Well, typically these output transistors are usually arranged in a push pull
type circuit. One transistor amplifies the positive side of the waveform
and the other does the negative side.

But I have to ask why. Emerson is junk. Not worth spending time and money
on. Buy a new one.

DaveL


"Dave" wrote in message
news:m1jbh.6192$dX4.81@clgrps13...
Last week I posted about an Emerson Research "home theatre" system that I
was given. Two 5A fuses on the CT transformer secondaries blown. This
unit has outputs for two rear speakers (L/R), a center channel (DIN
connector) and two front satellites (L/R). Now the inputs are analog RCA
audio so this box must be just simulating 5.1 using a delay or some other
trickery.

What I'm looking for here is an understanding of how the various output
components interact... in particular how the NPN/PNP pairs function

Anyway, there are six output transistors, 3 x TIP41C (NPN) and 3 x TIP42C
(PNP). It _APPEARS_ by looking at the board traces that one set goes to
the rear output RCA connectors (can't really see how it's connected as the
connector block takes up a lot of real estate on the board), one set goes
to the satellite RCA outputs, and one set goes to the subwoofer single
output which is enclosed in the cabinet with the PCB board. The 41/42
pair adjacent to the subwoofer output are the only two (of the six) output
transistors which are NOT blown.

Now, another poster suggested that I check the bias and driver
transistors, but I'm not sure which ones these are. here's what I've got,
I wish I were skilled in ascii art but I'm not so bear with me.

The output transistors are all in a row along a large aluminum heatsink
which runs the width of the PCB. They alternate --- In between each
PNP/NPN transistor is a smaller TO-92 NPN which is adjacent to the
heatsink as well, sort of stuck there with thermal grease. Adjacent to
each TO-220 Tip transistor but on the side opposite the heatsink, each
output transistor is paired with another TO-92 Like so:

blown pair blown pair good pair
|| || || || || || || || || || || || || || || || || || ||
----------------------------------------------------------
Tip41--c945--Tip42--Tip41--c945--Tip42--Tip41--c945--Tip42
| | | | |
|
c945 a733 c945 a733 2sc2245 2sa965
bad

I pulled ALL of these transistors and tested them. Of the TO-92's, all
test good except one of the a733's. My question is this: do I need to
replace BOTH a733's, i.e. to maintain matched set characteristics? I'd be
looking at an NTE290A as a replacement. Or are the c945/a733 some sort of
matched pair. I see that NTE offers either a) two matched 290A's or b)
matched sets of 289A/290A (289A is NOT listed as a replacement for
2sc945).

As I noted at the beginning, I'd like to understand which transistors
drive what if it's at all possible to determine from the information I've
provided. I can trace out the relationship between the transistors if
necessary but maybe the number, placement and type of units is enough.

Any replies greatly appreciated.

Dave