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|RADIOS SERVICED BY OBSERVATION
Sight, Hearing, Touch, Smell and Taste Are Valuable Instruments for Checking Receivers
By Lyle Treakle, Radio-Craft, September 1945
During about fifteen years of radio servicing I have noticed many beginners (and some not beginners!) tinkering with radios and getting nowhere. I have worked with a few so-called "engineers" and have. seen them search for many hours to discover trouble that would have been apparent at once if they had but used their knowledge and OBSERVED some things that are quite plain to see.
Careful observation will locate at least seventy-five per cent of all radio troubles. The following system is one I use all the time, and it leads me to the trouble quickly, in most cases. Old timers will agree that observation is well worth while, but beginners will find the system something they have wished for since they first became interested in "fixing" radios. These instructions are not likely to be as much use to the man who has so much confidence in his native luck that he plunges into a radio chassis with screwdriver, pliers and soldering iron and really "fixes" the set--so that it needs REBUILDING!
All information is as brief and as non-technical as possible so that the novice may derive all possible benefit from the information given.
Let us suppose that we have a six- to ten-tube superhet on the bench and that we are preparing to analyze the trouble. However, this system may be adapted to any other type of circuit also with proper consideration given to certain differences of circuit action.
First, see that all tubes are in their proper sockets. Often the owner has removed the tubes for testing (for free) and frequently replaces them in the wrong sockets.
Next, turn on the set. If tubes do not light, check line cord for breaks. On portables especially, check the switch.
Watch the rectifier tube for signs of over-heating, plates turning red, etc. Check for shorted filter condensers, shorted sockets or shorted transformer windings.
Turn chassis over and look for wires touching; burned out resistors, etc.
Have the dial set on a strong local station and the volume control set at maximum position.
Touch the grid cap of the first audio tube, or the grid terminal. This usually can be easily located as the grid lead comes from beneath the chassis. In the case of the single-ended tubes, touch a test prod to the center of the volume control to get the same results as though your finger were placed there. A loud clear buzz should be heard if all is well in the audio end.
If not, pull out the power tube. It should make a thump in the speaker if there is voltage on the plate of the tube.
If not, check the voice coil.
On midgets, make sure the pilot lamp is O.K.
Feel the output transformer. These often become warm when excess current is flowing through the plate winding.
The small tone compensating condenser connected from plate to cathode (or ground) may be shorted. Disconnect it and see. Or the coupling condenser may be leaking a positive voltage to the grid, causing the tube to draw excessive current.
Listen closely to the speaker. There should be some hum if there is any voltage at all on the power tube. If it is entirely quiet look for an open voice coil or broken leads to the voice coil.
Listen to the output transformer. You can hear it singing if the voice coil circuit is broken.
Watch any tuning indicator that may be present. If it indicates a signal the R.F. end is probably O.K. Electron-ray indicator tubes appear to burn red when no voltage is supplied to their anodes.
Have a test prod on the lead-in from a long antenna. Touch the grid of the I.F. tubes. Noise coming through will indicate the stage is in passable condition. Work back toward the antenna post.
Turn the wave-band switch to be sure it is set on the broadcast band. If the noise still comes through, but no signal, the oscillator is perhaps not functioning.
0ccasionally a strong signal will force its way through the I.F. Section when the oscillator has stopped. You can double check this by connecting the test oscillator to the grid of the first detector tube and setting it at a frequency of a local station plus the I.F. frequency of the receiver. The signal will come through if that is your only trouble.
Try adjusting the I.F. compensating condensers to be sure some home mechanic hasn't discovered they were loose and screwed them down tight. Mark the original setting and don't turn them far off without returning to the original--especially if you have no test oscillator. This procedure should not have taken over five minutes, and the service man should, with a little reasoning, have a good idea as to where the trouble lies--at least, in which stage it lies.
If you are without the test oscillator, you still can do a fair job of alignment on a receiver by using the noise pickup of your antenna. If you should be so (un)fortunate as to have your shop in an interference-free location, generate noise with a buzzer or spark coil.
Set the dial at a point where no station is heard. Turn up the volume control and adjust the I.F. trimmers for the highest noise level. The noise has very little effect on the AVC action and accurate adjustment can be made in this manner.
Next, tune in a station on the high frequency end of the dial and adjust the oscillator trimmer until the station is received best. Move the dial off the station and adjust the R.F. trimmers for maximum noise level. Lastly, set dial at the low frequency and adjust padder for maximum noise. The broadcast band is now aligned.
