Re: Audio Amplifier Problems

Charles Schuler wrote:

"PDI Dave" <david@xxxxxxxxxxxxx> wrote in message
news:1166913148.249329.228980@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx


Thanks for your help. well, i dont know if there are any shorts on the
transformer secondaries, and unless there is a way to check from the
main circuit, the labor involved to pull down each one to check would
be ridiculous.

I know that, and that is why I asked if there was normal output at all
locations before the amp shut down. A dead speaker would be a big clue. I
installed a 70.7 volt system in a church and it later turned into a
nightmare. Carpet installers shorted secondary circuits and ... you do not
need to know the rest. S**T happens!


Standard practice is one transformer per speaker, and mounted to the
speaker frame or cabinet. This prevents the secondary circuit from
being exposed.



as far as test signals i think i have a 1Khz Sine wave
signal on my laptop that i can try to load. is there someplace i might
find the test tone of which you speak?

You can indeed use a laptop because there are free-ware signal generators
out there ... Google is your friend.

also, where would i find a 100
watt resistor?

Mouser, Digi-Key, Allied and others ... Google is your friend.

i think the biggest iv'e ever seen is 5 watts. and i may
be showing some more ignorance here, but if i load/play the test tone
to the dummy resistor, then the scope shows a waveform & freq. that
match the test tone, does that mean the amps are ok? if so, then i just
need a huge resistor, test tone and scope :) also, do you know if there
is a way to find out the min/max frequencies that an amp could push to
such a circuit. i think i read that impedance is dependant on
frequency, and that if the impedance were to rise or fall too much,
either too much current or voltage would be needed and could cause
problems. is that to far-fetched?

Impedance is certainly not constant in these systems. But, that is not the
issue here. I suggested a triangular test waveform because, with an
oscilloscope across the output, it is easy to spot clipping and crossover
distortion.

I don't want to insult you, but you might want to consider some outside help
with this. Not via the Internet. An experienced audio technician to go to
the site with you.


I started servicing these types of systems in the mid '60s. We didn't
have anything fancy to troubleshoot with. Most problems were found to
be where someone had tampered with the wiring so try to raise or lower
the volume. I have seen messes that were so bad that they had to be
ripped out, and start over.

Personally, I would have split a system like this into zones of 10
speakers with a home run back to the equipment room for each zone. A
"66" series telephone stake block will handle 25 zones. You remove the
bridging clips to isolate a zone. Then if you have problems, you can
easily isolate it. A "Test tac" made for telephone work and a speaker
with a matching transformer (with a volume control) can be plugged into
any circuit to listen to the amp.

There are impedance bridges available to read the impedance @ 1 KHz,
but I used the test speaker and a small FM receiver and 5 watt amp with
both 25 and 70 volt outputs to feed unknown wiring. It wasn't uncommon
for some idiot to find a wiring cabinet and rip all the wiring apart,
and rip all the tags off, as well.
.

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