Scott 370 B Service Manual

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111 Powdermill Road, Maynard, Mass.

SERVICE BULLETIN
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MODEL 370-3 FM STEREO MULTIPLEX TUNER
SPECIFICATIONS

(These are minimum - all H. H. Scott Model 370 tuners must meet or exceed these figures).

Usable Sensitivity (IHFM) 3.5 microvolts
(For less than 3% total noise, distortion,
hum -30 db.)
Signal to Noise Ratio 60 db. below 100% modulation
Total Harmonic Distortion 1.0%
Frequency Deviation 0.02%
Frequency Response 30 to 15,000 cps i2 db.
Capture Ratio 6.0 db.
Selectivity 35 db.
Spurious Response Rejection 80 db.
I.M. Distortion (CCIF method) 0.3%
Hum 60 db. below 1 volt
AM Suppression 55 db.
Audio Output (100% modulation) .8 to 1.2 volts
Tuning Range 87 to 109 me.
Accuracy of calibration 0.5%
Output Impedance 20,000 ohms
Minimum Recommended Load Resistance 100,000 ohms
Maximum Recommended Cable Capacitance 500 mmfds
Maximum Recommended Length of Output Cable 25 feet
Range of Line Voltage and Frequency 105-125 volt/50-60 cps.
Power Consumption -- AC only -- DO NOT OPERATE 43 watts

WITH DIRECT CURRENT

GENERAL SERVICE NOTES

Service, 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 per-
formance. 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, available from H. E. Scott or any authorized warranty service
station. Other tubes will work, but only exact replacement tubes can give the full per-
formance 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.

D-370-2

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2.

Unscrew and remove the tuning knob and dial. This will permit access to the pilot light which is a VPL-lZ
bulb.

After the bulb has been replaced, use the following procedure:

1.

2.

3.

Close the condenser plates by rotating the condenser to the extreme counter-clockwise position.

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.

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

l.

7.

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.

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

Equipment set-up: Connect signal generator directly to the 300 ohm antenna input of the tuner using a match-
ing impedance network if necessary. The Channel A audio output of the 370 then feeds into the (+00 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 Dept.) the tuner can still be
serviced. However, it will not be possible to measure the tuner's "Usable Sensitivity" as per IHFM standards,
or align the detector.

Allow tuner and test equipment to warm up fully before beginning alignment. Adjust line voltage for 117
volts. Remove bottom cover of tuner. Always tune primary and secondary of LF. transformers at the same
time, using one alignment tool in each hand. Set 370 Selector Switch to Mono.

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 5 to 10
microvolts). The null filter should be switched out of the setup, so the tuner is feeding directly to the
VTVM and scope. Peak the IF's and the primary of the ratio detector (the bottom slug) for

maximum reading on the VTVM.

Increase generator output to 1000 microvolts and switch the 400 cps null into the circuit. 'lune ratio
detector secondary (top slug) for minimum reading on the VTVM.

Return generator output back down to 5 to 10 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.

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 mc.

MISCELLANEOUS SERVICE PROCEDURES AND TESTS - (Qualified technicians only)

Note: Replace bottom plate before testing.

1.

Test for IHFM Usable Sensitivity measurements: Use 117 volt regulated line, set generator and tuner at 92
me, with 10 microvolt input. Take a reference reading on the db. scale and check for 38 db. or more reduction
in output with 400 cycle null filter switched in to setup. Repeat at 106 me. If specification is not met,
tuner should be realigned and tubes checked. It is very important that the null filter be carefully adjusted
so that it is at exactly the same frequency as the 400 cps modulation being fed in. otherwise, it will be
impossible to obtain stated sensiti-

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vity figure. Note that 10 microvolt output of generator is equivalent to less than
5 microvolt on tuner input due to drop through the matching impedance network.

2. Audio output: With 1000 microvolt input, 75 kc deviation, maximum output should be
1.5 volts.

3. Automatic Gain Control: The maximum permissible difference in output between an in-
put of 10 microvolts and 1000 microvolts is 8 db.

4. F.M. Hum: At 100 mc., 1000 microvolts input, 400 cycle modulation, take a reference
reading and remove modulation. A 60 db drop should be noted.

5. A.C. Hun: With no modulation and level control at minimum, adjust hum pot for minimum
hum. Should not exceed .003 volts.

6. Oscillation Check: Check entire FM band for oscillation, with and without antenna,
at 105 and 130 volt line.

PRELIMINARY MULTIPLEX TESTS
Use same equipment described above, plus a good Audio Oscillator.

1. Connect the audio oscillator to the test point input on the multiplex section. Adjust
the output of the oscillator for 0.25 volts. Set the 370 selector to mono.

2. Measure the output of the tuner. It should be 1 to 2 volts with selector switch in
either mono or stereo.

3. Disable the multiplex section oscillator by shorting pins #9 and 11 of 6AG11 (V501)
to each other. With the audio oscillator at 400 cps take a reference reading. It
should be down maximum of 2 db at 20 cps., 2-4 db at 2 KC, 7-9 db at 5 KC, 12-14
db at 10 KC. Continue to 15 KC -- 15-18 db drop, a minimum of 21 db at 20 KC, and
minimum of 44 db at 40 KC on both channels.

4. Turn 370 selector switch to Stereo. Set oscillator at 67 KC, 0.25 volts output.
Adjust L-502 for minimum output from either Channel A or B. The null is quite broad.
If necessary, increase output of audio oscillator for better null.

STEREO MULTIPLEX SEPARATION TESTS AND ADJUSTMENTS

Equipment needed: In addition to all the equipment mentioned before you will need a
Hewlett-Packard 200CD Audio Oscillator or equivalent (with balanced output to ground --
oscillator with unbalanced outputs cannot be used), and the H. H. Scott Model 830 FM
multiplex stereo generator.

Equipment set-up: Use the same arrangement described with the tuner alignment procedure,
plus: Connect the Audio Oscillator to the 830 (as described in the 830 Instruction Manual)
and the 830 to the FM generator. Set the FM generator for External Modulation.

1. Set audio oscillator to 400 cps. and 830 for left channel (Channel A) signal only.
IMPORTANT: The 830 MUST be carefully adjusted for input signal and output phase
as described in the 830 Instruction Manual. The output phase must be monitored with
a scope to check for variances. If the phase is off even slightly, your alignment
will cause the tuner to have poor separation when tuned to station.

2. Set all controls on the 370 for stereo. Tune the 370 to the point on the dial where
you are transmitting your Channel A signal. Make sure you pick a point on the dial
that does not coincide with a local FM station.

3. Attach scope probe to junction of diodes and 220K resistor (T-501), short pin #9
and #11 at selector switch together. Turn selector switch to monitor position. Peak
L-501 and T-501 for maximum output as indicated on scope. Remove short from Pin #9
and #11. Turn selector switch to Stereo.