(This system will work only on sets with fixed padders in which no accident has caused the oscillator frequency to be "off." Where the padder has been screwed down so that the intermediate frequency generated by the oscillator is say--300 kilocycles, an attempt to align will leave the I.F. tuned to 300 kc instead of the normal 450-465 used on most radios. The result is that stations will come in only on that part of the dial at which the receiver has been "aligned." Attempts to "align" using the high-frequency end of the dial are equally dependent on the correctness of the oscillator trimmer.--Radio-Craft Editor)
Shortwave bands can be aligned also in this manner by using the government monitor station at 2.5, 5, 10 and 15 megacycles to set the oscillator trimmers, and the noise level to adjust the R.F. trimmers. Generally, the short wave bands should be aligned first.
A word or two on cut-out cases. These are in no way difficult. Locate the section giving the trouble. Then concentrate on that section.
A pair of headphones clipped in through a small condenser to the grid of the first audio tube will indicate whether the trouble is in the audio end of the receiver. If the signal is still coming through the phones, connect them to the grid of the second audio tube, if the set has one. If signal is also in the phones, go back to the detector. If signal is still heard, the trouble is not in the I.F or R.F. sections of the set.
If a test oscillator is available, connect it to the antenna and tune in the signal. Turn off the modulation. Turn up the volume control. Any loose connections can easily be heard by probing and tapping.
When a suspected open condenser is to be bridged with another one on a cut-out job, touch one side in the usual manner, then holding the other lead with the forefinger and thumb, touch the other terminal with the little finger, thereby charging the condenser slowly through the fingers before completing the connection. This will not cause sudden shock which will often make an intermittent radio start operating normally.
If the set is full of birdies, an R.F. or I.F. stage may be oscillating. This can be located best by touching the lead of a lead-pencil to the plate lug of the tube socket. A loud click will be heard when you have located the right one. If it proves to be an I.F. stage, everything apparently normal, put a resistor of as high a value as possible across the primary winding of the I.F. transformer. This will stop the oscillation. Usually 50,000 ohms will take care of it.
Distortion is the cause of many complaints so I will include some information which may be of some aid in locating the trouble, especially for the beginning serviceman.
Let us go back to locating the defective section, then the defective stage, and finally the defective part. After a little practice the serviceman will be able to distinguish by ear whether the trouble is in the R.F., audio, or speaker. However, touching the first audio tube grid may tell the story. A rattle will indicate speaker trouble. A distorted buzz proves the trouble is in the audio, a clear buzz indicates it is probably in the R.F. section.
If the distortion is traced to the audio amplifier, check all voltages carefully, especially the bias. Be sure it is correct before leaving it.
Check the voltage, grid to cathode. If it is resistance-coupled you won't get much indication of voltage, but must have some indication on the output stage.
The grid must have some negative voltage. If not, check the coupling condenser. If it is shorted or leaky the power tube may become very hot.
A shorted output transformer will cause poor tone. If it has been replaced with another, be sure the load matches the characteristics of the power tube.
If you are using the usual 1000 ohm-per-volt test meter, place your leads from grid to ground on all stages, as the grid may be floating. This may clear the tone. If so, replace the grid resistor. If the first audio tube is of the pentode type with a series screen resistor you may not read much voltage. However, turn your volt meter scales down. This may not increase the deflection, but it will lower the supply voltage to the screen. A high voltage here will cause distortion.
If the distortion trouble has shown up since you have been working on the set, you have probably caused it yourself. Check over the work you have done for defective parts, poor soldering or wrong connections.
R.F. distortion may he due to misalignment or to the wrong bias voltage. Check for both.
If distortion is only on strong signals, disconnect the antenna. If this clears up distortion you can be sure it is due to wrong bias voltage. On the older sets using 24's, etc., voltage under 25 volts on the screen or over 12 volts on the grid will cause the tube to be unstable It will be necessary to install super-control tubes or a local-distance switch to lessen pickup.
Again check the tubes as to their right positions in sockets. A sharp cut-off type like the 6J7 will not replace a super-control tube of the 6K7 type, where the volume is controlled by the C-bias either manually or with automatic volume control, which includes almost all sets in use today.
Be thorough. Don't skip a stage until you have checked everything. Particularly, don't take it for granted that the tubes are in the right places, even though you may have replaced them in the sockets yourself. CHECK THEM AGAIN
|Antique Radio Restoration Guide
Straightforward instructions guide reads through choosing the best radio to restore, troubleshooting and repairing the circuitry, and safety precautions when making repairs. Additional information includes sources for parts, information on common vacuum tubes, and a glossary of technical terms.
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