[Jack[_11_]]

Dave Platt wrote:
In article , Jack wrote:

I tried probing DC voltages across speaker output and ground at each
channel with a digital meter with the speakers connected and unconnected
and the readings were essentially the same, under 10 mv, left and right,
speakers connected and unconnected. I suspect that this is not an
effective test.

Well, it tells you that you don't have a serious DC fault in the
output stage, which is good news. Some sorts of failures can send the
full DC rail voltage (or near to it) to the output, and this can very
rapidly damage the woofers (high current flow, voice coil heats up,
magic smoke leaks out, bad smells occur, fire or burnt-out coil
results).

You're almost certainly going to need some way of actually looking at
the audio signal. You *might* be able to use an AC voltmeter, but an
oscilloscope will make the state of the signal a lot easier to
visualize. In either case, some sort of audio input signal will be
needed... FM tuner tuned to a station, or tuned in between stations
with the muting turned off, or a signal from a CD player, or etc.

Hint: if you have a CD burner, and a copy of Audacity (or a similar
audio-editing package) you can make your own "signal generator" for
cheap. Use Audacity to create a stereo 44.1 ksample/second 16-bit
audio file, containing a test tone (e.g. 1000 Hz). Store this on disk
and burn it as a one-track audio CD. Put it in your CD player, set
the player on "repeat", hit Play, and you've got a nice clean
consistent audio waveform that you can feed to the receiver and then
trace through the circuitry.

I downloaded a 30 second 1000HZ sine wave in 44100/16 format and I have
been playing that through the computer into the tape monitor playback of
the receiver.

If you use a lower frequency (e.g. 100 Hz) it'll be within the
specification range of almost any simple AC voltmeter, and should read
out properly... you can trace the signal through both channels, and
see where the differences first appear by comparing the voltage
readings.

I'll download the 100Hz signal now.

Try to borrow an o'scope, though... you'll see more about what's going
on, than you can deduce from voltmeter readings.

My main task is to determine where to probe. I decided that the
transistors that feed the outputs transistors is where to go next but
until I can get hands on an o'scope... and learn to use it, it's been a
while, I will have to rely on the volt meter. The smallest scale for AC
is 200 volts. I'll see if the 100Hz tone can be read with it.