McMurdo Silver, Inc. 802 Schematic

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Manufacturer:McMurdo Silver, Inc.

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DESCRIPTION: Model 802 receiver is designed to cover the 80, 10, 20, 16, 11/10 and 6 meter amateur bands only. Following A.H.R.L. Handbook teachings thru employment of controllable regeneration in its 735 kc. 1.f. amplifier to provide extreme and variable selectivity, its excellence in this respect is equalled by the extreme of band-spxead it provides upon each amateur band. Five new miniature u.h.f. tubes, three of them dual-function, yield effective 8 - tube complement. Each amateur band occupies the major portion of the eight revolution, 0/100 division 3 diameter main tuning dial, which with the secondary " tura counting " dial visible thru the panel window gives 88h of effective dial length. A series valve noise-limiter is effective in reducing ignition and other noise interference. Noise control, P3, is accessible upon the interior channel upon lifting cabinet lid.

The circuit consists basically of a 6BE6 pentagrid converter having antenna input circuit u Inductively tuned by the relatively non-critical ANT. knob, and local heterodyne oscillator with inductive tuning of L2 by the main TUNE dial. A single stage of dual-air-tuned 1.f. amplification extploys a 6BA6 tube, with regeneration controlled by REG, knob Pi, (which also carries an on-off. gwitch for control of external power supply). A 6J6 dual triode functions as non loading infinite impedance second detector and serieg-valve noise-limiter, This is followed by a second 636 functioning as first a.f. amplifier and beat-oscillator, pitch of which may be adjusted for single-signal cu reception by internal air capacitor c5d. A.f. power anplifier is a 6.86, with volume controlled by P2. A PX dynamic speaker and output transformer Tl are built into Yodel 802, together with headphone jack Jl. Band change is effected by ingerting appropriate plug-in coils in sockets accessible upon lifting cabinet lid. Coils are individually adjusted for each particular receiver, and are shipped with it. ACCESSORIES Accessories required, but not supplied as a part of Kodel 802, to put the receiver REQUIRED: into operation are as follows:

Antenna and ground-antenna may be a single wire 20 to 70 ft. long, including loadin (or may be a suitable doublet), erected as high in the air and clear of surrounding objects as practicable, Ground may be a connection to a well-scrapped cold water pipe. 1 each 6BE6, 6BA 6, 6AK6 tubes 2
6J6 tubes 1 pair of 2,000 ohm headphones (if desired) 1 Power supply to provide 100/205 volts at 35 ma. d.c., 180 to 250 volts d.c. at is mill. p and 6.3 volts a.c, or d.c, at 1.65 amperes. Power supply may be a well-filtered a.c, unit assembled from standard broadcast receiver replacement parts, a G - volt storage battery and suitable Vibropack or dry B batteries, or entirely dry batteries in emergency.

INSTALLATION: Insert vacuum tubes in their sockets as indicated in Fig. 1. Make sure each is fully seated in its socket, and that shield body encloses 6BA6 1.1. amplifier, Erect suitable antenna and connect its leadin to one of two wires projecting from ul socket thru hole in rear of cabinet. Connect ground lead to the second of these two rires and to BLACX rire (B A-). Connect power supply to twisted colored leads projecting from receiver cabinet, being careful to follow exactly color indications upon Fig. 2 for each wire. On-ofr switch upon REG, control is not wired into receiver circuit, but is brought out thru the two BLUE wires of power cable, It may be connected as desired to control the power supply, as in the 115 volt primary circuit of an a.c. power supply, etc.

Ingert coils for desired amateur band in sockets. Coil war ked " ANT * must go into upper left socket; coil marked " OSC " must go into center sockot. Seat coils fully in sockets, not at an angle, Take care in inserting or removing coils NOT TO BEND THE COPPER VANES controlled by ANT. and TUNE knobs. Tuning range and relogging calibration will be shifted if vanes are bent, and unless coils are inserted in sockets exactly the same my each time they are changed. Coils can be most easily removed by inserting a knife-blade or screw-driver tip between socket and coil base, then prying coil for back sufficiently to free it for easy finger removal. BEFORE CHANGING COILS BE SURE ANT. AND TUNE KNOBS ARE SO TURNED THAT THEIR COPPER VANES ARE OUT OF THE NAY AND DO NOT IN PEDE COIL REVOVAL OR INSERTION.

