![]() |
Email Home Page |
![]() |
Amplifier Failure:
----- Critically Important -----
Adobe has deemed that the Flash content on web pages is too risky to be used by the general internet user. For virtually all modern browsers, support for Flash was eliminated on 1-1-2021. This means that those browsers will not display any of the interactive Flash demos/calculators/graphics on this (or any other) site.
The simplest (not the best) fix, for now, is to download the Ruffle extension for your browser. It will render the Flash files where they were previously blocked. In some browsers, you will have to click on the big 'play' button to make the Flash applets/graphics visible. An alternative to Ruffle for viewing Flash content is to use an alternative browser like the older, portable version of Chrome (chromium), an older version of Safari for Windows or one of several other browsers. More information on Flash capable browsers can be found HERE. It's not quite as simple as Ruffle but anyone even moderately familiar with the Windows Control Panel and installation of software can use Flash as it was intended.
Output Basics:
The voltage at all points along the sine wave are the instantaneous voltages applied to the speaker. When you come across the power dissipation figures, keep in mind that 35 watts is enough power dissipation to keep a soldering iron at more than 700 degrees fahrenheit.
Output Voltage at Point "A":
Psp is the power dissipation in the speaker's voice coil.
Ptr = I*E
Output Voltage at Point "B":
Psp = E²/R
Ptr = I*E If the speaker load is 2 ohms, the current flowing through the PNP transistor would double to 17.5 amps. The power dissipation in the PNP transistor would not increase significantly because the voltage across it is very small.
Output Voltage at Point "C":
Psp = I*E
Ptr = I*E A 2 ohm load would result in 150 watts being dissipated in the NPN transistor (twice the power caused by the 4 ohm load).
Output Voltage at Point "D":
Psp = I*E
Ptr = I*E A 2 ohm speaker load would cause the current to double to 10 amps. Since the PNP transistor has 15 volts across it, the power dissipation would be 150 watts.
From the above statements, you should be able to see that bridging an amplifier into a load with a lower impedance than recommended will very quickly push an amplifiers components past their design limits.
(MOSFETs) The following diagrams will show you how to check an FET to see if it is bad. This is only for N channel enhancement mode FETs (the type used in most amplifiers' power supplies). Since I use Fluke meters exclusively, all measurements will show what you would see with a Fluke meter. Other digital meters should give very similar readings. Until you become extremely familiar with FETs, you should check the FET in this order from top to bottom. If the order is changed, you may not get accurate test results. To check P-channel FETs, you will need to reverse the meter leads. This image is checking to see if the gate is leaking or shorted. In this type of FET, the gate should be COMPLETELY isolated from the other two terminals when checking the transistor with a volt meter. During this first test, you are inadvertently charging the gate and turning the transistor on. The meter should read the same as if the leads are open (not touching anything). You should notice that there are two black leads. this means that you first check the transistor with the red lead on the first leg and the black lead on the second leg. Then you move the black lead to the third leg of the transistor (while leaving the red lead on the first leg). The readings should be the same in both positions. The meter is set to its 'diode check' position.
![]()
In this image, you are discharging the gate and making sure that the transistor is turned off for the next test. The meter should again read the same as if the leads are open (not touching anything). If it gives any other reading, the transistor is bad.
![]()
In this image, you're checking to see if there is any leakage between the drain and the source. The meter should again show no continuity between the drain and the source.
![]()
In this image, your meter should read approximately 0.4 to 0.5 volts on diode check (the same meter setting as the previous tests). Here the meter is showing the forward voltage of the intrinsic diode of the FET.
![]()
If you get any reading other than 'open' on the first 3 tests, the transistor is defective. If the readings on the last test is below approximately 0.4 volts, the transistor is likely defective. If it reads around 0.3 volts or lower, the FET is definitely defective. The transistor is bad in both of these examples. It shows that the drain and source are shorted together.
![]()
This type of transistor is used throughout all types of electronic equipment. To check PNP transistors, reverse the positions of the red and black leads. This is a close approximation of the junctions in both NPN and PNP bipolar transistor. I included it to help you to remember how/why the following tests are done.
![]() ![]()
This is the pin out configuration of a TO-3 transistor. The test procedure is the same as the bipolar transistor in the next diagram. Take note of the pins labeled on the diagram for the TO-220 case transistor.
![]() In this diagram, the volt meter applies a small voltage to the transistor's junctions. With the leads in this position, the junctions are forward biased. The reading should be between 0.5 and 0.7 volts. Readings outside this range are likely defective.
![]()
In this diagram, the junctions are being checked with reverse bias. No current should flow through the transistor's junctions with the leads in this position. The meter reading should be the same as it would read with open leads.
![]()
Both of these connections should also read as open. any other readings indicate defective transistors. The only exception will be with darlington transistors.
![]()
These first 2 diagram show how to check the dual positive rectifiers (diodes) and the associated meter readings. The windings of the transformer would go to the two outside legs and the center terminal would go to the positive rail capacitor.
![]()
![]()
The next 2 diagram show how to check the dual negative rectifiers (diodes) and the associated meter readings. The windings of the transformer would go to the two outside legs and the center terminal would go to the negative rail capacitor.
![]()
![]()
|
![]() |
![]() |
![]() |
![]() |