April 1933 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
Radio Service Data Sheets
These schematics, tuning instructions, and other data are reproduced from my collection of vintage radio and electronics magazines. As back in the era, similar schematic and service info was available for purchase from sources such as SAMS Photofacts, but these printings were a no-cost bonus for readers.
The superheterodyne series of Crosley Roamio sets; Radio Service Data Sheet No. 87 describes the T.R.F. series.)
Average operating potentials are given in the following tabulation. These figures are measured with the speaker connected; a high-resistance meter will be required to obtain correct readings.
*A. V. C. only.
The pole of the "A" battery which is not directly connected to the frame of the car connects through the insulated battery cable lead to the cable connector of the receiver, and thence to one pole of the power switch; this D. P. D. T. unit controls both the "A" and "B" circuits. After going through the switch, the "A" circuit branches, one branch going to the heaters (connected in parallel) and through them to the chassis, another going through the dial light to the chassis, and the third going through the speaker field, to the speaker cable shield, and then through the battery cable shield to the other side of the "A" battery which connects to the frame of the car.
Tube V1 incorporates the dual action of oscillator and first-detector. Tube V4 is connected as a diode and serves the dual functions of automatic volume control and A.V.C.
The grid of the output pentode, V6, is connected to the chassis through a grid leak of 0.3-meg. (in models using a type 38 pentode this resistor has a value of 0.5-meg.). The output volume is determined by the setting of the 3-meg, potentiometer, R1.
The A.V.C. potential is developed across resistors R2, R3, which connect to the plate of the second-detector, V4. The maximum negative potential (the total drop across R2, R3) is applied to the control-grid of the pentode I.F. amplifier, V2, through filter resistor R4. A lower potential is obtained at the center-tap of resistors R2, R3, and this voltage is applied to the control-grid of the screen-grid I.F. amplifier, V3, through filter resistor R5. Thus, resistors R2, R3 serve the dual functions of supplying A.V.C. potential, and at the same time acting as the load resistor combination across which is developed the A.F. potential which is applied to the .03-mf. coupling condenser and thence to the combination grid-leak and manual volume control potentiometer R1.
The two 8 mf. condensers are electrolytic units and are contained in one can.
To re-align the set, turn the station selector to 550 kc., and adjust the service oscillator to 181.5 kc., after first connecting it, through a condenser of 0.1-mf., to the control-grid of V1. Ground the other side of the service oscillator, and do not remove the control-grid clip-wire (of the set) from V1. Next, adjust the I.F. trimming condensers of I.F.T.1 for maximum reading, and then adjust the trimmers in shunt to the secondaries of I.F.T.2 and I.F.T.3, respectively, for maximum output-meter reading.
After this step has been completed, the antenna and oscillator circuits, respectively, may be aligned.
Police Roamio model 951 is the same as the model 95, except for the coils which are designed for the police band.
Following are the average operating potentials to be measured with a high-resistance meter.
The aligning procedure in connection with this receiver is the same as for the Model 95 chassis, except that the high side of the service oscillator connects through a condenser of 0.1-mf., not to the first tube in the set, V1, but to the first-detector, V2, for making the I.F. adjustments. After setting the station-selector dial to 550 kc., adjust the service oscillator to 181.5 kc., and proceed as previously described.
Then, tune the service oscillator to 1,400 kc., set the station-selector dial to 1,400 kc., and connect the high side of the service oscillator, through a condenser of 250 mmf. to the antenna lead of the receiver; the low side of the service oscillator connects to the chassis.
It is preferable to use a dummy antenna in making these adjustments. Align the I.F. and R.F. circuits for maximum reading on the output meter. The circuits of L1, L2 and L3 should not be adjusted until the I.F. circuits have been aligned.
The action of the A.V.C. section of this receiver model is a bit more complicated than that of the previously-described Roamio A.V.C. circuits. Consequently, the interested Service Man is referred to the September, 1932 issue of Radio-Craft, which contains a lengthy description of the diode-triode tube, in the article; "Still More New Tubes"; fundamental data regarding A.V.C. circuits in general are discussed at considerable length in the article, "Operation and Service of Automatic Volume Control Systems" (in the same issue).
The manual volume control resistor, R1, has a value of 3 megs.
Variation in tube current supplied by the automobile storage battery, due to fluctuations caused by the generator and load on the battery, is a cause of so-called "interference" in many automobile receivers. Although these Roamio models are designed to eliminate this effect as much as possible, in order to insure the best reception it may be advisable to pay particular attention to the connection of the yellow "A" lead, running it direct to the car battery rather than to other possible locations to the "A" supply.
Whether filter condensers will be required in shunt to the electric horn, electric windshield wiper, electric fan, etc., may be determined by shunting these units with a, test condenser of about 2 mf.
Posted January 28, 2015