I recently was given a "poor man's home theatre" system... it's made by a
company called Emerson Research (yeah, Emerson, not the best name in audio).
It had had the plug cut off, I assumed it to be non-functional.

this unit appears to contain a ported woofer or subwoofer, a crossover
network, and a switch for multiple stereo inputs and outputs as follows:

2 rear speakers
2 "micro" speakers - assume satellite tweeters with high crossover freq.
1 (7 or 9-pin) DIN connector labelled "center channel". ???

I opened it up and, yup, there are two 5A fuses, both blown, one on each
outside lead of a CT transformer prior to the rectifying diodes. Diodes
checked out fine, output transistors (there are six of them, TIP41C/TIP42C
pairs) are all blown.

Each transistor appears to be fed from either the + or - rail to the
collector. Each base connects to an output jacks via a 0.22ohm resistor,
none of the resistors are open.

These transistors are cheap, and I'd like to get the unit going to use with
my computer at work. I have a pair of unpowered satellite speakers and I
never listen to them because there is no bass whatsoever and because the
amp on my soundcard sounds like crap at the best of times. I know I'm never
going to get high fidelity but hey, the amp was free.

Before I replace the transistors, anything else I should check? Can anybody
tell me (without seeing a schematic) if crossing any of the outputs could
cause all of the transistors to go? As there's no SMPS, I can set it up
with my trusty ol' series light bulb I guess and see if there's any shorts.
Do I need to load the outputs with a dummy resistor? If so how what size
and rating?

And what's up with that "center channel" DIN jack?

Any replies greatly appreciated.

Dave

Dave wrote:
I recently was given a "poor man's home theatre" system... it's made
by a company called Emerson Research (yeah, Emerson, not the best
name in audio). It had had the plug cut off, I assumed it to be
non-functional.
this unit appears to contain a ported woofer or subwoofer, a crossover
network, and a switch for multiple stereo inputs and outputs as
follows:
2 rear speakers
2 "micro" speakers - assume satellite tweeters with high crossover
freq. 1 (7 or 9-pin) DIN connector labelled "center channel". ???

I opened it up and, yup, there are two 5A fuses, both blown, one on
each outside lead of a CT transformer prior to the rectifying diodes.
Diodes checked out fine, output transistors (there are six of them,
TIP41C/TIP42C pairs) are all blown.

Each transistor appears to be fed from either the + or - rail to the
collector. Each base connects to an output jacks via a 0.22ohm
resistor, none of the resistors are open.

These transistors are cheap, and I'd like to get the unit going to
use with my computer at work. I have a pair of unpowered satellite
speakers and I never listen to them because there is no bass
whatsoever and because the amp on my soundcard sounds like crap at
the best of times. I know I'm never going to get high fidelity but
hey, the amp was free.
Before I replace the transistors, anything else I should check? Can
anybody tell me (without seeing a schematic) if crossing any of the
outputs could cause all of the transistors to go? As there's no
SMPS, I can set it up with my trusty ol' series light bulb I guess
and see if there's any shorts. Do I need to load the outputs with a
dummy resistor? If so how what size and rating?

And what's up with that "center channel" DIN jack?

Any replies greatly appreciated.

Dave

If one pair is shorted, all may appear shorted in your test. Did you open
them up from the circuit for your test?

Typically, if the drivers and bias transistor check OK, you are just down to
replacing the outputs and any resistors which may have failed from the
over-current condition. These often include a resistor in the 220 to 470 ohm
range located between the emitters of the driver transistors, or resistors
in the 2.2 to 4.7 ohm range going to the base of each output transistor, and
sometimes a resistor in the range of 1 to 10 ohms feeding the B+ or - to one
or both of the driver transistors' collectors.

Mark Z.

Mark Z.

"Mark D. Zacharias" wrote in message
. net...

If one pair is shorted, all may appear shorted in your test. Did you open
them up from the circuit for your test?


I removed them all from the circuit, they all have either a) dead short
(zero voltage drop) between B-C-E or b) open between B-C-E, no drop either
direction between any leads.

Typically, if the drivers and bias transistor check OK, you are just down
to replacing the outputs and any resistors which may have failed from the
over-current condition. These often include a resistor in the 220 to 470
ohm range located between the emitters of the driver transistors, or
resistors in the 2.2 to 4.7 ohm range going to the base of each output
transistor, and sometimes a resistor in the range of 1 to 10 ohms feeding
the B+ or - to one or both of the driver transistors' collectors.

Each power transistor has a small transistor (TO-92) next to it, resting
against but not in any way affixed to the giant aluminum common heatsink,
those would be the bias transistors? I guess any driver transistors would
be located in the general vicinity as well. I'll keep looking.

I find it really weird that ALL of the transistors were baked, not just one
channel or one single output... even the pair that drives the internal
woofer which is not accessible from outside the case.

Dave

Dave wrote:

I recently was given a "poor man's home theatre" system... it's made by a

I opened it up and, yup, there are two 5A fuses, both blown, one on each
outside lead of a CT transformer prior to the rectifying diodes. Diodes
checked out fine, output transistors (there are six of them, TIP41C/TIP42C
pairs) are all blown.

Each transistor appears to be fed from either the + or - rail to the
collector. Each base connects to an output jacks via a 0.22ohm resistor,
none of the resistors are open.

I presume thats a typing mistake.


These transistors are cheap, and I'd like to get the unit going to use with

Before I replace the transistors, anything else I should check? Can anybody
tell me (without seeing a schematic) if crossing any of the outputs could
cause all of the transistors to go?

I dont see how it would. Does seem odd all 6 are gone.


As there's no SMPS, I can set it up
with my trusty ol' series light bulb I guess and see if there's any shorts.

Is this a capacitor coupled amp or dc coupled? IF the former, you only
need check the output stage is ok. If its a dc coupled amp, bin it now,
theyre nightmares to work on.


Do I need to load the outputs with a dummy resistor? If so how what size
and rating?

No.


NT