Baseline of performance

[electra225]

If you decide you are going to try and repair your instrument yourself, one thing I like to do is to establish a baseline of performance. This procedure basically involves powering up the device to see what its performance is like.

A trick I was taught by my stepdad, my first elmer, was to do a "safety check" on the power supply. This test is not a scientific one, but, rather gives an indication of the condition of the power supply and the B+ (high voltage) circuits in the device. This test is more critical when you are dealing with a transformer power supply. You measure the resistance between the positive terminal of the first filter capacitor (verified by consulting the schematic) and common negative. Common negative on transformer supplies is typically the metal of the chassis. If this resistance reading is in the megohm range, it is probably safe to power up long enough to get a baseline. If the reading is down around 500,000 ohms or below, the rectifier is "seeing" high resistance caused by leaky filter caps or other parts in the B+ circuit. This condition would indicate a near-short in B+, putting the power transformer or other parts in jeopardy. You'd need to replace the filter caps before proceeding with applying power. Of course, the earlier suggestions of using a dimbulb tester to limit current should be employed. If you are working with a transformless power supply, this test is not as critical, since there is no transformer to injure and there are fewer parts to damage. If you measure less than about 50,000 ohms from the positive of the first filter cap to common negative, typically one terminal of the power switch, there may be high resistance in B+, necessitating replacement of filter caps before you power up.

After you have determined the power supply is safe to use, or after you have replaced the filter capacitors in the power supply, and still using your dimbulb tester in series with the device, power up and see what you get. If you have access to a Variac, a variable AC transformer, that will work even better. You can start at a lower voltage, say, around 60 volts and see what happens. This may not be enough voltage to let the rectifier conduct, but it will let you know, by observing how bright the dimbulb tester is, whether all is well. Of course, you don't want to see sparks or smoke. If all is well, ramp up the voltage. If all is well there, go to about 100 volts. This should let the instrument work somewhat, unless it has a record player motor. This voltage may be too low for the motor to run. Ramp up voltage then to nearly full voltage. Now you should get some idea of what you have. Do you hear any hum? Do you hear music? Do you see smoke? This procedure should give you some idea what needs attention, then, consulting the schematic, you can perform directed voltage and resistance tests.