Troubleshooting Components - Vulcan-Hart A-130 Service Manual

SECTION - TROUBLESHOOTING - -STAGE
. . 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 receives +12V pulses, to reduce the rpm's.

. 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. (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.
. . 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.
. . 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.
. . LOGIC LEVELS
Microprocessor:
74LSXXX:
4XXX (CMOS):
5 – 2
High +3.5 - +5.0V
Low 0.0 - +1.0V
High +2.0 - +5.0V
Low 0.0 - +0.5V
High +3.5 - +5.0V
Low 0.0 - +1.5V