Service Shortcuts December 1932

Due to a technical problem, this post was initially published with no content on Tuesday, December 29th. Retrovoltage regrets this error.

Hints for the service shop, from December 1932’s Radio Retailing:

A.V.C. Accessory for Screen-Grid Sets
by Joseph E. Soos

A simple automatic volume control arrangement which works well with most sets using screen-grid type 24’s in r.f. stages is shown in the diagram. All models in which I have installed this system perform excellently.

A 27 tube is connected in such a manner that the voltage applied to the r.f. grids is automatically varied in relation to the signal input. Wired on a small panel, the 27 unit is connected to the receiver by simply breaking the screen-grid lead to the r.f. tubes and connecting the plate lead of the 27 to this point instead, and by tapping the grid lead of the 27 into the detector grid circuit (second detector in case of a super) without breaking the original grid lead.

dec32-1

In order to control the volume properly the screen-grid potential must be made variable over a considerable range. This is achieved by adjusting the bias of the volume control tube with a 50,000 ohm potentiometer. The plate current passing through the 35,000 ohm resistor provides the necessary drop to vary the voltage over the required automatic range for control of r.f. amplification. A signal applied to the grid of the 27 control tube reduces its bias and consequently increases plate current, providing an automatic decrease in gain.

The constants of the circuit must be proportioned as to function rapidly, while electrical inertia must still be great enough to avoid any possibility of swamping out low-frequency modulation as this is actually slow changing of signal input.

Since the volume control tube must have its plate at the same potential as the screens of the r.f. amplifier it is necessary in order to obtain the correct voltages on the 27 to take off voltage taps at minus 60 and minus 80 (with respect to ground) on the voltage divider of the receiver’s power supply unit. This puts a potential of approximately 135 volts on the plate with respect to the cathode. Ground all r.f. tube cathodes.

Curing Hum With the 56
by Boris S. Naimark

Many of the older sets using 27 detector and 26 first a.f. stages have an annoyingly high hum-level. The trouble arises in the audio stage and the remedy is to substitute a heater type tube for the 26. A 27 could be used for this purpose were it not for the fact that the 2.5 volt transformer winding could probably not supply filament current for two 27’s without serious overload.

The answer is the 56. Substitute 56s for both detector and first a.f. tubes, running their heaters from the 27 transformer winding. This places only 1/4 amp. overload on the winding, which it will probably stand without danger.

The 56 is installed as a detector in the same manner as the original 27 with the single exception that the cathode is connected to the center tap of a 50 ohm resistor placed across the heater winding as shown in the accompanying diagram. Use wafer sockets for the substitution and it will be found that the smaller height of the 56 will permit tubes and wafer socket to fit in the average set with room to spare.

dec32-2

Substituting 47’s for 45’s
by J. P. Kennedy

Adapters which permit substitution of 47’s for 45’s do not ordinarily change the value of the C bias and consequently do not permit maximum results from the pentodes. This can be corrected in two ways: The first is to substitute a 420 ohm resistor, in the case of a single power tube, for the 1,500 ohm C bias resistor used with the 45, or a 210 ohm resistor for the 750 ohm unit in a push-pull or double power-tube arrangement. This means opening the chassis, an awkward thing to do, especially when trying to sell a customer the changeover idea.

The second method consists of bridging a 20 ohm center-tapped resistor across the filament circuit of one power tube by means of an external adapter and shunting a 600 ohm (584 is the exact value) one-watt resistor from the center tap to the chassis or ground post. Inasmuch as the two bias resistors – the original and the new – are now connected in parallel this gives a net value of 428 ohms, which is close enough to the recommended 420 to work satisfactorily. When two power tubes are used, half the resistance (300 ohms) with a two-watt power raging will bias the 47’s properly.

As a further improvement, for the sake of tone quality, a 10 mfd., 25-volt electrolytic condenser across the external C bias resistor will effectively by-pass the lower audio frequencies. As the above changes can be made quickly without opening the hcassis it should be easy to sell an adapter, consisting of resistors and by-pass condenser, plus new tubes by actual demonstration.

I.F. Alignment Kink
by Lloyd H. Harder

The i.f. stages of a superheterodyne may be readjusted to the proper frequency without a special oscillator if another super using the same intermediate frequency and an ordinary broadcast-band oscillator are available.

