Fedral Signal siren Interfering with MCS2000

[Wile E. Coyote]

OK - we have seen this so many times now, I am curious if anyone else has seen this same problem:

When using a Federal Signal SS2000SM siren, the MCS-2000 radio would suddenly stop transmitting intermittently. Even the TX light would go out even though you still had your hand on the PTT button.

What we found is when the SS2000SM siren is on, it is dumping a TON of noise back on the 12V line and finds it's way back to the radio. When the siren is at the low part of the audio cycle, the radio will stop transmitting. when the audio is higher, it recovers briefly.

We did everything we could think of to resolve this issue. From moving all power lines involved directly to the battery, and ensuring we were using a good ground point. Nothing worked until we placed a noise filter on the SS2000SM. On top of that, it's happening with all of the newer cars in the fleet!

This leaves me with two questions. 1 - is it normal for the siren to dump all that noise on the 12V line? 2 - why is the MCS2000 Vulnerable to this noise in the first place? would'nt it have some kind of flitering on board to keep that from happening?

anyone else seen this happen?

~WEC

[Wile E. Coyote]

Hmm - I guess nobody else has heard of this.

[thebigphish]

we didn't have a fed sig siren, (we had an HFS295) but i can definately agree w/ the MCS2k (ours was UHF) being incredibly sensitive to the 12v line. We had intermittent problems with ours essentially restarting itself from line noise...i'm not talking voltage drops or anything (we monitored the lines something fierce), but just weird problems until we put a line filter on the 12vdc feed, and all the problems went away...

strange.

[dbcomm]

How close is the 2000SM to the radio?
What did you use to look at noise on the DC?

FS sirens use circuitry that "protects" the AMP from overloading should a speaker be removed/shorted/etc. Speaker disconnected, you can still hear the siren tones through the windings. Do you have the correct speaker installed? A speaker not rated for the power output of the siren could be causing it to dump some audio to ground, creating noise.

Just a couple of thoughts.

[Wile E. Coyote]

the SS2000SM is physically close, but the power wires of both meet at the battery. (we did this to try to eliminate this issue the first time - no luck)

for measuring the noise, we used an o-scope - 3 of them just to be sure. Motorola 2600 Monitor, HP-1741A analog, and a Fluke handheld.

The speaker we are using is a Federal Signal MS-100. We have tried another older driver rated for 100W and we had the same result. So far the only cure is to filter the radio - or in our case the noise source.

We have even seen MCS2K radios acting up from noise being induced by fluorescent lights (in the back of an ambulance). we placed a filter on the power line going to the lights and the radio's started working fine.

FYI - we are using UHF radios also. Thanks for the input so far. any other info will help a lot.

~WEC

[Microwave Mike]

What is needed to be determined is what frequency range the “ noise” is in.
A method to do this is to use a dc current probe on the power line in question.
The probe will have a voltage output in ratio to current drawn. A lot of noise
Can be current spikes as well as voltage spikes.
Find what frequency range the noise is in and use a pi filter, C-L-C. To reduce the noise.
There are a lot of good commercial filters out there at different voltage and current ranges.
I use surplus 50 amp filters rated at 24vdc 50 amps for each of my radios after the fuse
From the DC mains.
Also diode isolation of the siren amp might help.
The current probe with a good scope can also find failing alternators. With a good clean
DC power you should see only a flat line on the scope display. Most current probes have a frequency limit of 100 KHz. Real good ones, like Tektronics, have a frequency limit of 50 Mhz.

Hope this helps

mm

[Wile E. Coyote]

what we saw on the scope was a reproduction of the audio wave on the main B+ line. it was large enough to swing it more than a volt. weird stuff.

[jackhackett]

I've seen something similar, in an ambulance the radio wouldn't transmit, worked fine on the bench. Turned out to be a voltage inverter putting noise on the vehicle A+. I don't recall the type of radio, could have been an MCS or a CDM.