atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services Security, Alarms and Paging Hum on Page Horns

I replaced a Norstar compact key system with a BCM. I used the existing Bogen amp, horns and cabling.

As soon as the bridging clips went on, we started to hear a hum from every speaker. I tried isolating different speaker runs and couldn't narrow it down. Checked with a meter and found it is a 60 Hz hum.

At the connecting block I removed the bridging clips. No 60 Hz on the speaker side. Nothing on the ksu side. Put the clips back on and the 60 Hz was back.

None of this was there on the old system. Any ideas on what to do next?

Thanks.
Jim

I'd get a few different variations of non-polarized capacitors at Radio Shack and place them across the paging input pair. I've encountered this in the past as well and it really is a trial and error process.

I'm pretty sure that the isolation transformer within the BCM's output is bad (leaking to ground), hence the noise, but a capacitor might help to absorb it.

The caps may stop it but I would install a an isolation transformer and make sure the impeadence matches both units.

I'm with Merritt on this one. An isolation transformer like a Bogen WMT-1 is good at resolving these kind of problems.

How do you have the PA system hooked up? Line port? Trunk port? What kind of amplifier input?

Sam

I got the Bogen WMT-1.
The Bogen amp is connected to the page port of the BCM. They just page from their phone sets.

However, when I connect the WMT-1, I get no hum, but I also get no page output.

???
Jim

Something is not right. Make sure the volume control is turned up.

Instructions

Bridging clips connecting what to what?

-Hal

Hal: Connecting BCM page out to amplifier in.

Merritt, I'm looking at the same instructions taht came with my unit. But, something just hit me between the eyes. I put the WMT-1 between the BCM page out and the amp input. The rca plug going to a phono plug and wired to the amp input. I think this is wrong.

I'll wire it so we go from amp out to the rca plug of WMT-1. Then screw terminals out to page horns.

I feel silly, but hope this is the answer.

Jim

No! The WMT-1 is a matching transformer that matches the impedance of the 600 ohm telephone line to either a 150 ohm mic input or a 15,000 ohm aux input.

The telephone system goes on the input of the WMT-1 and the output of the WMT-1 goes to the input of the Amp. The output of the Amp goes to the speakers.

Sam

No, that is correct but you do have the transformer backwards. Phono plug to the amp in, screw terminals to the KSU page out.

-Hal

Sam and Hal,

Then I did have it connected correctly the first time. But I still get nothing.

Thanks for the link Merritt. I actually have the WMT-1AS, which must be the current model. Here is the paper on it: WMT-1AS Instructions

I'll have to double check all of my connections, jumper setting, etc.. I'm using the page output connection from the phone system.

Before, the phone system page out was connected to the amp's Tel Line input. Do I still use this, or should the rca plug output go to the mic input or aux input, as Sam mentioned?

Thanks.
Jim

Both switches in line position. Connect phone page output to 600 Ohm screws and RCA plug to AUX input on amp.

If no sound then connect butt set to screws to check page output from KSU. If that works connect to RCA plug and see if sound is going through transformer ok. If there is no sound on screw terminals then you could try turning transformer around as the 600 ohm may be to low for the KSU output.

If 600 ohms is too low try moving the slide switch to the spk position which will make the screw input 48k ohm. But you are going to have to crank the volume up on the amp input because it puts a pad on the input to get the impedance up.

-Hal

Got it fixed! It's the little things that get you.

I checked my wiring and it was correct. Next, I removed the WMT-1A (thought I had the 1AS, but it's a minor point) from the wall. On the back side, the shorting block was missing from the pins that select either line or mic input! I remember checking this when I first got the unit, but apparently the shorting bar fell off.

While I was looking at the three bare pins, I heard something rattle inside. Turned it upside down and the shorting bar fell out into my hand. I put a little pressure on one pin so it fit tighter, and everything works like it should.

So, the moral of the story is: make sure it's plugged in before more involved troubleshooting.

Thanks everyone, for all of the help and suggestions. You're the best!

Jim

Thanks for letting us know what the problem was.