Film-Tech Forum ARCHIVE: Testing Amplifiers

This is topic Testing Amplifiers in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Manny Knowles (Member # 1171) on 02-13-2003, 05:51 PM:

Maybe I got a bad batch of used amplifiers (QSC 1400). Or maybe there's something I am missing here...

I noticed yesterday that the amplifier for the subwoofer would not turn on. I tried using other AC outlets but that didn't help. The outlets were tested with another amplifier in the rack. All outlets are working. So that amp must have developed a problem. Anyway, that first amp is dead.

I replaced it with one of the used spares I recently received. It powered up, but I got no noise in the theatre. I double checked the DIP switches (1,2,7 & 8 are up) and I tried in both Bridge and Stereo modes. Nothing in the theatre.

I did a quick check of the speakers and cabling by playing some (low-volume) pink noise (from the CP-45) and temporarily plugging the SW speaker cables into the amp for the Center LF. I got sound in the theatre, so the cables and speakers don't seem to be the cause of the trouble.

So that amp had to come out. I replaced it with another used amp (both came out of a UA theatre) and lo! and behold...nothing.

Could it be that I tried to replace a dead amp with TWO dead amps in a row? I have other spares, but I'm wondering if there's something else I should check before hauling out a third (or fourth) amp.

For instance, I did a quick check of the processor output. With pink noise playing I get a small (mV) reading. I could observe the numbers increase/decrease as I adjust the volume control up and down. So I *think* I'm getting a signal at the output. I checked it at the processor end and I observed the same reading, so I believe there is nothing wrong with the hookup wires from the processor to the amplifier.

Any suggestions? Should I just bite the bullet and try another amp?

In the meantime, I'd appreciate a checklist for doing a quick-and-dirty testing of used amps before buying and after uncrating.

Posted by Paul G. Thompson (Member # 655) on 02-13-2003, 06:01 PM:

Does the green light come on? And, has the circuit breaker (the little black button one) next to the power switch been reset?

There is nothing else I can think of other than two bad amplifiers. Even if the speaker outputs were dead shorted, it would not prevent the amplifier from powering up.

Posted by Michael Schaffer (Member # 1204) on 02-13-2003, 06:04 PM:

Do you have a booth monitor between processor and amplifier?
edit- of course, you have checked the gain controls of the amplifier. I feel a bit stupid to mention it, but sometimes we overlook the most obvious things in the heat of battle.

Posted by Manny Knowles (Member # 1171) on 02-13-2003, 06:15 PM:

Thanks for adding that, Michael. Yes, I checked the gains on both of these amps.

Paul I did not check the reset breaker on the first amp. I will test it out tomorrow.

Wouldn't that be a funny thing if I tried to replace an OK amp with two bad spares?

So how should a person test a used amp to make sure that it's OK before buying or installing?

Posted by Gordon McLeod (Member # 33) on 02-13-2003, 06:37 PM:

I usually place a amp on a dummy load on the bench (several ohmrite resitors 200watts each in a big bank) and drive it for several hours full out with both a sine wave and a pinknoise source
On the USA or 1400 I have had the bridge fail on some as well as the current limiting resistor

Posted by Manny Knowles (Member # 1171) on 02-13-2003, 07:41 PM:

If the bridge dies, will it still work in stereo mode?

Posted by Gordon McLeod (Member # 33) on 02-13-2003, 07:54 PM:

bridge rectifier failure

Posted by Michael Rourke (Member # 1142) on 02-14-2003, 12:51 PM:

Unless the crossover plugs right into the amp I don't think the DIP switches matter. Could it be a blown fuse? Maybe whoever sold you the amps pulled the fuses out for backups.

Posted by Gordon McLeod (Member # 33) on 02-14-2003, 05:51 PM:

If you have the dip switchs mis programed it will not work as a basic stereo amp

Posted by Ray Derrick (Member # 1001) on 02-14-2003, 07:21 PM:

Manny, here are some quick and dirty tests you can do with just an audio signal generator and a CRO.

1. Connect a basic audio signal generator to the inputs (and nothing to the outputs), turn the amp on it on and look at the outputs with a CRO. With a 1kHz sine wave input you should see a clean waveform on each output. As you wind the level up and down you should not see any splikes or glitches on that sine wave. When the level reaches clipping, the waveform should square off nicely without any spikey or unstable artefacts. Try sweeping the frequency from 20Hz up to 20kHz and look for any glitches in the waveform.

2. Connect a speaker alternatively to each output and listen to the tone (at low volume). The tone should sound pure and clean with no audible harmonics, distortion or noise modulation. Once again, try sweeping it from 20Hz to 20kHz. The tone should always sound pure.

3. Disconnect the sig generator and turn the volume full up. Listen closely to the speaker for any excessive noise, crackles, pops or other garbage.