 MHZ. Large signal slew rate is approx. 150 usec.
The input signal is pre-distorted by the components lacated within
the input bridge comprised of D600,D601,D602,D603, to compensate for
inherent CRT non linearity in deflection. Contros R600 and R602 are used
to set size an linearity. The gain corrected signal is applied to the base
of the input defferential amplifier whose constant current source is formed
by Q604,Q606,D607 and R611. The reference diodes are also shared by a
second constant current source Q607>
The amplified signal is applied to a cascade stage formed by Q605,
Q606 and then applied to the basses of output transistors Q608,Q609.
These transistors are operated class B in an emitter follower configuration.
Current is coupled through F600 to the yoke and then to ground through
the sense resistor R620. Very heavy feedback is applied from R620 to the
base of Q603, to correct for any non-linearities in the amplifier. A
considerable amount of power supply ripple can be tolerated because of
the push-pull arrangement of the output transistors and the canceling
effect fo such a stage on any common ripple component. R621 serves as
a critical yoke damping resistor.