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[EDWARDEFFECT1]

i believe i read a previous posting somewhere about checking ac voltages on the chip.i have an oscilloscope.how do you do it?i know all output chips are different,but there must be some simularities in  the testing proceedure!

[J M Fahey]

Chips are very complex inside, so a simple test with a multimeter does not tell us much.
The useful test is a functional one, meaning : "does it do what it's supposed to do?"
Suppose you have a popular TDA200x :
1) Does it have +V and -V in the correct pins? (check datasheet)
2) Does it have essentially 0V DC on the output pin?
3) If I inject signal in the input pin, do I get audio in the output one?
There's not much more than that, besides visually checking for a split plastic case or (quite common) a cracked leg .
The oscilloscope is useful to check the input/output signal test, although just hooking a speaker in the output and touching the input with a finger is almost the same.

[EDWARDEFFECT1]

thanks again jm for the reply....ed!

[EDWARDEFFECT1]

jm where do you insert a signal on a fet transistor (gain,source,drain).i have a mfj signal injector tracer.if i put  a signal into the source terminal ground the second lead on the tester would i get the output at the drain terminal on a good fet?....thanks....ed

[J M Fahey]

In fact, whether transistor, tube or bipolar, you have 3 ways to connect it, with different results.
As a general rule, these are 3 terminal devices, where 1 of them is the input; the 2nd one is the output, and the 3rd is where said audio signals are referenced to, and is therefore called "common", it's often AC grounded..
1) the most common one (pardon the pun):
"common emitter/cathode/source"
Emitter goes to ground, you inject signal in the base, you get your amplified signal in the collector (or in all 3 cases the functionally equivalent terminal)
You usually have both voltage and current gain.
2) "common collector", base is input, emitter is output.
You have no voltage gain but a huge current one.
3) "common base": input is emitter, output is collector.
No current gain but some voltage one.
Much used in Radio Frequency circuits because of the extended frequency response and separation between input and output circuits.
No time to draw, maybe somebody can draw all 3 cases and post here.

[gbono]

For a Field Effect Transistor: Gate is usually the input - drain and source are interchageable