OPERATION: With tubes and coils for desired band in place, power supply connected and power turned on, antenna and ground connected, proceed as follows: Set B.F.0. switch OFF, N.L, switch OFF, VOL. to 10. Advance REG. knob from 0 toward in until a rushing noise is heard from the speaker indicating that the inf. amplifier is regenerating, possibly oven oscillating. Turn REG. knob up just enough so stations may be tuned in on the main TUNE dial as a squaal. Set AMT. knob for greatest volume, retarding VOL. knob if volume is so great as to cause micro phonic howling from speaker.

Greatest sensitivity and selectivity is with REG, knob set just below the point where signals are heard as squeals. KEEP IT BELON THIS POINT OF OSCILIATION. Thile CW signals can be received with B.F.O. OFF by pushing REG. Knob up to oscillating point of i.f, amplifier, this does not give best results. Use REG, knob to control sensitivity and selectivity alnys set it below point of i.f. oscillation, else when B.F.0. is turned ON more squeals and " birdies " will be heard than signals.

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Having operated Model 802 for possibly half an hour to find out how it works and controls, fusing oscillating 1.f. as above in this initial test only to make CN signals audible, or to locate weak phone stations by their carrier gqueals), set Red. about division below point of oscillation and turn B..0. ON. Tune in CT signals as on any receiver; locate weak phone stations by tuning to their squeal, then turn B.F.O. OFF to make speech audible and cisar. Note the extrert selectivity possible the muffling or speech quality as REG, knob is advanced toward 1.f. oscillation, This is side-band cutting due to the extraordinary selectivity possible to a re generative i.f. system.

It is this extrene sensitivity that gives Model 802 the advantages of " single-signal " CW telegraph reception which mutes one of the two audio beats found on each side of carrier resonance. B.F.0. may be set by adjustment of c5d, either to exact 1.f. resonance for locating weak phone stations, or preferably about 1,000 cycles to one side of resonance for " single-signal " CH. When so set, if RED. knob is operated just below i.f. oscillation, one of the two audio beats associated with each ci signal will be muted almost to inaudibility giving crystal selectivity without the annoying " pingingh quality associated with crystal filters. To set B.F.o. for single-signal reception, tune in a weak phone station, or a telegraph station which can be heard as key-clicks or " thumps with B.F.o. off and REG. set Just below squeal point. Turn ON B.F.O. and rotate rotor of C5d using the eraser on the end of a lead pencil to turn game, until the audible beat note is at about 1,000 cyclas. Now tune thru the signal, and noto that the second audio beat note is practically inaudible much weaker than the other. This condition can obtain for two settings of c5d; use the one with the rotor of C5d turned furthest in.

It is desirable to read the section of the A.R.R.L. Handbook describing operation of series valve noise limiters to gain understanding thereof. Tho series-valve noigo limiter of Model 802 Receiver (right hand triode of center 66, Fig. 2) may be thought af as a " gate which may be opened, partially closed or fully closed by adjustment of P3. Full open, the gate pusses both signals and noise; closed it will cut off both; partially closed it can cut out noise louder than a desired signal to improve signal readability.

To use the noise limiter, set NOISE switch si to ON after tuning in a weak signal markedly ndow " in local noise interference. Slowly rotate P3 until signal volume is neither reduced nor signal quality distorted but noise is reduced to a point where it is no louder than signal. Advancing P3 further will reduce both noise and signal volume together, Operate with p3 set to reduce noise as much as possible without impairing signal. Adjust P3 for each signal received, Set NOISE switch sl OFF in tuning for signals, gwitching noise limiter into circuit only when required. Leaving NOISE switch ON when tuning will result in distortion of signals louder than the one for which it was set.

MAINTENANCE: Hodel 802 is so basically simple that, outside of customary long-time tube detori oration, there is Httle to go wrong with it. Ita relatively high 735 kc. 1.1. frequency results in good image rejection without an r.f. stage. When operated near powerful local broadcast stations operating or 730 or 740 kc., interference therefrou may sometimes be experienced. This may be eliminated by tuning the i.f. amplifiers (adjust capacitors C5, C5a, 050, C5c and C5d) to a new i.f. frequency just gufficiently away from 735 kc. to cut out auch interference.

Care must be taken NOT TO BEND the copper tuning vanes actuated by ANT and TUNE knobs. Each affects the field of the coil (11, 12) with which it is associated. If vanes are bent, or coils are not seated fully and identically in their sockets each time they are changed, logging of atations tuned in previously will not repeat exactly upon TUNE dial. Accidental bending of vanes can shift tuning range go it will no longer be as originally established. Should this occur, vanes may be repositioned by gently bonding them to or away from coil ends. Do this ONLY if they are accidentally bent, and then to bring tuning back carrectly upon ANT and TUNE scales for stations first at high, then at low, frequency ends of each coil tuning range.

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