Vulcan-Hart 3-130, Service Manual

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5 – 2
SECTION 5 - TROUBLESHOOTING - 3-STAGE
5.3.7 FAN DRIVE (OPTIONAL ACCESSORY)
The 12V DC fan is controlled by a FET device which is
activated by the microprcessor. At its highest setting
(9), the fan receives the full 12V DC. At lower settings
the fan re12V pulses, to reduce the rpm's.
5.4 TROUBLESHOOTING COMPONENTS
5.4.1 DIODE
A diode (except a zener) is defective if there is greater
than 1 Vdc (typically 0.7 Vdc) forward voltage across it.
5.4.2 OPERATIONAL AMPLIFIER
Generally the “+” and “-” inputs of an operational
amplifier will have less than 15 mV voltage difference
when operating under normal conditions. (U1). When
the output of the amplifier is connected to the “-” input
(voltage follower connection), the output should be the
same voltage as the “+” input voltage; otherwise, the
amplifier is defective.
If the output voltage stays at maximum positive
(typically 1/3 of the supply voltage), the “+” input
voltage should be more postitive than the “-” input
voltage.
5.4.3 TRIAC
The gate to power line return voltage (K1) under load
measures typically 1-2 Vac, while the MT2 to return
voltage measures between 1.3-1.8 Vac.
A triac without connections 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.
5.4.4 CAPACITOR
Shorted capacitors have 0V across their terminals.
Open capacitors can be located by using a good
capacitor connected in parallel with the capacitor under
test and observing the resulting effect. Leaking
capacitors will often have a decreased voltage across
their terminals.
5.4.5 LOGIC LEVELS
Microprocessor: High +3.5 - +5.0V
Low 0.0 - +1.0V
74LSXXX: High +2.0 - +5.0V
Low 0.0 - +0.5V
4XXX (CMOS): High +3.5 - +5.0V
Low 0.0 - +1.5V