Zenith Case Histories of Receiver Troubles and Their Remedies
Section 3

Models 760 through 990 plus the 'Stratosphere' models

INDEX

Zenith 760, 765, 767 (chassis 2054)

Zenith 770 (chassis 2032)

Zenith 770B (chassis 2059)

Zenith 775 (chassis 2032)

Zenith 775B (chassis 2059)

Zenith 801

Zenith 805, 807, 809

Zenith 835 (chassis 1001, 1001A)

Zenith 870 (chassis 5701-2-3)

Zenith 880 (chassis 1001, 1001A)

Zenith 970, 975 (chassis 5902)

Zenith 980, 985, 990 (chassis 1201 1201A)

Zenith "Stratosphere" 1000 Z (chassis 2501)

Zenith "Stratosphere" 16-A-61, 16-A-63 (chassis 1601-C, 1601- P)

Zenith "Stratosphere" 25 tube model (serial nos. 174001 to 174100)

Zenith "Stratosphere" 25 tube model (after serial nos. 75406)

Zenith 760, 765, 767 (chassis 2054)

Intermittent reception. Fading. Motorboating sound.

1. open-circuiting r-f, first detector and i-f secondary-return by-pass condensers.

2. check 15,000-ohm resistor (No. 63-301) for decreased value. Replace with 10-watt unit.

Whistling or squealing. Hum.

1. add 8-mfd electrolytic filter condenser after first speaker field to chassis.

2. add additional filter choke between 80 tube filament and first speaker field, and connect 8-mfd electrolytic filter condenser from 80 filament to chassis.

Hum (after regular values of filter condensers have been installed).

1. by-pass the .5-mfd condenser next to the primary of the input transformer with an 8-mfd electrolytic condenser.

Hum at resonance.

1. cathode-heater leakage of type 56 oscillator tube.

Distortion.

1. by-pass the center-tap of the volume control resistor to ground with a .00015-mfd mica unit, thereby reducing the r-f load on the grid of the first a-f tube and allowing greater a-f amplification.

Distortion. Poor AVC action.

1. leaky or short-circuited secondary-return by-pass condensers.

Zenith 770 (chassis 2032)

Inoperative.

1. open-circuited shadowgraph.

2. open-circuited audio coupling condenser.

Very weak. Distorted reception.

1. open-circuited audio coupling condenser.

No AVC action. Distortion at resonance.

1. open-circuited AVC grid-coupling condenser.

Motorboating sound between stations.

1. open-circuiting r-f first detector and i-f secondary-return by-pass condensers.

Hum at resonance.

1. cathode-heater leakage in type 56 oscillator tube.

Intermittent reception. Cutting off upon vibration.

1. type 58 tube bias resistor grounding lug loose.

Inoperative. Shadowgraph functions normally. Intermittent reception.

1. open-circuiting audio coupling condensers.

2. open-circuiting diode audio coupling condensers.

Intermittent reception. Fading. Whistling or squealing. Station hiss.

1. open-circuiting grid filter condensers in r-f, i-f, and first detector secondary-return circuits.

Intermittent reception.

1. leaky insulation in AVC resistor by-pass condenser lead. Slip a piece of spaghetti over this lead.

Zenith 770B (chassis 2059)

Inoperative.

1. check for open 1/4-watt 99,000-ohm diode load resistor on 56 diode second detector tube.

Zenith 775 (chassis 2032)

Same Case Histories as listed for Zenith 770 receiver.

Zenith 775B (chassis 2059)

Same Case History as listed for 770B receiver.

Zenith 801

Weak reception.

1. if all parts and tubes check OK, replace filter condenser block.

Zenith 805, 807, 809

Inoperative over part or all of tuning range.

1. leaky .01-mfd oscillator plate coupling condenser (connected between oscillator plate prong and oscillator coil. Replace with 600-volt unit.

Insensitive. Inoperative above 900-kc.

1. short-circuited oscillator plate condenser. Replace with .01-mfd unit

Fading. Distortion.

1. gassy 42 tube.

2. faulty .02-mfd coupling condenser. Replace with 600-volt unit.

Distortion (model 807).

1. occasional short in .005-mfd condenser which connects band switch to ground by one of the many terminals on the switch.

Zenith 835 (chassis 1001, 1001A)

Intermittent reception. Fading. Inoperative.

1. open-circuiting .02-mfd audio coupling condenser.

2. rear right chassis bolt tightened too much.

Whistling or squealing. Motorboating sound.

1. open-circuited .1-mfd r-f screen by-pass condenser.

Zenith 870 (chassis 5701-2-3)

Inoperative below 700 kc.

1. check for shorted or leaky .01-mfd 400-volt condenser in oscillator plate circuit.

Zenith 880 (chassis 1001, 1001A)

See also Case Histories listed for Zenith 835 receiver.

Distortion. AVC tube blocking.

1. short-circuited .0004-mfd condenser in first r-f coil can (No. 22-285).

Zenith 970, 975 (chassis 5902)

Inoperative. Screen voltages on r-f, i-f, and translator tubes only 20 volts. Positive indication from control grids to ground.

