Specifications

SECTION 5 MACHINE TESTING / TROUBLESHOOTING / SERVICE

5.8.1 PCB1 Control Board and Bias Supplies

BIAS POWER SUPPLIES

The PCB1Control Board creates 4 bias voltages for circuit function. These voltages are +/- 12 volts DC, +15 Volts

DC and +5 volts DC. This provides operating voltages to the Pulse Width Modulator (PWM), the control circuit, the

torch switch circuit, the over current protection circuit and the arc on circuit.

Positive 12 and 15 volts are created on the board by VR1 and VR2 respectively. This circuit receives 20 Volts AC from

the PCB module control transformer on the connector plug P1 pins 3 and 4 (P1-3, P1-4). The voltage is applied to

BR2, a full wave bridge rectier and converted to pulsating DC. The pulsating DC is then ltered by lter capaci-

tor C32, a 1000 Microfarad capacitor, and fed to a 3-pin voltage regulator, VR1, an LM 7815 +15 volt regulator. The

positive DC is input on pin one of the regulator and outputted on pin 3. Pin 2 of the regulator is tied to ground.

The 15-volt output is then connected to C31, a 47 Microfarad capacitor for ripple and noise suppression, and then

distributed throughout the board.

Filtered DC from the rectier circuit is also fed to VR2, an LM7812 3 pin voltage regulator. The capacitor C63, a 1000-

microfarad capacitor, is installed for additional ltering before applying voltage to pin 1 of the regulator. The Posi-

tive 12 volt output is derived from pin 3 of the regulator, and then fed across C30, a 47 Microfarad capacitor for ripple

and noise suppression, and then distributed throughout the board.

Capacitor C33 is installed ahead of the rectier bridge for noise suppression

Negative 12-volt power is created by VR3, An LM7912 3 pin voltage regulator. 12 Volt power from the mains module

control transformer is supplied to the rectier bridge via connector plug P1 pins 5 and 6 (P1-5, P1-6). The voltage is

applied to BR3, a full wave bridge rectier and converted to pulsating DC. The pulsating DC is then ltered by lter

capacitor C35, a 1000-microfarad capacitor. The ltered DC is then applied to pin 2 of the LM7912, a negative 12 volt

3 pin voltage regulator chip. Pin one is connected to the negative output of the Rectier Bridge and negative 12 volts

is distributed throughout the board via pin 3 of the LM7912. A 47 microfarad capacitor is applied across the output

of the LM7912 for ripple and

noise reduction.

The 5 volts for the board is

derived from the reference

voltage pin, pin 16, of the

UC3525an chip, IC6.

Testing:

With the unit energized and

the protective cover of the

PCB removed, perform the

following voltage checks:

POSITIVE SUPPLY TESTING

If the +12 and +15 volt rails

are missing, check for AC

voltage at the bridge recti-

er BR2 – 20 VAC. If there is

AC voltage is present, check

for DC voltage from the output of rectier BR2 – approximately 25VDC.

If there is AC voltage at the input of the bridge, but no DC output, replace the PCB module.

CONTENTS