Connect the ground posts of the two sets together. Then fasten a wire between the grid of the standard receiver’s second detector and the grid of the repair’s first detector. Connect the r.f. oscillator to the standard set in the normal manner and tune both oscillator and standard to 1,400 kc.

Connected in this manner the output of the standard receiver’s i.f. amplifier, a signal of the desired intermediate frequency, feeds into the i.f. amplifier of the set under test in precisely the same manner as would a special i.f. oscillator. Thus the repair job may be balanced in the usual fashion, its i.f. stage trimmers being adjusted for maximum output.

It may be necessary to cut out the oscillator of the repair by grounding the cathode tap of its oscillator coil, particularly if a combination first detector and oscillator are employed. It may also be necessary to disconnect the permanent grid lead to the set’s first detector.

Twin Speaker Installation
by W. T. Golson

Two dynamic speakers may be connected in a twin arrangement, one reproducing treble notes best and the other bass, by utilizing two output transformers and two .002 mdf. condensers. Select a large cone for the bass and mount it on a large baffle. (In consoles the built-in unit is usually suitable.) Obtain a small cone for the treble and mount this on a small baffle, or in the bottom of the console cabinet facing the floor. Connect the output transformers and condensers as shown in the diagram.

The condenser, in series with the primary of the transformer feeding the treble speaker will pass high frequencies better than low to thus unit while the condenser shunting the primary of the transformer feeding the bass speaker short circuits high frequencies out of this cone. In one satisfactory installation I used a 13 in. cone for bass and a 6 in. unit for treble, both being dynamics.

dec32-3

Curing Critical Volume Controls

Many supers that use 51’s and 35’s in their i.f. stages employ a 10,000 ohm volume control to simultaneously vary cathode bias and also to shunt the antenna circuit. A characteristic of such sets is a rather critical point between minimum and maximum volumes. This is often so critical that the user cannot control the volume of dx signals.

Observe the position of the volume control arm at the critical point and with an ohmmeter measure the resistance from cathode to ground. Wire a fixed resistance having approximately twice the observed value from the cathode of tube to ground. The control will then work smoothly over its entire range.

dec32-4

Miscellaneous Hints

STROMBERG 29. On-off switch and tone control unit is electrically and mechanically identical to the phonograph and pickup switch and volume control unit. When on-off switch contacts are discovered to be burned out, and the set is not equipped with a pickup, interchange the two units, making sure that the “jumper” across the pickup input is in place. This saves $1.95 until the customer wants to use a pickup.

PHILCO 70, 90. If airport beacons operating on 260 kc. cause interference, readjust the i.f. compensating condensers and the oscillator compensating condenser to either 250 or 270 kc.

RCA M30. Lack of complete manual volume control is usually an indication of trouble in the a.v.c. circuit.

RCA R34, R35, R39, RE57. 90 per cent of the trouble experienced with these receivers may be traced to the 70,000 ohm red and green resistor in the plate circuito f the first audio stage, the 1 1/2 meg. red and white resistor in the detector control grid circuit and the 1 1/2 meg. blue and green resistor located under the resistance board.

CROSLEY. Certain Mershon condenser models hum and may be repaired by drilling a 1/4 inch hole in the bakelite top of the electrolytic unit, being care3ful not to damage the “innards”, filling with distilled water to a point about 1/4 inch from the top and closing the hole up again with sealing wax. Discharge the condenser before drilling.

AK. Volume controls which become noisy need not always be replaced. Remove them from the chassis, swab the winding with a cloth saturated with alcohol, bend the slider arm so that it makes firmer contact with the winding and also tighten it against the tension spring.

FADA. Some of the older models use special knobs equipped with tension springs which fit into a notch cut into the shaft. To hold these in place while replacing knobs first put a little soft pitch or candle tallow in the notch.

COLONIAL 32. Loss of volume accompanied by poor tone is usually due to an open first audio bias resistor. These are of the flexible type and breaks generally occur near either end. Unwind a few turns of resistance wire to cut out the break and resolder.

SPARTON 410. Type 45 power tubes may be substituted for the 183’s by rewiring the output stage filaments in series, including a half-ohm resistor in the circuit.

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