[RADIO Q&A

RESTORATION
TIPS, HINTS,
& KINKS

INDEX
Checking Signal Generator Calibration
When aligning a receiver, it is important that the frequency dial of the signal generator be accurately calibrated. This is to assure that you align the I-F and R-F circuits to the frequencies as specified in the alignment procedures. It is not necessary to check the calibration of the generator at all points of the dial, as there are just a few test frequencies that are use in the alignment of most receivers. These include several I-F frequencies, and a few points in the broadcast band.
A quick and accurate check of the calibration would be to use a frequency counter which would give a direct readout of the generator frequency. Most generators have adjustments that allow the frequency to be shifted, so it would just be a matter of setting the dial on the frequency to be checked and then adjusting either the coil or trimmer capacitor to make the frequency agree with the dial setting.
Lacking a frequency counter, there are other ways to check the calibration. Stations on the broadcast band maintain their frequency to very close tolerances, and we can use these stations on known frequencies to check the generator.
Lets see how we go about making the check suggested above. Tune a receiver in to a station of known frequency on the low end of the broadcast band, somewhere around 600 kHz. Lets say we have one at 610 kHz. Connect the output of the signal generator to the antenna through a small value capacitor (around .0002). If the receiver has a loop antenna, you can couple the signal by making a loop of a few turns of hook-up wire, which is connected to the generator output and then brought near the receiver loop.
With the receiver tuned to the station, tune the signal generator toward 610 kHz. As the signal generator frequency approaches the frequency of the station, a high-pitched whistle will be heard in the receiver, and will become lower as the generator approaches closer to the station frequency. This phenomenon is known as beating of the two signals. The tone heard is the difference between the two frequencies. When the signals are 1,000 cycles apart, in other words the signal generator frequency is 611 kHz, a 1,000 cycle note is heard, when 500 cycles apart, a 500 cycle note is heard and so on. When the two frequencies are the same, the note disappears. The two frequencies are now zero-beat with each other.
If the dial of the signal generator agrees with the station frequency, then it is in calibration, but lets suppose that the zero-beat occurres when the generator dial reads 620 kHz. This would mean that the generator is off by 10 kHz. There are a couple of options to set the calibration. One is to adjust the frequency of the generator so that when the dial is set to 610 kHz, the generator frequency agrees with the station frequency. This can be done by adjusting the compensating circuits in the generator. Some of the more economical units have only an adjustable slug in the coil to make any adjustments. This arrangements makes it difficult if not impossible to get the frequency accurate over any wide portion of the band. Better units will have an adjustable slug in the coil to set the low end of the band and a trimmer capacitor to set the high end. If this is the case, then it is just a matter of setting the generator dial to indicate 610 kHz and then adjusting the coil slug until the two signals again zero-beat. The same procedure is use for the upper end of the band using a station somewhere around 1,500 kHz and adjusting the trimmer capacitor. After setting the high end of the band, re-check the lower end. The manual for the signal generator should give the proper procedure for setting calibration.
The I-F frequencies, being below the broadcast band, require a little different procedure, since we can't tune in a station this low in frequency. For this, we will use harmonic frequencies. A harmonic is a frequency that is a multiple of the fundamental (main) frequency. The fundamental is called the first harmonic, the second harmonic would be twice the fundamental, the third harmonic, three times the fundamental, and so on. Every oscillator produces harmonic frequencies, which includes our signal generator. So, if our generator is set to 455 kHz, it is not only putting out a signal at 455 kHz, but is also producing signals at 455 X 2 = 910 kHz, 455 X 3 = 1,365 kHz, 455 X 4 = 1,820 kHz, ect. It should be pointed out that each harmonic signal is weaker in signal strength than the previous one.
Now, since the second harmonic of 455 kHz falls within the broadcast band, we can use this to check the calibration of the generator at this lower I-F frequency. Set the signal generator up as before, with the band switch set for the band that tunes 455 kHz, and tune the receiver to a station somewhere on the low end of the band. Lets suppose there is one at 880 kHz. When the signal generator is tuned to zero-beat this signal, the dial should read 440 kHz as 880 kHz is the second harmonic of 440 kHz. If zero-beat is obtained at 445 kHz, the signal generator is off by 5 kHz. Even lower frequency ranges, such as 175 kHz, can be calibrated the same way by using the higher harmonics. For instance, the 4th harmonic of 175 is 700 kHz, the 5th is 875, ect.
A special precaution must be noted when checking the calibration in the I-F ranges. If the receiver has an intermediate frequency equal to the frequency being checked on the generator, a double zero-beat may be obtained since the signal generator will also be beating against the intermediate frequency of the receiver. A receiver with an R-F amplifier stage, or one with an intermediate frequency different from the one being checked, will help to reduce this problem.
In the case where the calibration cannot be set accurately at the different ends of the band scale, a note should be made of the calibration error at that particular point of the dial so that the error can be compensated for when using the generator.
Index
Rider's Manuals

The following chart gives the approximate years covered by the Rider's manuals for volumns 1 through 19. This information comes from various advertising literature.

VOL   YEARS      VOL   YEARS      VOL   YEARS
1   1920s to
late 1930		      8   Oct 1936
Oct 1937		      15 1946
2   late 1930
late 1931		      9   Oct 1937
Oct 1938		      16 late 1946
early 1947
3   late 1931
late 1932		      10   Oct 1938
Aug 1939		      17 1947
4   Nov 1932
Oct 1933		      11   Aug 1939
Jun 1940		      18 Nov 1947
Nov 1948
5   Nov 1933
Oct 1934		      12   To April
1941		      19 Nov 1948
late 1949
6   late 1934
Nov 1935
      13   late 1941
early 1942		           
7   Nov 1935
Nov 1936		      14   late 1942           

Index
Rebuilding Philco Bakelite Block Capacitors

There is an excellent article on rebuilding Philco Bakelite block capacitors on Chuck Schwark's website at:
http://www.philcorepairbench.com/capbuild.htm
plus lots of other good information on Philco.

