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Re: Repair advise [message #19133 is a reply to message #19064]

Tue, 13 August 2013 18:16

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Start by turning off and unplugging the amp.

Transistors are three element devices. They have a base, a collector and an emitter. On the schematic they are drawn like a letter Y on it's side. The single element side is the base and the side with the two lines are the collector (plain) and the emitter (arrow).

To test the transistor you set the meter to diode test and then connect the red lead to the base and then touch the black lead to the collector. You will either get a reading or it will test the same as having the meter not connected to anything. Then you will reverse the two connections. If the junction is working properly there will be a low reading maybe .6-.7 volts in one direction and a wide open reading in the other direction. Then you need to repeat the same test, but this time touching the leads to the base and the emitter. And again touching the collector and the emitter.

There are two types of transistors in the preamp, NPN and PNP. The NPN will read low when the let's say the red lead is connected to the base and high when the black lead is connected to the base. The opposite will be true of the PNP transistors they will read low when the black lead is connected to the base. This will actually be dependent upon the voltage polarity of your meter, but I hope you get the difference.

Now test all of the transistors on the board. They all should have high and low readings from base to collector and base to emitter. And they should all have open readings from collector to emitters.

There may be a few cases where the reading will be skewed by resistances in the circuit that parallel the transistor elements. If you can't get a good reading, you may need to remove the transistor from the circuit to get a final answer.

I don't know if you have a known good transistor available to test out of circuit. If you do, it may help you to understand the process better.

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[Message index]

		Repair advise By: Jonomega3 on Fri, 09 August 2013 15:44
		Re: Repair advise By: chicagobill on Fri, 09 August 2013 16:28
		Re: Repair advise By: Jonomega3 on Fri, 09 August 2013 19:56
		Re: Repair advise By: chicagobill on Fri, 09 August 2013 21:36
		Re: Repair advise By: Jonomega3 on Sat, 10 August 2013 07:52
		Re: Repair advise By: pleat on Sat, 10 August 2013 08:15
		Re: Repair advise By: Jonomega3 on Sat, 10 August 2013 08:31
		Re: Repair advise By: Jonomega3 on Sat, 10 August 2013 18:09
		Re: Repair advise By: Jonomega3 on Sat, 10 August 2013 23:30
		Re: Repair advise By: Jonomega3 on Sat, 10 August 2013 14:50
		Re: Repair advise By: chicagobill on Sat, 10 August 2013 12:55
		Re: Repair advise By: chicagobill on Sun, 11 August 2013 14:34
		Re: Repair advise By: Jonomega3 on Sun, 11 August 2013 14:46
		Re: Repair advise By: Jonomega3 on Sun, 11 August 2013 16:17
		Re: Repair advise By: chicagobill on Sun, 11 August 2013 22:20
		Re: Repair advise By: Jonomega3 on Sun, 11 August 2013 22:41
		Re: Repair advise By: chicagobill on Sun, 11 August 2013 22:53
		Re: Repair advise By: Jonomega3 on Mon, 12 August 2013 12:19
		Re: Repair advise By: chicagobill on Mon, 12 August 2013 12:57
		Re: Repair advise By: Jonomega3 on Mon, 12 August 2013 22:43
		Re: Repair advise By: chicagobill on Tue, 13 August 2013 00:26
		Re: Repair advise By: Jonomega3 on Tue, 13 August 2013 10:34
		Re: Repair advise By: chicagobill on Tue, 13 August 2013 11:35
		Re: Repair advise By: Jonomega3 on Tue, 13 August 2013 12:06
		Re: Repair advise By: Jonomega3 on Tue, 13 August 2013 13:33
		Re: Repair advise By: chicagobill on Tue, 13 August 2013 13:38
		Re: Repair advise By: Jonomega3 on Tue, 13 August 2013 13:48
		Re: Repair advise By: Jonomega3 on Tue, 13 August 2013 15:03
		Re: Repair advise By: chicagobill on Tue, 13 August 2013 15:08
		Re: Repair advise By: Jonomega3 on Tue, 13 August 2013 15:23
		Re: Repair advise By: chicagobill on Tue, 13 August 2013 16:16
		Re: Repair advise By: Jonomega3 on Tue, 13 August 2013 16:37
		Re: Repair advise By: chicagobill on Tue, 13 August 2013 17:16
		Re: Repair advise By: Jonomega3 on Tue, 13 August 2013 17:37
		Re: Repair advise By: chicagobill on Tue, 13 August 2013 18:16
		Re: Repair advise By: pleat on Tue, 13 August 2013 21:18
		Re: Repair advise By: Jonomega3 on Tue, 13 August 2013 21:52
		Re: Repair advise By: pleat on Wed, 14 August 2013 07:23
		Re: Repair advise By: Jonomega3 on Wed, 14 August 2013 09:07
		Re: Repair advise By: chicagobill on Wed, 14 August 2013 12:33
		Re: Repair advise By: Jonomega3 on Thu, 15 August 2013 08:59
		Re: Repair advise By: chicagobill on Thu, 15 August 2013 13:26
		Re: Repair advise By: Jonomega3 on Thu, 15 August 2013 13:41
		Re: Repair advise By: chicagobill on Thu, 15 August 2013 15:17
		Re: Repair advise By: Jonomega3 on Thu, 15 August 2013 15:40
		Re: Repair advise By: chicagobill on Thu, 15 August 2013 16:15
		Re: Repair advise By: Jonomega3 on Thu, 15 August 2013 16:29
		Re: Repair advise By: chicagobill on Fri, 16 August 2013 12:16
		Re: Repair advise By: Jonomega3 on Fri, 16 August 2013 13:12
		Re: Repair advise By: chicagobill on Fri, 16 August 2013 13:36
		Re: Repair advise By: Jonomega3 on Fri, 16 August 2013 13:39
		Re: Repair advise By: chicagobill on Fri, 16 August 2013 14:55
		Re: Repair advise By: Jonomega3 on Fri, 16 August 2013 15:27
		Re: Repair advise By: chicagobill on Fri, 16 August 2013 15:52
		Re: Repair advise By: Jonomega3 on Fri, 16 August 2013 16:00
		Re: Repair advise By: chicagobill on Fri, 16 August 2013 16:41
		Re: Repair advise By: Jonomega3 on Fri, 16 August 2013 20:44

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