(Continued from Part 2) Most good quality Multiplex Receivers and Power Amplifiers will have a set of fuses to protect the power output sections. These fuses will (usually) be located near the output devices (being a Chip or set of Transistors) mounted on a large Heat Sink. If not, they can be found in the Power Supply Section. These fuses should be replaced with the same type and rating for continued protection against a (possible) fire hazard. If the fuse (or fuses) blows right away, you have a bad output chip or shorted output transistor(s).
I have a Fisher Studio Series Power Amplifier that has selectable inputs and a 3 band graphic equalizer. A pretty basic unit, but only one channel works. The other channel sends a B+ (Direct Current Supply Voltage) right to the speaker terminals! This is rare, but can (and will) destroy speakers (almost) instantly. The fuse installed was over-rated and didn’t protect the transistor(s) from a shorted speaker wire, as the man who sold it to me was a DJ back in the day, and he told me what he did. (I purchased this unit at a Garage Sale for $10.00. A ‘Boat Anchor’ Special…someone elses’ junk). I have it on a shelf awaiting a ‘rainy day’. I mention this because there’s a lot of different equipment out there and I want to cover all of the bases, sort of speak.
As for finding out the specification(s) of parts that are (presumed to be) defective, you can get a cross refferance book (or a disc that can load into your computer) from http://www.mcmelectronics.com for a good price, if you order a few other items as well; for they have a minimum order of $25.00. NTE, (a supply company of electronic components) has the most replacement parts inventory that I’ve seen. MCM Electronics will be able to help you. They carry the NTE line of components. Each component is tested and will work as a direct replacement if handled and installed correctly. They only (usually) cater to Service Centers, Factoriy Maintenance Departments and Schools as they offer special prices to these folks. You can order from these people if you wish at the same prices that they pay. I’ll cover more about Parts and other sources where you can find them later in this series.
If the unit powers up, will have sound from an audio CD, but nothing from the Radio, check and make sure that all of the front panel switches are correct to operate the receiver. If still no sound, you will need an Oscilloscope, Vacuum Tube Volt Meter or a Digital Meter that has a scale on it (like an Analog Multimeter) because of it’s high input resistance. The test points will be at the last IF, or intermedate frequency stage of the Tuner. This will be found (traceable from) the switching network to the preamp of the tuner’s output, that is the last IF. You will have to ‘walk backwards’ until you find a signal that you can use (amplify). This will be noted by a good signal on your O-Scopes’ screen or your VTVM. Very rare that a component failure would happen here, because of today’s circuitry being done on two (or more) IC’s (Integrated Circuits).
As usual, feel free to e-me if you have any questions, comments or concerns about this (or any other) article that I’ve published to: [email protected]