McMurdo Silver, Inc. 801
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|Manufacturer:||McMurdo Silver, Inc.|
DESCRIPTION: Model 601 Receivers hew rigorously to the line of maximum sensitivity and general utility for the minimum of tubes, circuits and power consumption. They differ only in power requirements and frequency range. Model 801 employs new miniature 6.3 volt heater tubes for operation from batteries or d.c. power supply.
The two circuit diagrams show the basic similarity of the two models. Each consists of a high-gain pentode r. f. amplifier stage tuned by Li and C1, the latter controlled by the ANT. knob. On short waves the tuning of this r. f. stage is not particularly critical, and the ANT. knob may be regarded as a trimmer rather than as a second tuning control. Following the r. f. amplifier is a rentode regenerative detector. It's tuned circuit 1s L2, cla. At and around critical regeneration, detector tuning is very sharp. Cla 1s controlled by the main dial marked TUNE upon the panel. Connected to cia through 16: 1 preloaded gears, the main dial makes eight revolutions for each tuning range to yield 800 well-spaced dial divisions per band. Revolutions of this dial are counted by a secondary dial numbered o through 8, with the number of dial turns in use indicated by the figure visible through the panel window at the upper left of the dial. Logging of stations received is effected by noting down the numbers of the coils in use at L1 and L2; the setting of the ANT. knob and the window-visible number followed by the TUNE dial setting. Setting of the REG. knob may also be recorded, since it affects TUNE dial setting slightly Detector regeneration is controlled by varying screen voltage by means of Pi. Regenerative tickler is in detector plate circuit, with a. f. load R5 isolated from r. f. by two-section filter C3, R2, C3A. This method of control gives extremely smooth variation of regeneration Just below, at and immediately beyond critical regeneration. Stickiness, drag and fringe-howl are pleasingly absent.
The detector is followed by a. f. VOLume control P2 and two stages of high-gain pentode audio aap 1 1fi cation. The 2nd (power output) stage feeds the self-contained loud-speaker: unless a 2 - circuit phone plug is inserted in the PHONTS jack when the speaker is muted and output fed through the plug to head phones. ACCESSORIES Accessories required, but not supplied, to put either receiver into operation are as REQUIRED: follows:
MODEL 801; Antenna and ground antenna may be a single wire about 50 Pt. long plus 20 to 30 ft. of leadin, and well up in the air. Ground connection should be to & cold water pipe scraped bare at point of connection. 1 each 6BA6, 6AU6, 6C4 and 6AK6 tubes 1 a. c. power supply furnishing 6.3 volts at. 9 amperes and 180 volts at 35 ma. filtered d. c. storage or dry batteries (4 Burgess # 4FA, or equal, 1t volt batteries connected in series to provide 6 volts for tube heaters and 4 Burgess # 5308, or equal, 45 volt "B" batteries in series to provide 180 volts). INSTALLATION: Insert tubes in sockets as indicated in rear-view illustration. Erect antenna using
# 14 to # is bare, ename) ed or insulated wire as high and clear of surrounding objects as practicable. If a single wire is used, fasten far end to glass or ceramic insulator fastened to tree, pole or building with rope. Thread leadin end of antenna wire through second insulator, twist back on itself, and carry free end of wire down to receiver (antenna and leadin one piece of wire). Fasten second insulator to tree, pole or building as convenient with rope. Keep antenna and leadin free and clear of nearby objects. Connect leadin to wire projecting from receiver and terminated at # 2 contact of L1 socket (upper left, as seen from front).
Insert coils for desired frequency range in Li and L2 sockets. NOTE that while the two coils for each range are identical, small electrical differences will affect dial logging if they are interchanged. Therefore, scratch identifying marks, such as "Li" L2 " upon each coil of each pair so as to be sure
Connect batteries to MODEL 801 receiver (a. c. power supply to MODEL 801 only) by means of color-coded twisted cable projecting from receiver cabinet. Follow wire colors and connection legend found at bottom of appropriate circuit diagram. Connect ground lead to BLACK receiver cable wire at batter y or a. c. power supply connection point. ON - OFF switch in YELLOW wire opens filament circuit of Model. 8018; switch in RED wire of Model 801 opens Bt circuit when REG. knob is turned to OFF.
