Troubleshooting a Multiplex Receiver can be intimidating at first glance. Before we start, let’s talk about safety. We will be working on LIVE equipment at some points, and you must know what you can touch and what not to touch with your bare fingers. When replacing any parts such as fuses, always unplug the unit and place the power cord plug end so that you can see it at a glance to remind you that it is unplugged and safe to work on. If there’s no power to the unit, the first thing that should be done is to check the fuses in the power supply section. Always replace blown fuses with the same type and rating for continued protection against fire and additional component damage. If there are no blown fuses, you will need a Multimeter to do some preliminary checking. Any multimeter that has an AC range of 250 volts AC will work for this purpose. If there’s a Power Transformer, check the voltage across the Primary Winding. To find the Primary, trace the power cord into the unit. The hot wire will go to a fuse and then to the power switch. This is where the primary winding of the power transformer (X-FMR.) will connect. (The other side of the ‘line voltage’ will be ‘common’ and is not connected to any other part of the unit). If the meter reads zero, you have a blown fuse or bad power switch. Un-plug the unit and, with your multimeter, set the selector to Ohms (R X 1K) Scale. Check across the switch for resistance. A good switch will measure about 0.3 Ohms. This reading includes the ‘leads to the meter’ resistance. A bad switch will read very high or ‘infinity’ resistance. If there’s an AC Voltage across the Primary Windings of the Pwr. X-Fmr (about 120 Volts AC) but still no power, we need to check the Secondary Windings of the Pwr. X-Fmr. These may be several voltages that are required to operate all of the different sections of the unit. Use your meter on the AC scale set at 250 Volts AC. The secondary winding will connect to the power supply section that will have rectification and filtering. If there’s no voltage in the secondary windings, you have a bad Pwr. X-Fmr. If there’s voltage across the secondary windings, there may be an issue with the power supply section. Un-plug the unit and check for fuses in this section. If you find a bad fuse, replace it as indicated above. To be continued in Part 2 of this series.
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