Index
Cleaning Tuning Capacitors
To clean tuning capacitors, I soak them in a solution of half and half of Naval Jelly rust remover and water. Place enough solution in a plastice container to cover the capacitor and let is soak for an hour or two. After soaking, remove the capacitor and rinse thouroughly with water. A tooth brush is good to help remove any left over residue.
I use an air compressor to blow dry being careful of the mica under any trimmers, then set in the sun for an hour or so to fully dry, or place in an oven set to warm. Be sure to lubricate the bearings before reinstalling.

William Bittle says that one of the problems he has noticed with tuning capacitors is the loss of connection between the tuning shaft and the wipers at the end of each section. He finds that spraying the shaft area with a commercial tuner cleaner, then washing that off with isopropyl alcohol, and finally putting few drops of "Deoxit" on the ground wipers will restor the continuity. Thanks for the tip William.

Index
Speaker Repair
Minor tears in speaker cones can be repaired by brushing on a coat of varnish then layering with facial tissue paper and coating each layer of paper with varnish. I usually apply at least four or more layers of tissue. If the speaker cone is brittle, it can be strengthened by painting the entire cone with rubber cement. The cement will be absorbed by the paper yet allow the paper to remain flexable.

An off center voice coil that is rubbing the pole piece will cause the speaker to rattle and distort the sound. You can check for this by gently moving the cone up and down, using equal pressure on opposite sides of the voice coil. If there is drag, you can feel it as well as hear it if you hold the speaker up close to you ear. The cure for this is to re-center the voice coil. Some speakers, if you look down into the center of the cone, you will see a spider with a screw in the center that screws into the pole piece. By loosening this screw you can re-center the voice coil.
Other speakers, may have the spider mounted under the cone and is attatched to the speaker frame with screws, which can be loosened and the spider moved to adjust the voice coil.
If, however, the spider is glued to the speaker frame you can sometimes use a q-tip and apply acetone to the edges and un-glue the spider from the frame. Re-center the voice coil and re-glue.
To help in re-centering the voice coil, cut small strips of paper that you can use for shims to place between the voice coil and the pole piece. If there is a dust cap in the center of the cone over the voice coil, it will have to be removed. A circular piece of felt can be cut and glued in place for a new dust cap.
Assuming the voice coil properly centered, rattling, especially on bass notes, may also be caused by loose components such as; cone un-glued from edges of frame, voice coil loose from the cone, spider un-glued from frame, or tears in the cone. The cure is to re-glue the loose components.

George Clare of Aldergrove B. C., Canada offers the following tip on centering the cone.
An added improvement to centering the voice coil and excellent for removing buzz or rattle is to rig up a 110v to 6v transformer with a pot (wire wound) so you can adjust the 6v a/c side to zero, feed this through a 25 ohm 5 watt resistor, to the voice coil of speaker,adjust volume with pot and listen for any buzzing or rattling and adjust centering until you hear a smooth a/c hum.

Index
Repairing IF Transformers
There is an excellent article by Daniel Schoo in the September 1992 issue of Antique Radio Classified on the repair of IF transformers.
Index
Troubleshooting
Harley L. Miller contributes the follow time tested method that is often good for finding those loose connections and bad solder joints that cause intermittent problems.
"I was working on a Philco 112, and couldn't find the source of a buzzing, arcing sound. I used an old Army trick of tapping components 'gently' to get a response, and found one connection which had been wrapped but not soldered. Still had an arcing sound, so continued with the tapping and found the sound to be comming from the "max/normal" switch. Spraying the switch with contact cleaner and working the switch briskly cleared up the problem."
Be sure and use a non-conducting tool to do the tapping. - BH
Index
A Replacement for the 1L6 in Zenith TransOceanics
In the T/O's that use the miniature all-glass tubes, the 1L6 pentagrid converter can be hard to find and expensive. One trick is to replace it with a 1R5. The electrical characteristics of the 1L6 and 1R5 are different and this can lead to problems of low sensitivity on some of the higher frequency bands and alignment problems.
Fred Gordon of Fred's Old Radio Emporium in Corsicana, Texas offers another solution.
Upon checking his RCA tube manual, Fred discovered that the 1LA6, which is used in the earlier T/O models, is an electrical equivalent of the 1L6. The 1L6 is a 7-pin miniature, while the 1LA6 is an 8-pin loktal.
As an expirement, Fred wired the 1LA6 loktal tube base to a 7-pin male plug that will insert into the orginal 1L6 socket on the T/O chassis. The wiring between the 8-pin loktal socket and the 7-pin male plug is pin-for-pin except for one wiring change. The interconections between the male plug and the loktal socket are shown below. 

1L6  Socket Pin #  1  2  3  4  5  6  7  
                   |  |  |  |  |  |  |
1LA6 Socket Pin #  1  2  3  4  5  6  8
Fred says that after completing the modification the set worked on all bands from broadcast to 18 MHz with normal sensitivity. The dial calibration on this particular set was a bit off, maybe due to the extra wire length used in the modification, which upsets the original local oscillator portion of this circuit. This was corrected by a touch-up of the oscillator coils for each band.
Index
Battery Cable Lead Color Code - Standard RMA Color Code for the wires comprising the cables used for connecting battery-operated receivers to batteries. All leads are solid color.
Index