Allen B. Dumont Labs., Inc. 183X
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|Manufacturer:||Allen B. Dumont Labs., Inc.|
ALIGNMENT AND PRODUCTION TESTING OF TELEVISION RECEIVERS.
1. AUDIO CIRCUITS
Operation of the audio frequency amplifier may be checked by touching the grid of the 607 and noting hum pickup in the
peaker. The first video IFT should be adjusted roughly as it affects the sound I.F. characteristic.
Next, the I.F. Amplifier should be aligned, using an oscillograph connected to the screen of the 2nd I.F. tube screen by pass removed) and a wobbulator connected to the proper points to indicate the desired characteristic. The 2nd I.F.T. should be adjusted first with the wobbulator connected to the grid of the first I.F. tuds (1851), and then the first I.F.T. Adjusted with the wobbulator connected to the converter grid. Next the 6J7 screen by phas should be put back, the oscillograph shifted to the diodo output at the first audio coupling condenser, the.001 de-emphasis condenser opened, and the discriminator transformer adjusted.
The output of the wobbulator should be of the order of 5000 microvolts which do y be obtained from the RCA wobbulator using the low tap with a 10 ohm resistor Bhunting it to ground. signal generator may be used as a marker connecting it to the wobbulator output (10x tap) through a 100 ohm resistor.
The appearance of the characteristics are indicated by the sketches below. The exact Shape of these curves will vary somewhat with individual receivers.
A check on alignment should be made using a Ferris signal generator with about 5000 microvolts output. Tuning the signal generator through the band two equal peaka (20 %) and a null point should do observed. The null point should be at 8.25 mo (+ 15 kc).
Sensitivity should then be checked using the Farris 818 nal generator connected to the converter grid, and the oscillograph connected to the plato of the 6V6G audio output tube. Tho input for an average output for the two peaks of 60 V. p. to p. (1 direct on 168) should be from 30 to 100 microvolts. (At this low input the two peaks may not be exactly equal due to the fact that signal level affects the I.F, tuning to some extent.)
2. VIDEO I.F. CIRCUTIS
An oscillograph is attached to the 6H6 video detector load. An I.F. wobbulator is connected successively to the last I.F. stage, next to the last, and so on, back to the mixer grid with adjustment of the corresponding I.F. transformer at each stop. In this alignment the overall curve is approximately that shown below. This sketch is illustrative of several receivers but the exact amount of dip is somewhat variable and the final adjustment generally involves use of an actual test pattern received by R.F. It is desirable that the video I.F. alignment shall have the 6db attenuation at the carrier to provide successful reception of the single side band transmission. 4 of the 5 picture I.F. transformers are triple tuned while the first I.F. transformer is a double tuned unit. When tuning the video I.F. transformer in the plate of the mixer tube, the R.F. circuits should be disconnected from the grid of the mixer before attaching the I.F. signal wobbulator to this grid go as to insure flat input.
The trap to reject the adjacent channel picture carrier and the traps to project the associated sound carrier are all pro tuned and need no further adjustment. These traps are tuned in manufacture using a Q-meter.
The R.F. circuits are aligned by using an input wobbulator having relatively high voltage of the order of l volt covering the channels as follows:
1 50-56 Mc
2 60-66 Mc
4 78-84 Mc
6 96-102 Mc
To determine the characteristic of those RF circuits Independent of I.F. response, an oscillograph is connected with its grounded terminal to the B plus supply (using care not to touch the oscillograph) and with its vertical input amplifier connected to the mixer screen. In this way the mixer screen response represents quite adequately the band pass characteristics of the R.F. Circuits. This high level wobbulator is applied to the antenna terminals, following which the R.F. antenna coil and the mixer grid coil are tuned in the corresponding condensers for each band. The response curve for each band is represented by the sketch above, showing the response for one of the Bands which is typical of all of them. The higher channel are somewhat broader than this. During this alignment the oscillator tube has been removed.
Alignment of the oscillator itself is made by using a signal generator tuned to the carrier frequency for the sound channel. Then the oscillator trimmers are adjusted for each of the 4 channels mentioned above so that the sound carrier is received as indicated by the loudspeaker. To insure that the oscillator is tuned above the desired carrier the signal generator is then tuned to the picture carrier and a check of received signal is made through the video channel. Another check it to see that the minimum capacity of the oscillator trimmer is used where it is possible to get 2 oscillator frequencies which pass a sound signal. This adjustment of the 0x011lator is made with the front knob trimmer set at 1/2 capacitance. A final sensitivity measurement is now made using the signal generator on the carrier frequencies for sight and sound for all 4 channels.
all 4 Channels..
