H.H. Scott 340B
A vacuum-tube receiver with a service manual written for the person holding the soldering iron. Scott expected you to open it up and keep it running.
Overview
The 340B is a tube stereo receiver. The first thing that stands out in its service documentation is how much of it is written for the person with the soldering iron. The manual's general service notes open by saying that, beyond pilot lights and vacuum tubes, service "is usually not required." Then it walks through a full alignment procedure for the FM front end, IF strip, detector, and multiplex section. It also warns that a sweep generator should never be used to align Scott equipment, because it will produce false indications of alignment through loading of the individual circuits. That is a manufacturer writing to technicians, not to a warranty department.
The amplifier section is rated at 35 watts per channel music power (IHF) and 30 watts per channel steady state, with maximum total harmonic distortion of 0.8 percent at rated output. Frequency response is quoted at 20 to 20,000 cycles ±1 dB, and hum and noise sit 80 dB below rated power. The power bandwidth figure is given as 20 to 20,000 cycles by the IHF method. Scott's manual notes that all of the company's amplifiers and preamplifiers have a low-frequency roll-off filter that becomes fully operative below 20 cycles. It keeps subsonic rumble and record eccentricity from eating output-stage headroom, so the amplifier's full power goes to the audible range.
The preamp section has the usual mid-century input spread. Tape head and magnetic low inputs present roughly 50 K impedance and need 3 mV for rated output. Magnetic high also sits near 50 K but wants 9 mV. High-level inputs are 500 K and 0.4 V. Flat frequency response runs 20 to 20 kc ±1.0 dB. Treble controls, measured at 10,000 cps, offer 15 dB of boost and 15 dB of cut with ±2 dB of trim. Bass controls, measured at 30 cps, are specified the same way. The scratch filter rolls off at 6 dB per octave above 5 kc.
The tuner is where the numbers get interesting. Usable sensitivity is specified at 2.2 microvolts (IHF), signal-to-noise at 60 dB below 100 percent modulation, and total harmonic distortion at 0.8 percent. Frequency deviation, or drift, is listed at just 0.02 percent. Stereo frequency response is quoted as 50 to 15,000 cps ±1 dB, with a footnote saying this is the limit of FCC stereo broadcast specifications and that all H. H. Scott tuners have far wider frequency response. Capture ratio is 6.0 dB, selectivity 35 dB, AM suppression 55 dB, and stereo separation is stated as better than 30 dB. Tuning range is 87 to 109 mc with 0.5 percent calibration accuracy. Spurious response rejection and cross-modulation rejection are listed together at 4b 08. That is an OCR garble in the surviving scan, and the Archive cannot resolve it to a clean figure.
The tape output is rated at 0.5 V into a minimum recommended load of 100 K, with cable capacitance kept under 1000 mmf and a maximum recommended cable length of 40 feet. Power consumption is listed at 200-280 watts on 117 volts at 60 cps, AC only. The manual says no significant change in characteristics should be expected across normal line-voltage variation or at 50 cycles. The unit is designed to run on 105-120 volts, 50-60 cycles.
The maintenance details in the manual are unusually specific. Bias adjustment calls for a milliammeter on each channel's cathode current test point and a target of 70 milliamps per side. The two adjustments interact, so they should be performed at least twice to land on correct settings. DC balance is set by pulling the phase-splitter tubes V3 and V103, connecting a 16-ohm load box, letting the unit warm up, and adjusting each channel's balance pot for minimum hum as read on a VTVM. The gain-per-stage table is measured with a 16-ohm load box on the 16-ohm tap, referenced to chassis ground, and the manual points out that the "0" speaker output tap is not at chassis ground. Pilot light replacement requires lifting the control shield. It has wires attached, so it should not be pulled far. The dial lamps are bayonet-type #47 bulbs whose beam shields must be transferred to the replacements. The meter bulb is a factory-order item, VPL-47-Blue.
The alignment procedure is documented in full. The FM section is checked with output taken from the TAPE OUT jack and the level control full counter-clockwise. Recommended test gear is an oscilloscope, a VTVM, a distortion analyzer or 400-cycle null filter, and a Measurements 210AB FM signal generator or equivalent. IF alignment is done by peaking top and bottom slugs simultaneously with a non-metallic tool in each hand. The generator is set for 400 cps modulation at 75 KC deviation, with output trimmed to a barely adequate sinewave on the scope, on the order of 3 to 6 microvolts into the tuner. Detector alignment peaks the primary for maximum audio output at low signal, then raises the generator to 1000 microvolts and adjusts the secondary for minimum distortion. Multiplex alignment involves shorting the 19 KC test point to kill the pilot level and adjusting T502 for zero beat. Then the separation pots are worked back and forth across both channels until maximum separation is achieved, which the manual warns may take several passes. Last, input is reduced to 10 mV and the threshold adjustment is set until the stereo light comes on.
On tube replacement, Scott is blunt. Defects frequently do not show up in a tube tester, and only operation in the tuner/amplifier will confirm a tube is functioning properly. Exact replacement tubes from Scott or an authorized warranty service station are recommended. The manual admits other tubes will work, but says only exact replacements deliver full performance.
The 340B's documentation is a service bulletin covering serial number 235581 and above. That tells collectors Scott revised this receiver in production, and the manual in circulation applies to later units. Whether earlier serial numbers differ in circuit or specification is not documented in the material the Archive has located.
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