Service manual

5 –2
SECTION 5 - TROUBLESHOOTING - 3-STAGE
5.3.7 FAN DRIVE (OPTIONAL ACCESSORY)
The V DC fan is controlled by a FET device which is
activated by the microprcessor. At its highest setting
(), the fan receives the full V DC. At lower settings
the fan receives +V 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  Vdc (typically . Vdc) forward voltage across it.
5.4.2 OPERATIONAL AMPLIFIER
Generally the “+” and “-” inputs of an operational
amplifier will have less than  mV voltage difference
when operating under normal conditions. (U). 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 / 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 (K) under load
measures typically - Vac, while the MT to return
voltage measures between .-. Vac.
A triac without connections can be checked for a go-no
go condition with an ohmmeter. The gate to MT
resistance for a power triac (-A) should be between
 and  ohms; there should be infinite resistance
between MT and MT.
5.4.4 CAPACITOR
Shorted capacitors have V 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 +. - +.V
Low . - +.V
LSXXX: High +. - +.V
Low . - +.V
XXX (CMOS): High +. - +.V
Low . - +.V