For anyone who would like the measure the primary impedance of the output transformers in their amplifier/receiver but are unfamiliar with the process, here is the procedure to do that. The measurement procedure is performed with the unit's power cord UNPLUGGED, and NO LOAD attached (turn any speaker selector switches ON so no internal loads are attached if applicable), while the output tubes can remain installed, but do not need to be.
The minimum equipment needed is a DVM, and an ordinary line cord with an unterminated end. If the line cord has a green wire ground lead, it can be connected to the chassis. Ideally, the hot and neutral leads will have insulated clips installed, but it is not absolutely necessary. Here are the steps then to measure your transformer:
1. DISCONNECT the red transformer lead from the unit and tape it off to isolate it.
2. Connect the hot and neutral leads of the line cord to the plate leads of the transformer being tested. For Fisher equipment, these will ALMOST always be the BLU and BLU/TRACER transformer leads that connect directly to the output tube sockets serving a particular transformer. For units employing 7591 tubes, this will be pin #3 of each output tube socket for the transformer being tested. For units with 7868 tubes, these leads will be connect to pin #9 of each output tube socket. It doesn't matter which transformer lead connects to which line cord lead. Ideally, you will just be able to clip the line cord connections to these terminals, but if your cord doesn't have any clips, then temporarily tack solder them to the terminals.
COMMENT: Do not be concerned about plugging the primary winding of your output transformer into your home's AC power. The transformer routinely "sees" more voltage than this in normal operation. To punctuate this fact, such a voltage impressed upon the primary winding of a typical 7581/7868 output transformer will only cause the secondary to deliver slightly over 2 watts RMS into any secondary terminal with a rated load impedance attached. Transformers for 6BQ5 class tubes will deliver even less power. Heck, I listen to Deep Purple's "Hush" with a heck of a lot more power than that! Also, do not be concerned that the test is made with no load attached. Driving the transformer primary from the low impedance source of the AC power grid is very much different than driving it from the much higher impedance source of the output tube plates.
3. Check that your connections are good, and the scenario conditions are met, and then, plug the line cord into the wall socket. With line voltage now available inside the unit at the OPT connections, exercise ALL appropriate caution as you proceed.
4. Measure the value of the AC line voltage applied to the OPT primary winding, and record it.
5. Measure the value of the AC voltage transformed into the full 16 ohm secondary winding. For virtually all Fisher equipment this will be the BLK and YELLOW leads. Do NOT assume that the BLK lead is connected to the chassis -- often it is not. Therefore, connect your meter leads directly to the BLK and YELLOW leads where they connect into the unit to make your measurement, and record the result. Unplug the line cord connected to the transformer.
6. Math: Divide the voltage measured on the secondary winding into the line voltage measured that was applied to the primary winding. The result will be the "turns ratio" of the transformer. Example. 6.00 vac measured on the secondary winding divided into 120.0 vac applied to the primary winding = a Turns Ratio of 20.
7. Square the turns ratio, and then multiply that product by the rated impedance of the secondary winding that was being measured in step 5. In this case, that would be 16. Therefore, in this example, 20X20 = 400 X 16 = 6400. The primary impedance of the transformer in this example then is 6400 ohms, since impedance is equal to the square of the turns ratio of the transformer. However, that is only true if the secondary impedance is 1 ohm, which of course is not the case, so after the turns ratio has been squared, it must then be multiplied by the impedance that the secondary winding actually represents.
8. Disconnect your line cord from the plate leads and reconnect the RED B+ transformer lead, which will return the unit to normal operation.
With this information then, you can determine the primary impedance of any unknown transformer, as long as you have good information as to what the secondary winding was designed to represent impedance wise.
I hope this helps!
Dave