OPERATION: Set ANT. knob to about 50, TUNE dial to about 4-0, set VOL. imob to about 10. Advance REG. knob slowly from OFF toward 10, listening carefully for the beginning of a faint rushing sound in the speaker. Advance REG. knob to just beyond the point where this rushing noise is just heard, and which indicates that the detector has passed the point of critical regeneration and has gone into oscillation. Rotate TUNE dial slowly until a squeal is heard evidencing reception of a signal. If this squeal is broken up into dots and dashes it is a CW telegraph station: 1f the squeal is continuous, it is a broadcast or phone station.
Adjust ANT. knob for loudest signal. Carefully adjust TUNE dial and REG. knob for loudest telegraph signal. Adjust VOL. Knob for desired loudness of signal,
To initially locate phone or broadcast stations, proceed as above (REG. knob advanced so signal is first heard as a continuous uninterrupted squeal). Once the signal 1 $ 50 found, retard REG. knob together with the slight adjustment of TUNE dial required to " hold * the signal until the detector stops oscillating, the squeal disappears, and voice or musie is heard.
Greatest sensitivity and selectivity obtains when the REG. knob is advanced to just above the point at which a signal is heard as a squeal when it is a CW telegraph signal. Greatest sensitivity and selectivity for phone, broadcast and modulated telegraph reception obtains when REG. knob is set to just below the point where such signals turn into squeals. The range of mechanical movement of the REG. knob is much greater than its useful electrical range. Its useful operating range is from just below to just above the point where signals are heard as squeals. This point will move about upon the REG. scale for different coils and different frequencies. ALWAYS operate with REG. knob just below signal-squeal point for phone, just above for CW telegraph reception. DO NOT control volume with REG. knob control volume with VOL. knob.
As with all t. r. f. - regenerative detector receivers, selectivity is a function of Regeneration setting, is greatest for weak signals and at critical regeneration. For the regular broadcast band selectivity is adequate to separate weak distant from strong local stations only when using a very short antenna maybe in to 15 ft. long. The 50 ft, antenna may be electrically shortened by disconnecting It metalic ally from the receiver antenna lead, then twisting the leadin about the insulated receiver antenna wire for a couple of turns-just enough to give maximum selectivity without serious loss of sensitivity. On short waves this problem is not particularly serious, and good selectivity will usually obtain with a 50 ft. antenna. Good reception can be had with a shorter antenna in most cases, however.
PHONES jack is for private listening does not yield more volume than loud speaker. If more volume on headphones is desired the receiver output circuit may be altered upon addition of a 10,000 ohm, 1 watt resistor and a. 1 mld., 400 volt tubular condenser. To make such alteration disconnect lead of T1 going to long spring of J1; disconnect short spring of J1 from speaker voice-coil lug. Connect freed lead of Ti directly to speaker voice-coil lug, leaving both lugs of J1 momentarily unconnected. Dis connect lead of Ti going to plate of 6AK6 (or 184) tube socket and connect this lead to short spring of Jl. Connect long spring of Ji to one lead of. 1 mfd. condenser: other lead of condenser to plate lug of 6AK6 (or 184) socket. Connect 10,000 ohm resistor between plate lug of 6A86 6r 184) socket and receiver Bt circuit (to B + 180 V. for Model 801; to B + 90 v. for Model 801B).
NOTE: At and around critical REGeneration the detector circuit is extremely sharp in tuning. Upon loud signals from the loud speaker mechanical vibration feedback between speaker and cia may occur. This may cause howling, with howl masking signal. Do not turn VOL. knob up so signal becomes loud enough to cause such feedback or use headphones for reception.
ALTERNATE ANTENNA: If it is desired to use a V 2 - wave doublet, or other antenna having a two-wire leadin, disconnect contact # 1 from contact # 3 on L1 socket. Connect two-wire lead in to Li socket contacts # 1 and # 2. Use or omit ground connection to water-pipe ground as results dictate.