The picture 14nsitivity should be approximately 200 micro Tolts Input signal on all channels to yield 15 volts peak to peak at the final video 6T6 amplifier plate, using an oscillograph for measurement and being a signal generator with 30% modulation.
While an attenuation ratio of 100 at the sound carrier WAS sufficient with A.M. sound, it is not adequate with F.M. sound. The Bignal generator should be tuned through the sound band which is 150 KO (75 Kc) and the attenuation ratio should be at least 100 throughout this band at R.F.
Adjacent Channel Sound Rejection
Previously rejection ratios of 1000 to 1 At R.F. was attained, measurements should be made by tuning through the band as above and the ratio should be over 500 throughout the band. The sound sensitivity at R.F. should be approximately the same at 1.
VIDEO AND SWEEP CIRCUITS
This alignment of the video amplifier and the sweep circuits can be made either with an over-the-air test pattern or with a test pattern from a coaxial line. When an over-the - Air transmission is used the signal is applied to the antenna terminals.
However, when a coaxial line signal is used, it is necessary to observe the precaution of a suitable input network for Applying the signal to the grid of the 1851 first video amplifier tube, This tube has a fixed blas within the set to which its grid lead is returned and its cathode is grounded. It is therefore desirable to insert a coupling condenser of at least 0.1 mfd from the coaxial line and supply a grid leak from the 1851 grid load of at least megohm between the 1851 grid cep and the 1980 wir from beneath the chassis which would otherwise normally be connected to the 1851. In this way the proper fixed bias is still applied to this tube.
After alignment has been made as outlined above there are certain tests and precautions that should be followed closely in order to eliminate the possibility of shipping either defective receivers or those that are not up to standard in efficiency and quality. A co-ax line carrying composite video signal to be used for checking video amplifier and sync circuits should be monitored to make sure that Horizontal Blanking is no more than 16 % and front porch comprises 2 % of total. Vertical Blanking should be from 7 to 8 %.
The 1851 tube and 6V6 tube of the 8 atago video frequency amplifier have their frequency constants such that the overall response to the cathode-ray tube grid is essentially flat from 30 cps to 3 1/2 megacycles with a gradual drop to approximately 1 megacycles at which time the response is down to about 30 %. this original design was checked with the video frequency wobbulator and it has been found unnecessary to check each receiver individually except for general observation of a test pattern which is adequate to show up any actual mistake in the circuit wiring of the peaking coils, etc.
The sweep circuits are tested to do tarullo the adequacy of amplitude and frequency range. Linearity adjustment is made with the two linearity controls on the sweep deck. In case these adjustments do not cover sufficient range additional small capacitances are placed in parallel with the bottom condenser of the potential divider which feeds the grid of the sweep amplifier tube. This added condenser is actually placed from grid to ground of the sweep amplifier tube. In this way the ratio of signal from the oscillator to the signal from the amplifier will be controlled, thus correcting the linearity so that an overall linear sawtooth is produced by combination of sweep oscillator output which is exponential and a sweep amplifier output which by its grid characteristic produces a reverse curvature.
After linearity has has been adjusted the horizontal amplitude control should have at least one inch additional amplitude available. The vertical amplitude control should have several inches of additional amplitude available.
The black sweep control knobs, which are connected by turning the sweep selector switch on the front panel counterclockwise (to position 2), should be checked to insure that the vertical frequency range includes 30 and 60 fields per second with adequate overlap, and that the horizontal frequency range includes 8000 and 15,750 lines per second with adequate overlap.
The black knobs should be set up at the standard 525 lines 30 frames.
The red sweep control knobs, which are connected by turning the sweep selector switch clockwise (to position 1) should be capable of being adjusted to the following color combinations:
(a) CBS color pictures use 375 lines per frame at 60 frames per second which requires a horizontal scanning rate of 22,500 lines per second, and a vertical scanning rate of 120 field scans per second.
(b) NBC has transmitted color with 441 lines per frame and 60 frames per second, requiring 26,460 scanning lines per second, and 120 vertical fields per second.
The Du Mont sync transformer should be adjusted as follows:
A Du Mont picture signal should be applied to the 1850 first video grid in accordance with the previous instructions, or received over the air. A diode rectifier with its output connected to an oscillograph should be very loosely coupled to the grid of the horizontal oscillator (green lead on Du Mont sync transformer). This may be done by clipping a battery clip around an insulated portion of the green lead. The oscillograph sweep should be synchronized to the 60 cycle power line, the beam of the CRT should be cut off, and the sweep oscillator tubes of the television receiver removed. The Du Mont sync transformer should then be adjusted for maximum amplitude of the envelope of the H.F. burst pulse as indicated on the oscillograph.
