Good radio/bad radio comparison tests

One of the best resources for troubleshooting a defective radio is the good radio/bad radio testing technique. Having a reference radio that is known to be operating properly can be an extremely valuable reference tool when troubleshooting a defective set.

Two live-sets When troubleshooting a defective set and comparing measurements from the bad set to the good set, it is inconvenient to constantly transfer the power hookup and RF connector cable from one set to the other. A simple way around this is to power up both radios simultaneously, as shown in Figure 1, top left. Also, for receiver testing, a signal combiner (hybrid or simple resistive type) can be used to connect the signal generator to both receiver inputs. Actually, a simple “tee” connector could be used to connect both receivers to a single signal generator. A mismatch will result, but because we are only interested in relative measurements, the mismatch should not cause a problem. Both sets should be equally affected. This eliminates all the cable swapping that can become annoying during extended testing periods. For checking comparison points in both receivers simultaneously, a dual-trace oscilloscope can be used, as shown in Figure 2, center left.

For transmitter tests requiring the transmitter to be keyed, a simple coaxial switch can be used to divert a dummy load or service monitor from one set to another, as shown in Figure 3, bottom left. The coaxial switch is much more convenient than cable swapping. An alternative is to use two dummy loads, unless you need to monitor the power output or other modulation. Then you must use two service monitors or power meters if the instruments are available.

The other problem with transmitters is having to key them. If you need two free hands for testing, it will be inconvenient to use the microphone for keying. One alternative is to use a foot switch for keying by wiring up a switch for transferring the foot switch between the good set and the bad set, as shown in Figure 4 on page 40. If two hands are not needed, the microphone can be used to key the transmitter.

Reference measurements If the two-live-sets method isn’t convenient, reference measurements made on a radio known to be in good operating condition can be used for future reference. The advantage of making your own reference measurements instead of using the manufacturer’s reference measurements (if available) is that you are using the equipment for the reference measurements that you would use in actual troubleshooting. Thus, any loading effects of this equipment will be the same as the equipment used for the reference measurements. Basically, we are interested in the signal measurements and not the operating voltages of the various circuitry.

Because a fully equipped radio shop has a computer anyway, the reference measurement data can be stored on computer for quick and easy recall. A simple table can be generated within the computer program to record the pertinent reference data. Besides the actual measurement data, the make and model of equipment used to measure the data should be included as notes with the table, as well as the conditions under which the data were measured. For example, a table for signal levels measured with 280dBm signal into receiver RF input might look like this:

Note: Measurements made with signal level of 280dBm at receiver input using Model “X” low capacity scope probe with Model “X” spectrum analyzer.

Sure, the loading effect may be serious and the absolute measurements may not be valid. So what? We are not worried about absolutes here_only relative levels of the bad set to the good set. Loading effects will be the same for both sets so the comparisons will be valid.

Using the techniques presented here will be much easier if you prepare the workbench setups beforehand. Unless arrangements are made convenient it is unlikely that they will be used at all. Adequate workbench space and switching setups are necessary in order to use these procedures to best advantage.

Until next time-stay tuned!

Kinley, a certifiied electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is a member of the Radio Club of America.Je is the author of Standard Radio Communications Manual: With Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. Kinley’s email address is hkinley@aol.com.