Hammond Model A, Manual, Page: 272 / 298

3-3. TUBE TESTING. When the trouble is traced to a specific stage, test tubes in
that stage. If tubes are satisfactory, make a point-to-point voltage check in
accordance with paragraph 3-4.
3-4. VOLTAGE AND RESISTANCE MEASUREMENTS. Make voltage and resistance measurements
on the individual components of the stage (as shown in specific console schematic).
Make all capacitor checks with capacitor analyzer if available. Always disconnect
capacitors before making tests; otherwise the readings will be affected by a
possible shunt circuit. Replace any capacitor which shows a deviation of 20
percent or more.
3-5. RESISTORS. Resistors used in Hammond Organs are marked with the standard EIA
(Electronic Industries Association) color code, as shown in Table II. In this code,
the body color or first color ring (starting from the outside edge) indicates the
first digit of the resistor value. The second ring denotes the second digit, and the
third ring represents the number of zeros after the second digit. Thus a resistor
marked with brown, green; and yellow rings (in that order) would have a value of
150,000 ohms. Gold and silver rings represent percentage tolerance, gold indicating
5 percent tolerance and silver indicating 10 percent tolerance. Replace resistors
differing by as much as 30% from their rated values.
3-6. COIL MEASUREMENTS. For the DC (direct current) resistance value of chokes and
audio transformers, refer to the appropriate circuit diagram. An open winding in the
choke or transformer will be indicated by no ohmmeter indication. Check the power
and filament transformers by comparing their measured voltage with the voltages
given in the circuit diagram. All voltage values are given for a 117- volt (or
234-volt) AC input. If the input voltage varies, a corresponding change will be
noted.
3-7. SECTIONALIZING TROUBLE
3-8. AMPLIFICATION SYSTEM TROUBLES.
3-9. Such troubles as loss of volume, poor quality, excessive hum, noisy operation,
or no signal are usually traceable to the amplification system. For example, if
distortion is noted in the loud speakers, connect a headset across terminals marked
G - G on the console preamplifier. If the quality is good on the headset, the cause
of the distortion will be found in the amplification system following the console
preamplifier. Should distortion occur at the preamplifier terminals, replace all
tubes. Should distortion in the console preamplifier continue, test each
preamplifier stage individually with the headset. When the defective stage is
located, test each capacitor and resistor for deterioration.
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