Re: Any SS foks here : 15003 readings

On Mar 23, 3:11 pm, "Peter Wieck" <p...@xxxxxxx> wrote:
On Mar 23, 10:52 am, jpopel...@xxxxxxxx wrote:
The concept of an in circuit tester is pretty simple. It is an ohm
meter that applies less voltage to make its measurement, than it takes
to forward bias a silicon diode (somewhere between .1 and .4 volts,
open circuit). This allows you to check if a silicon diode junction
is short circuited (no normal diode drop needed to produce a low
resistance value) or to measure the values of resistors connected to
transistors and diodes, while keeping all the junction voltages low
enough that nothing gets forward biased enough to conduct and distort
the ohm readings.

Diode check meters (usually show a diode symbol on the selector) apply
a limited current through the probes, while displaying the open
circuit voltage (often with an implied decimal point). So if you
connect the leads across a diode or base to collector junction, etc.
you may get a reading of 50 to 70 (or 500 to 700, depending on where
the implied decimal point is), indicating .5 to .7 volts forward bias
drop. A shorted junction will read very close to zero and an open
circuit usually blanks the display, instead of showing the open
circuit voltage. Testing diode junctions in circuit will include the
effect of any other current paths on the voltage drop across a
junction, so it is not generally an in circuit test.

Yabbut...

Those two transistors (cited in the OP) are very nearly in parallel.
So any results are questionable if in-circuit. A simple diode tester
may not be enough to tell one from the other. Those resistors are only
5 ohms.

I'm agreeing with that. I was describing the limitations and
application of in circuit testing, andthe difference between in
circuit and diode junction tests. I recommend pulling the power
transistors and testing their junctions with a diode test function, to
discover any shorted junctions. While all 4 output transistors are
pulled, I think the rest of the amplifier can be tested for normal
bias operation with no load on the base driver section. The 4 output
transistors are essentially a high current voltage follower for the
base driver outputs.


.

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