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Testing Output Transistors

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Anonymous
August 6, 2004 1:16:06 PM

Archived from groups: rec.audio.tech (More info?)

Hello,

I have acquired an Amber Series 70 amp, I am relatively new to electronics
and am attempting to debug a problem with the left channel (it simply does
not work).

I removed the output transistors and tested them with a DVM. Not sure which
is the Base or the Emitter but on the diode check I get approximately 425 on
one pole and 1.0 on the other. I tested all of the output transistors and
they all are around the same reading 425-440. Is there another way to check
the transistors? Also how would I go about checking the output driver which
is an LM391-100 if possible with a simple DVM?

Thanks in advance,
Dan
Anonymous
August 7, 2004 3:39:44 AM

Archived from groups: rec.audio.tech (More info?)

solex wrote:

> Hello,
>
> I have acquired an Amber Series 70 amp, I am relatively new to electronics
> and am attempting to debug a problem with the left channel (it simply does
> not work).
>
> I removed the output transistors and tested them with a DVM. Not sure which
> is the Base or the Emitter but on the diode check I get approximately 425 on
> one pole and 1.0 on the other.

I have no idea what you mean by " 425 on one pole and 1.0 on the other ". Pole
of what ?

> I tested all of the output transistors and
> they all are around the same reading 425-440. Is there another way to check
> the transistors?

Both the base emitter and base collector junction of bipolar devices measure
like big diodes - you'll get typically more like 500mV forward voltage IME and
*very* high resistance in the non-conducting direction.

If you find a short instead anywhere - the device is definitely dead. If you
don't get the diode characteristics either, then the device is also u/s.

You have NPN and PNP devices to consider, so bear in mind that the NPN device
will conduct with the base positive and the PNP with the base negative.

Have you looked at the transistor part numbers and googled them ? You'll find
data sheets for most popular devices on their manufacturer's websites and
elsewhere too. That'll help you find the connections !

> Also how would I go about checking the output driver which
> is an LM391-100 if possible with a simple DVM?

No you can't. It's an IC and far to complex to test with a DVM

Data here http://www.national.com/pf/LM/LM391.html

Graham
Anonymous
August 9, 2004 1:08:14 PM

Archived from groups: rec.audio.tech (More info?)

In article <41177a30$0$3099$afc38c87@>, solex <solex@nowhere.com> wrote:

> Do you think that it would be possible to switch the output drivers
> (LM391-100, from one board to the other) without causing any harm to the
> system or the "good" output driver? My concern is that the components on
> each of the boards are some how configured "balanced" for each output driver
> and their respective components, and switching then would result in damage
> to the ouput driver.

In many transistor designs, pairs of outputs need to be matched
for gain. That is the HFE value for each transistor, and it
actually needs to be measured, not read off of a data sheet.
You can swap entire groups of outputs right for left, but don't
move them to other positions, or get them mixed up. Many amps
have adjustments that need to be made after a change like this,
such as the DC Offset and gain controls.

-john-

--
====================================================================
John A. Weeks III 952-432-2708 john@johnweeks.com
Newave Communications http://www.johnweeks.com
====================================================================
Anonymous
August 9, 2004 1:20:47 PM

Archived from groups: rec.audio.tech (More info?)

Graham,

Thank you for your response, and please excuse my ignorance.

I will look more closely at the data sheet to determine the characterstics
of the ouput transistors, but have tested each one of the transistors and
have recieved similar results when tested in the same fashion, which leads
me to believe that the are not bad since one channel works and the other
does not.

Do you think that it would be possible to switch the output drivers
(LM391-100, from one board to the other) without causing any harm to the
system or the "good" output driver? My concern is that the components on
each of the boards are some how configured "balanced" for each output driver
and their respective components, and switching then would result in damage
to the ouput driver.

Thank you,
Dan


"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:411408AF.F9C2784B@hotmail.com...
>
>
> solex wrote:
>
> > Hello,
> >
> > I have acquired an Amber Series 70 amp, I am relatively new to
electronics
> > and am attempting to debug a problem with the left channel (it simply
does
> > not work).
> >
> > I removed the output transistors and tested them with a DVM. Not sure
which
> > is the Base or the Emitter but on the diode check I get approximately
425 on
> > one pole and 1.0 on the other.
>
> I have no idea what you mean by " 425 on one pole and 1.0 on the other ".
Pole
> of what ?
>
> > I tested all of the output transistors and
> > they all are around the same reading 425-440. Is there another way to
check
> > the transistors?
>
> Both the base emitter and base collector junction of bipolar devices
measure
> like big diodes - you'll get typically more like 500mV forward voltage IME
and
> *very* high resistance in the non-conducting direction.
>
> If you find a short instead anywhere - the device is definitely dead. If
you
> don't get the diode characteristics either, then the device is also u/s.
>
> You have NPN and PNP devices to consider, so bear in mind that the NPN
device
> will conduct with the base positive and the PNP with the base negative.
>
> Have you looked at the transistor part numbers and googled them ? You'll
find
> data sheets for most popular devices on their manufacturer's websites and
> elsewhere too. That'll help you find the connections !
>
> > Also how would I go about checking the output driver which
> > is an LM391-100 if possible with a simple DVM?
>
> No you can't. It's an IC and far to complex to test with a DVM
>
> Data here http://www.national.com/pf/LM/LM391.html
>
> Graham
>
!