Eico HF 12 Instruction Manual page 7

connection to the cable shield from turntable frame, pick-
up arm, and cartridge case), dired hum pick-up by the
magnetic cartridge from the record player motor (try using
a rubber mat on the turntable to Increase the separation of
the pick-up from the motor), or pick-up from a power trans-
former or other magnetic field in the vicinity (try moving
phonograph away from suspected source). Check also that
the phono input cable shielding
Is
grounded to the ampli-
fier chassis at one point only through the skirt of the Input
connector where it plugs Into the amplifier. Finally, try
a good building ground such as a connection from a cold
water pipe terminated under speaker terminal
II
Gil on the
amplifi er. Do not connect such a ground wire to other
components In the system.
Excessive hum on other inputs may be checked in a similar
manner. Disconnect the Input cable In question and short
the particular input jack to the chassis. If the hum dis-
appears, the trouble is extemal to the amplifier. Note
that on all inputs, the braid of the Input cable should con-
nect to the amplifier only through the skirt of the Input
connector. The cause and remedies for the following symp-
toms are then based on the assumption that checks made in
the manner described above have eliminated the possibll ity
of the trouble being external to the amplifier.
If the trouble is no output or low output, checkAC signal
voltages and DC operating voltages starting at the input
and working step-by-step toward the output. Set the
VOLUME control to maximum (10), and the BASS and
TREBLE controls to their mid-points (0). Use a 1000 eye Ie
sinewave signal, such as supplied by the EICO 377 Sine &
Square Wave Audio Generator. In addition, use a pre-
cision 100: I attenuator to permit obtaining a level of
0.01 volt fedintoMAG. PHONOfrom an audio generator
output of 1 .0 vol t, which can easily be measured on the
lowest ACvolts range of yourVTYM(also improves signal
to hum from generator). Use a high input Impedance VTYM
for allAC signal voltage measurements (such as the EICO
232,249,221, or 214) and aVTYM or20,000n/voltVOM
for DC voltage measurements.
If
the troubl e is an excessively distorted output, try tube
replacement, signal tracing, or procede directly to vol tage
and resistance measurements.
When the defective stage is localized, precede to a resist-
ance and vol tage check of the stage, using the data In the
VOLTAGEand RESISTANCEchart. Disconnect the ampll-
-fler from the power line and discharge capacitors prior to
making any resistance check and prior to removing either
or both of the EL84 output tubes V4 and V5. Do not turn
the amplifier on with either of the output tubes removed.
6
TROUBLE-SHOOTING A TYPICAL TUBE STAGE
1. Check tube.
2. Check plate and cathode resistors.
3. Check coupling capacitors for leakage or short.
4. For output stage, check de resistance of transformer
windings.
5. Check grid leak resistor for open.
6. Check cathode by-pass capacitors for short.
7. If no or low B+ voltage on tube, check decoupling
path for open or defective R18, R31, R32, R33 and filter
capacitor C17 or C18.
8.
If
wiring and circuit components including the tube
check O .
K. and B+ voltage is excessive, check the de-
coupling path for short or defective R18, R31, R32, R33.
Suspected trouble in the equalization, tone, and volume
controls and networks should lead to specific resistance
and capacitance checks to localize the trouble. In gen-
eral, if the user suspects poor frequency response, defec-
tive equal ization, or defective operation of the tone con-
trols, the amplifier should be tested thoroughly with audio
generator, vtvm, and scope.
SERVICE
If trouble develops in your instrument which you ccn not
remedy yourself, write to our service department listing
all possible indications that might be helpful. Note num-
ber appeari ng in red under the word" ManuaP' on the front
cover. If there is no number, state this. If desired, you
may return the instrument to our factory where it wi II be
placed in operating condition for $7.50 plus the cost of
parts replaced due to their being damaged in the course
of construction. NOTE: Before returning this unit, be
sure all parts are securely mounted. Attach a tag to the
instrument, giving your home address and the trouble with
theunit . Pack very carefully in a rugged container, using
sufficient packing material (cotton, shredded newspaper,
or excelsior), to make the lonitcompletely immovable
within the container. The original shipping carton is sat-
isfactory, providing the original inserts are usedor suffi-
cient packing material is inserted to keep the instrument
immovable. Shipby prepaid Railway Express, if possible,
to Electronic Instrument Co., Inc., 33-00 Northern Blvd. ,
Long Island City 1, N. Y. Return shipment will be made
by express collect. Note that a carrier cannot be held
liable for damages in transit if packing IN HIS OPIN ION,
is insufficient.