1. screen dropping resistor short-circuiting to diode load resistor. Clear by moving.

Intermittent reception. Popping noises.

1. open-circuiting diode audio coupling condenser.

2. intermittent short-circuiting of screen voltage dropping resistor to diode load resistor.

3. open-circuiting r-f grid filter condenser.

Zenith 980, 985, 990 (chassis 1201 1201A)

Weak. Noisy. Broad tuning.

1. check d-c resistance of primary and secondary windings of i-f transformer. If it is much above 5 or 6 ohms, replace transformer.

Zenith "Stratosphere" 1000 Z (chassis 2501)

Weak distorted reception.

1. check driver transformer (1) for open or short. The audio signal will be weak, and a hum will be evident if this transformer is shorted.

2. open 1,500-ohm resistor (R-28) in cathode circuit of first audio stage.

Poor low-frequency note reproduction. High-frequency note reproduction over-accentuated.

1. open center-tap on driver transformer (1).

2. open condenser (C-14) in cathode circuit of 85 tube.

3. grounded 50,000-ohm resistor (R-4) on tone control.

4. shorted section in "low-boost" transformer.

Motorboating sound on all bands.

1. open .5-mfd condenser connected to the 85 tube socket.

Fluttering as tone control is rotated.

1. open or shorted .1-mfd condenser (C-4) in the tone control circuit.

Zenith "Stratosphere" 16-A-61, 16-A-63 (chassis 1601-C, 1601-P)

AVC blocks.

1. open AVC resistor or grounded circuit, noticeable by distortion. Usually clears up when aerial is removed.

Distorted and weak.

1. shorted i-f grid circuit.

Signals tune sharp.

1. noise level low off the signal - check 6H6 tube.

Pointers will not turn.

1. Belt off pulley.

Split second hand will not turn.

1. binding on cabinet or loose planetary lugs - adjust carefully.

Lack sensitivity with oscillation on "D" band around 12 megacycles.

1. check for open antenna choke of antenna coil.

Distorted.

1. check tubes.

2. 20-mfd condenser (no. 22-438) shorted.

3. AVC grounded or open.

4. open resistor (no. 63-478)

5. One-half of push-pull transformer open.

Distorts at full volume (do not confuse with overloading which is a natural characteristic on strong input signal).

1. check condenser .05-mfd (no. 22-171) for short. Set will be weak if open.

No minimum volume control and also lacks bass response.

1. check high fidelity choke (no. 95-321) for open. There will be a lack of high response if shorted.

Dead as tone control is turned on bass only.

1. check .01-mfd condenser (no. 22-196) for short. Set will lack bass response if this condenser is open.

Lack bass.

1. check .03-mfd condenser (no. 22-287) for open.

Graph will not operate.

1. defective 6C5 tube or open graph - burned out pilot bulb.

Zenith "Stratosphere" 25 tube model (serial nos. 174001 to 174100)

Hums.

1. check tubes and filters.

Hums, weak and distorted.

1. check transformer #95-250 for open winding, weak audio if shorted.

Too much high, lacks low notes.

1. driver transformer #95-250 open center tap.

2. 5-mfd 25-volt condenser open.

3. 50,000-ohm resistor grounded on tone control to bottom plate.

4. one-half of low boost shorted secondary.

Weak and distorted on all bands.

1. 1500-ohm resistor across #22-189 25-volt 2-mfd open.

Blocks up - tendency to slow motorboat.

1. check red wire from 10,000-ohm resistor (#63-407) to 10,000-ohm resistor on 5-meter coil for ground. Set seems alive but no signal if this resistor is open.

Motorboats on all bands.

1. .5-mfd condenser (#22-223) to 85 socket open.

2. i-f shorted, graph won't narrow and set will distort on edge of carrier.

Set dead.

1. check tubes, filters, coils and transformers and i-f for grounds or open.

Weak audio and high notes (hum level increases slightly if you pull one push-pull 42 tube).

1. open secondary on low boost #95-251. Disconnect high boost and tone control circuit before testing.

Tone control no effect on highs, set flutters as tone control is rotated to bass.

1. check .1-mfd condenser #22-377 for open or short.

Zenith "Stratosphere" 25 tube model (after serial nos. 75406)

Hums.

1. check tubes, filters, voltages, ect.

Hums, weak and distorted.

1. check transformer #95-250 for open winding, weak audio if shorted.

Too much high, lacks low notes.

1. driver transformer #95-250 open center tap.

2. 5-mfd 25-volt condenser open.

3. 50,000-ohm resistor grounded on tone control to bottom plate.

4. one-half of low boost shorted secondary.

Weak and distorted on all bands.

1. Audio bias resistor across 20-mfd 25-volt condenser #22-189 open.

Weak audio and high notes (hum level increases slightly if you pull one push-pull 42 tube).

1. open secondary on low boost #95-251. Disconnect high boost and tone control circuit before testing.

Tone control no effect on highs, set flutters as tone control is rotated to bass.

1. check .1-mfd condenser #22-377 for open or short.

Set dead.

1. check tubes, filters, coils and transformers and i-f's for grounds or open.