Line speakers and amps.

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pre65
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Line speakers and amps.

Post by pre65 » Wed Apr 03, 2019 1:13 pm

I've been asked to look at an amp and speakers that have been donated to the church in my village.

The speakers have line transformers fitted, but when wired up to the 100v line output on the amp there is no sound. :?

I put a multimeter on the 100v and 70v taps and no voltage is shown, should there be ?
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Nick
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Re: Line speakers and amps.

Post by Nick » Wed Apr 03, 2019 2:50 pm

I put a multimeter on the 100v and 70v taps and no voltage is shown, should there be ?
DC, no, AC, when playing music, Yes.
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Dave the bass
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Re: Line speakers and amps.

Post by Dave the bass » Wed Apr 03, 2019 7:42 pm

Be very wary of getting a shock if the Amp is working when working on 100V line stuff Phil, it really hurts, I speak from unfortunate experience.

In the mid 80's I was working on a schools 'combined broadcast system' which was 2 x 100W 100V line amps that fed PA announcements, class change 'pips' and the fire alarm signal over the entire site. I was trying to find a line fault with the system live (I know, I know...) and some oik set off the Fire Alarm which was a 400Hz squarewave fed into the amps running flat out. I copped it good and proper.

It's still the most painful and upsetting electrical shock I've ever had. I felt very ill and disorientated afterwards.

Be careful Popz.
Dirt eating, low-voltage boy but "'Occasionally phenomenal'"

chris661
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Re: Line speakers and amps.

Post by chris661 » Sun Apr 07, 2019 12:39 pm

If the amp has 100v line outs, then I've definitely expect to see AC voltage (as Nick notes) there.

The amp is also likely to have 8ohm terminals, bypassing the step-up transformer. You could try putting a test speaker (nothing too fragile) across those and see what you get. Check for DC voltage there beforehand.

If that checks fine, the next step would be continuity between the 8ohm terminals and the 100v tap. I'd expect a few hundred ohms at most.

Chris
If you can see the cone move regularly, you haven't got enough speaker.

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