Section Troubleshooting A-Control; Factory Repair; Before You Start; Troubleshooting Guides - Vulcan-Hart A-130 Service Manual

SECTION - TROUBLESHOOTING - A-CONTROL

. FACTORY REPAIR

DENTSPLY Ceramco maintains a factory repair
department for those customers not possessing the
necessary personnel or test equipment to maintain the
VULCAN. If a unit is returned to the factory for
calibration or repair, a detailed description of the
specific problem should be attached to minimize
turnaround time. Call factory for PR number before
shipping at 1-800-835-6639.

. BEFORE YOU START

Since no troubleshooting guide can possibly cover all
the potential problems, the aim of this guide is to give a
methodology which, if applied consistently, will lead to
the problem area. Therefore , it is necessary to
familiarize yourself with the VULCAN by reviewing the
functional description in conjunction with the
schematic. Successful troubleshooting depends upon
understanding the circuit operation.
WARNING:
With covers removed, dangerous voltage points may
be exposed. Contact with any of these points could
cause serious injury.

. TROUBLESHOOTING GUIDES

WARNING:
When measuring voltages use battery operated test
equipment unless an isolation transformer is
connected between circuit input and AC line.

. . POWER SUPPLY

Rectifier diodes D2 and D3 convert the AC line voltage to
a DC voltage, while resistors R11, R12 reduce the current
flow thru the 8.6V internal zener diode of U2 and the
6.8V external zener diode D1. Capacitors C1 and C5 filter
the rectified voltages. Most of the circuit's current
consumption is taken up by the triac gate drive.

. . ANALOG CIRCUITRY

The thermocouple voltage which ranges from 0-45mV
(25°C - 1100°C) is multiplied by amplifier U1
approximately 100 times (depending on the setting of
potentiometer RP1) and added to the negative setpoint
voltage from RP2. A sawtooth voltage signal is
generated by U2 from a constant current source
charging capacitor C4. The value of C4 determines the
burst period of the triac output (typically 8 seconds).
The triac gate output pulse current is about 60mA. The
triac is triggered in quadrants II and III. Synchronization
is provided by resistor R10. Its value determines the
trigger pulse length.

. . TRIAC DRIVE

A current pulse from U2 pin 6 of about 60mA will turn
on the gate of the muffle triac which in turn will then
carry the full load current. The voltage across the triac
is now at 1-2 Vac. In order to comply with norms limiting
the frequency at which a kW size load may be connected
to the main line (fluorescent tubes "flickering") a
proportional temperature control is provided by means
of burst firing the triac.
NOTES:
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. TROUBLESHOOTING COMPONENTS

. . DIODE
A diode (except a zener) is defective if there is greater
than 1 Vdc (typically 0.7 Vdc) forward voltage across it.
. . OPERATIONAL AMPLIFIER
Generally, the "+" and "-" inputs of an operational
amplifier will have less than 15 mV voltage difference
when operating under normal conditions. If the output
voltage stays at maximum positive (typically 1/3 of the
supply voltage), the "+" input voltage should be more
positive than the "-" input voltage. If the output voltage
stays at minimum (typically 1-5 mV), the "-" input
voltage should be more positive than the "+" input
voltage.
. . TRIAC
The gate to power line return voltage under load
measures typically 1-2 Vac, while the MT2 to return
voltage measures between 1.3-1.8 Vac. A triac without
connections and no power applied can be checked for a
go-no go condition with an ohmmeter. The gate to MT1
resistance for a power triac (20-40A) should be between
50 and 100 ohms; there should be infinite resistance
between MT1 and MT2.
. . CAPACITOR
Shorted capacitors have 0V across their terminals.
Open capacitors can be located by using a good capaci-
tor connected in parallel with the capacitor under test
and observing the resulting effect.
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