Scott 340 Service Bulletin

Page 1

SCOTT

111 PowdermiH Road, Maynard, Mass.

SERVICE BULLETIN FOR
MODEL 340 TUNER - AMPLIFIER

SPECIFICATIONS
ese are minimum-all H. H. Scott Model 340 tuneramplifiers must meet or exceed these figures.)

TUNER (FM-MPX)

'--_115able Sensitivity (IHFM) 2.5 microvolts
(For less than 5% total noise, distortion, Hum-30 db.)
Signal to Noise Ratio 60 db. below 100% modulation
Total Harmonic Distortion 0.8%
Frequency Deviation 0.02%
Frequency Response 20 to 20,000 cps 2 1 db.
Capture Ratio 6.0 db.
Selectivity 35 db.
Spurious Response Rejection 80 db.
1. M. Distortion (CCIF method) 0.3%
AM Suppression 55 db.
Tuning Range 87 to 109 me.
Accuracy of calibration 0.5%
PRE-AMPLIFIER
Gutput
ated voltage output to tape recorder 0.5 V.
Minimum recommended load resistance 100 K.
Maximum recommended cable capacitance 1000 mmf.
Maximum recommended cable length 40 ft.
Input
ag. Low - Input impedance 47 K.
Signal for rated output 3 mv.
Mag. High - Input impedance 150 K.
Signal for rated output 9 mv.
High Level Inputs - Input impedance 500 K.
Signal for rated output 0.5 V.
Frequency Response in flat position 20-20 KC, 2 1.0 db.
Treble Controls measured at 10,000 cps, boost or cut 15 db. 9 2 db.
Bass Controls measured at 30 cps. boost or out 15 db. 2 2 db.
Scratch filter (Cuts 6 db. per octave over
5000 cps.)
NAB Tape Equalization (Within 2 db. of published
standard)

AMPLIFIER
___-fliiifium power output each channel at 1000 cycles:
Music waveforms................. 30 watts
Steady state..... ........ 25 watts
Maximum total harmonic distortion at rated output...... ........... 0.8%
Frequency response for 25 watts steady state at less than 2.0%
total distortion......... ....................................... 20 to 20,000 cycles 3 0 db.
Power bandwidth at rated di ortion [IHFM method) below l9 cycles to *
above 20,000 cps
(limits of test equipment)
Intermodulation distortion. ............... below 0.8%
Hum and noise........................................................ 80 db. below rated power

u...

(These characteristics are measured at a line voltage of 117 volts rms and line frequency of 60 cycles
per second. No significant changes of characteristics should be experienced for normal variations of
line voltages or a line frequency of 50 cycles per second.)

Range of line voltage and frequency.................................. 105-125 volts, 50-60 cycles
Power consumption - 117 volts at 60 cps (A.C. only).................. 240-300 watts

* All H. H. Scott amplifiers and preamplifiers incorporate a low frequency rolloff which becomes full
operative below 20 cycles. This is designed to prevent overload of the output stage and the loudspeaker
due to subsonic rumble frequencies and record eccentricity. This means that the full power of the ampli-
fier can be concentrated into the audible range.

D-340-2 1 (See inside also)

Page 2

TUNER SECTION

GENERAL SERVICE NOTES

Serv1ce, other than replacement of either pilot lights or vacuum tubes, is usually not required. If
the tuner is not operating properly, all external connections should be checked to make sure that the
difficulty is in the tuner. Generally, it is advisable to replace the tuner with a tape recorder or
similar device to check out the amplifier performance. If the difficulty appears to be located in the
tuner, the level control should be first checked to insure that it is rotated away from its extreme
counter-clockwise position. Then, the vacuum tubes should be checked by replacing them with new ones,
one by one. The tubes should be tight in their sockets and provided with shields where applicable.
Tube defects frequently do not show up in a tube tester. Only operation in the tuner will insure the
proper working of a vacuum tube. In replacing tubes, if possible, use exact replacement tubes, avail-
able from H. H. Scott or any authorized warranty service station. Other tubes will work, but only
exact replacement tubes can give the full performance that the tuner is capable of.

The pilot light may be simply replaced by the following procedure:

1. First, set the tuning dial so that the heavy white lines line up with the pointers on the dial
and the logging scale is towards the top of the panel. In this position, the tuning condensers
are closed.

2. Unscrew and remove the tuning knob and dial. This will permit access to the pilot light which
is a #47 bulb.

After the bulb has been replaced, use the following procedure:
1. Close the condenser plates by rotating the condenser to the extreme counter-clockwise position.

2. Replace the dial, lining up the heavy white lines of the dial with the pointers and keeping the
logging scale toward the top of the front panel.

3. Replace the tuning knob.

ALIGNMENT INSTRUCTIONS FOR FM SECTION
Note: No attempt should Be made to align the tuner or repair it unless the person so doing has
had extensive experience in tuner alignment and repair procedures and has the necessary

laboratory equipment. Without proper experience or equipment, the repairman may seriously
damage the tuner.

1. Equipment required: VTVM (AC), FM Signal Generator (must be Measurements ZlOAB or equivalent),
Oscilloscope, 400 cps null, and insulated alignment tools.

2. Equipment setup: Connect signal generator directly to the 300 ohm antenna input of the tuner
using a matching impedance network if necessary. The left Channel (A) tape recorder output
of the 340 then feeds into the 400 cps null and from the null to the oscilloscope and VTVM in
parallel. If no null is available (a schematic for making one of these very simple and useful
devices is available from the Engineering Department) the tuner can still be serviced. How-
ever, it will not be possible to measure the tuner's "Usable Sensitivity" as per IHFM standards
or align the detector.

3. Allow tuner and test equipment to warm up fully before beginning alignment. Adjust line vol-
tage for 117 volts. Remove bottom cover of tuner. Always tune primary and secondary of I.F.
transformers at the same time, using one alignment tool in each hand. Set 340 Input Selector
Switch to FM Mono, AGG switch to Mono.

4. Set generator and tuner for 92 mc. Generator should be modulating a 400 cps signal at 75 kc
deviation. Adjust output of generator so that a barely adequate sine wave appears on the
scope (on the order of 3 to 6 microvolts input to tuner). The null filter should be switched
out of the setup, so the tuner is feeding directly to the VTVM and scope. Peak the I.F.'s
for maximum AGC reading on the tuning meter of the 340 (not the VTVM).

5. Increase generator output to 1000 microvolts. Peak the primary of the ratio detector (bottom
slug) for maximum audio output (on the VTVM). Switch the 400 cps null into the circuit. Tune
ratio detector secondary (top slug) for minimum reading on the VTVM.

6. Return generator output back down to 3 to 6 microvolts. Adjust antenna coil (under chassis)
for maximum output. Tune generator and tuner to 106 me. with same output and deviation, and
adjust antenna trimmers (on top - FM ANT.) for maximum output. Repeat this operation until
best results are obtained.

7. If calibration is off, repeat "6" except adjust oscillator coils (under chassis) for correct
tracking at 92 mo. and oscillator trimmer (FM OSC - on top) for correct tracking at 106 me.

(Circuit diagrams on inside pages) 2