McMurdo Silver, Inc. 801 Schematic

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Product Information:
Model:801
Manufacturer:McMurdo Silver, Inc.

Schematics Content:

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Page 1:

McMURDO SILVER PAGE 20 I
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DESCR|PT|0|| llodel 801 Receivers hew rigorously to the line of maximum sensitivity and
generalutlllty for the mlnlmum of tubes, circuits and power consumption They differ
only ln power requirements and frequency range Model 801 employs new mlnlature 6 8 volt heater tubes
for operation from batterles or a c power sunply
The two clrcult diagrams show the basic similarity of the two models Each consists of a high gain
pentode r f amplifier stage tuned by L1 and Cl, the latter controlled by the ANT knob On short waves
the tuning of this r f stage ls not particularly crltical, and the ANT knob may be regarded as a
trimmer rather than as a second tuning control Following the r f amplifier is a pentode regenerative
detector It's tuned clrcult ls L2, Cla At and around crltlcal regeneration, detector tuning is very
sharp Cla is controlled by the main dial marked TUNE upon the panel Connected to Cla through 16 1
preloaded gears, the main dial makes elght revolutions for each tuning range to yleld 800 well spaced
dlal divisions per band Revolutions of this dial are counted by a secondary dlal numbered 0 through
S, with the number of dlal turns in use indicated by the figure visible through the panel window at
the u per left of the dlal Logging of stations received ls effected by noting down the numbers of the
coils in use at L1 and Lz, the setting of the ANT knob and the window visible number followed by the
[UNE dial setting Settlng of the REG knob may also be recorded, since lt affects TUNE dial setting
slightly
Detector regeneration is controlled by varying screen voltage by means of P1 Regenerative tlckler ls
ln detector plate circuit, with a f load R5 isolated from r f by two section filter CS, Rl, CBA
This method of control glves extremely smooth variation of regeneration just below, at and immediately
beyond crltlcal regeneration Stlcklness, drag and fringe howl are pleaslngly absent
The detector ls followed by a f VOLume control P2 and two stages of high gain pentode audio ampllfi
catlon The 2nd (power output) stage feeds the self contained loud speaker unless a ? circuit phone
plug is inserted ln the PHUNFS jack when the speaker ls 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 ft long plus Z)
to 30 ft of leadln, and well up ln the air Ground connection should be to a cold water pipe scraped
bare at point of connection
1 each GBA6, SAUS, 604 and GAK6 tubes
1 a c power supply furnishing 6 3 volts at 9 amperes and 180 volts at 35 ma filtered d c or
storage or dry batteries (4 Burgess #4FA, or equal, li volt batteries connected ln series to provide
6 volts for tube heaters and 4 Burgess #5308, or equal, 45 volt "B" batteries in series to provide
INSTALLATION Insert tubes ln sockets as indicated in rear view illustration Erect antenna usxng
#14 to #18 bare, enameled or insulated wire as high and clear of surrounding objects
as practicable If a single wire ls used, fasten far end to glass or ceramic insulator fastened to
tree, pole or bulldlng with rope Thread leadln end of antenna wire through second insulator, twist
back on itself, ana carry free eau or wire down to ree lver (ante a and leaaln one piece uf wire)
Fasten second insulator to tree, pole or bulldlng as convenient with rope Keep antenna and leadln
free and clear of nearby objects Connect leadln to wire projecting from receiver and terminated at
#2 contact of L1 socket (upper left, as seen from front)
Insert colls for desired frequency range in Ll and L2 sockets NOTE that while the two colls for each
range are identical, small electrical differences will affect dlal logging lf they are lnterchanged
Therefore, scratch identifying marks, such as 'L1", "L2' upon each coll of each pair so as to be sure
to get the same coll ln the same socket everv time
(C)John F. Rider

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P E 20-2 MCMURDO SILVER V
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
battery or a. c. power supply connection point. ON-OFF switch ln YELIDW wlre opens filament circuit
of Model, 8018; switch in RED wire of Model 801 opens B+ circuit when REG. knob is turned to OFF.
OPERATION Set ANT knob t about 50, TUNE dlal to about 4 0, set VOL knob 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
8-Yld h&S g0\'1'? iuf-0 0SC111Bti0l1 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 if 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 is so found, retard REG knob to
gether with the slight adjustment of TUNE dial required to 'hold' the signal untll the detector stops
oscillating, the squeal disappears, and voice or music is heard
Greatest sensitivity and selectivity obtains when the REG knob is advanced to just above the point
at which a signal ls heard as a squeal when it is a CW telegraph signal Greatest sensitivity and
selectlvlty 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 usehxl operating range is from just
below to just above the point whee 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
slgnal squeal point for phone, just above for CW telegraph reception D0 N01 control volume with REG
knob control volume with VOL- knob
As with all t r Y regenerative detector receivers, selectivity is a function of REGeneratlon setting,
ls greatest for weak signals and at critical regeneration For the regular broadcast band selectivity
ls adequate to separate weak distant from strong local stations only when using a very short antenna
maybe 10 to 15 ft long The FD ft antenna may be electrically shortened by disconnecting lt metalic
ally from the receiver antenna lead, then twisting the leadln about the insulated receiver antenna
wire for a couple of turns just enough to glve maximum selectivity without serious loss of sensl
tivlty On Short waves this problem ls not particularly serious, and good selectlvlty will usually
obtain with a 50 ft antenna, Good reception can be had with a shorter antenna ln most cases, however
PHONES jack ls for private listening does not yield more volume than loud speaker lf more volume
on headphones ls desired the receiver output circuit may be altered upon addition of a 10 000 ohm, 1
watt resistor and a 1 mfd , 400 volt tubular condenser To make such alteration disconnect lead of
T1 going to long spring of J1 disconnect short spring of Jl from speaker voice-coll lug Connect freed
lead of T1 directly to speaker voice-coll lug leaving both lugs of J1 momentarily unconnected Dis
connect lead of T1 going to plate of GAKB (or 1S4 tube socket and connect this lead to short spring
lug of 6AK6 (or IS4) socket Connect 10,000 ohm resistor between plate lug of 6AK6 br1S>>4) socket and
receiver Be clrcult (to B4 180 V for Model 801. to Bw 90 V for Model 80113)
NOTE At and around critical Rlmeneratlon the detector circuit ls extremely sharp in tuning Upon
loud signals from the loud speaker mechanical vibration feedback between speaker and Cla 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 1/2-wave doublet, or other antenna having a two wire
leadln, disconnect contact#l from contact #3 on L1 socket Connect two wire lead
in go L1 50,-ket contacts #1 and #2 Use or omlt ground connection to water pipe grozmd as results
'"???'? PARTS L|sT
(Exact values subject to change)
58 mmfd mica capacltor +2O%
C5a OI mfd 400 V paper capac|tor+2O%
of J1. Connect long spring of Jl to one lead of . l mfd. condenser: other lead of condenser to plate `
(C)John F. Rider

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MCMURDO SILVER PAGE 20 3
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R2a 25 KQ, ? watt res|stor +2O%
H052 5 watt resnstor +2O%
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R a 50 KQ, ? watt res|stor 20%
|50 KS2, i watt reslstor +201
R7a 750 KQ, 3 watt res|stor+2O%
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(C) John F.
Rider

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