The test pattern should be clean and crisp with no signs of any breakdown visible. Breakdown will cause intermittent black lines which jump back and forth vertically or horizontally tear out similar to that produced by noise, which is particularly noticeable at the black circle of the test pattern.
Very often faulty coupling condensers in the deflection circuits will cause this trouble and tapping them with an insulated red will help locate the faulty party.
DU MONT TELEVISION RECEIVER
TRE FOLLOWING information was compiled by the Service Department and is based on actual experience acquired in the field over the last three years.
ERECT the antenna in the clear whenever possible, as high and as far back from the street as possible.
ANTENNAS Over 13 feet high should be guyed securely. Mount antenna securely on a chimney or wall, using the hardware and mounting brackets supplied by the antenna manufacturer.
INSTALLATION crew should consist of two men. One man of the roof to rotate the attensa and locate the position. Another man at the set to watch the results. These two men should be in constant communication. Philco phones will serve the purpose. Connect the speaker of the Philco phones to as teet of transmission wire with a pair of insulation-piercing clips this enables you to clip on the lead-in wire without breaking the insulation. Connect the master station in series with the antenna lead - in wire and antenne plus at the set, thereby giving constant communication with the operator on the root without using extra lead-in wire for the phones.
THE DIPOLE (both rods) should be equal to one-half wave-length of the radiation to be received, for maximum result, and connected to the receiver by means of a transmission line-twin conductor.
FREQUENCIES assigned to each of the New York stations and dipole needed to match the wavelength are listed below:
THIS does not necessarily mean that a separate antenna is required for each station. It has been found that in most parts of the Metropolitan area. a satisfactory signal can be picked up from all three stations on an antenna tuned to WQSW's frequency, 90 inch dipole (45 inches euch rod) with reflection
HOWEVER, in some of the outlying points in Long Island, Westchester, and New Jersey it may be necessary to erect a second antenna tuned to WABD's frequency, 72 inches (36" each rod) to pick up their signal, until such time as WABD's power is raised to normal strength.
WHEN the receiver is located at a considerable distance from the transmitter, better pick-up and directional properties are required, and a second rod connected parallel to and 1/2 wave length behind the dipole will reflect the signal back and aid signal strength considerably. Tho reflector will also help reduce reflections.
THE LENGTH of the reflector rod should be slightly over the overall length of the dipole (see table),
WHEN TWO antennas of varying lengths are used, remember that the distance between the dipole and the reflector rod of each antenna should approximate wave length of the transmitted signal, or the length of the dipole.
METAL structures, large buildings in the path of the signal, will reflect the transmitted waves and tauge multiple " ghost " images on the screen of the receiver. These " ghosts " are very annoying and should be eliminated by rotating the antenna or changing the location. The use of reflector roda may at time serve the purpose. SOMETIMES, however, the reflected warta are a blessing in disguise, specially in large cities where low buildings are sandwiched in between high buildings. Very often it is possible to pick up a reflected signal below the line of sight, or turn the dipole completely away from the 1196 - of aight to eliminate " ghosts " and pick up a reflected signal with better results than on a direct pick-up, IN CERTAIN locations, in large cities, signals radiated by the various stations are reflected from many angles and it will be impossible to eliminate " ghosts " on all stations. 11t such cases, & second antenna will have to be erected. But, bear in mind that a satisfactory signal either direct or reflected, can always be picked up within the transmission radius with the proper equipment. TRANGNI SSI ON LINE TRE TWISTED pair transmission line or lead in wire used should have an inte pedance of 78 ohms per 100 feet. This type of transmission 11ne can be used in most installations, but it must be remembered that there is an appreciable loss of signal strength in ordinary twisted wires approximating 20 % for lengths from 100 feet to aoo feet. IN CASES where the signal strength to low to begin with, or where an or. ceptionally long lead-ia is required, co-axial cable it recommended. There is very little loss in comparatively long leagths 01 coaxial cable, WHEN the signal level ia weak the constrast or sensitivity control of the het her to be turned on " fu11 " and all kinds of noises will be picked up, latertering with the picturn. This kind of interference will cause small white spots and flasher similar to a snow storm on the acreon, and is known 46 " snow in the picture IF CO - AXIAL cable to used in such cases, the alqnal level will be raised and the noise level louared proportionately, giving a clear picture at all times
RERE are some of the most common service problems encountered in the field:
1.- Receiver dead... Sound but no picture
Plotate but no sound Poor sound
Poor synch picture tara out 6. Bright spot on screen of CRT Sound Ox