Think I got my smoke alarm problem licked!

[WX2S]

A month or two ago, I got a linear, and immediately afterward started setting our hardwired smoke alaems off.

My first attempt was cheap inline split ferrites from Amazon. No luck.

Second, after some research, I thought it might be differential mode noise. So I made up delta networks of bypass caps, three caps per detector, and put them between the neutral, hot, and interconnect lines. No luck.

Finally, with TX5K running and 4 AM my only clear shot at working 'em on 40, I did some research. Turns out that the interconnect line between the detectors, the one that sets them screaming in unison, is only a 9V DC signal. With 1500w being thrown into the air and my house wiring about 20 meters long, it occurred to me that it could be common mode noise on the interconnect line.

My third attempt was therefore L networks on the interconnect line at every detector. I reused the 0.47 mfd, 1000v caps that I'd bought from Newark ELectronics in my second attempt. The caps went between the interconnect line and the ground wire. For the inductors, I took some big split ferrites, Type 31 mix, that I'd gotten from DX Engineering, and threaded six turns of hookup wire through them. These went between the cap and the detector, so any induced QRM would be steered through the cap and away from the detector. (For the prototype, I['d built a pi network, but this was so bulky that I left out the first cap for the others.)

The detector nearest the shack got a big ferrite around the power leads, just for good measure.

So far it's worked. Got TX5K on 40 tonight, which sweeps 80-10 in CW.   And not a peep out of the detectors. A really thorough test of the setup (key down for at least 30 seconds) will have to wait until the XYL is out of the house, or at least awake.  

For those who are having the same issue, here is where I got the parts.

Caps: http://www.newark.com/jsp/search/productdetail.jsp?SKU=03J6973 . They are quite bulky for the application; since I didn't go to the power lines, I'd use a physically smaller cap with a lower voltage rating if I was doing this from scratch.

Ferrite beads: http://www.dxengineering.com/parts/dxe-csb-275p These are also pretty large, but I see no way of miniaturizing here. If you substitute this part, make sure to get one made from Type 31 ferrite mix. I won't mention the cheapies because they didn't do the job; I think they're likely made from a different mix that's only good at VHF.

I'd appreciate hearing from anyone else who has had, and successfully attacked, this problem. As I said, I don't want to declare total victory just yet.

73,
- WX2S

[WX2S]

P.S. My detectors are BRK/First Alert model 7010B, http://www.amazon.com/gp/product/B000H3AGZO/ref=wms_ohs_product?ie=UTF8&psc=1 .

Side note: This afternoon, I called First Alert to try to get some help. They were very nice to me, but had never heard of RFI problems from hams. Qne of their engineers talked to me and explained that they make their RFI tests with a 5-watt walkie-talkie.  . I personally happen to be in the business of testing large-scale fire-safety systems, so this was pretty surprising; we throw the whole spectrum at ours from DC to daylight. RFI is a big problem with long runs, especially from electrically noisy manufacturing processes and the like.

 For those wishing to inform them about RFI problems or seek solutions, their number is 1-800-323-9005. Please call them and help them improve their product!

73, -WX2S

[KF7BBJ]

Thanks for sharing this.

73,
Jonathan W6GX

[WX2S]

Sadly, it's a no-go. Key down at 1250 watts sets off the detectors in about 10 seconds.

Edit: The problem is limited to a single detector. I unplugged them one by one until they stopped squealing when I keyed the amp. Now into the circuit to see if I made any mistakes.

Edit 2: Problem isolated to a defective detector.

Moral of the story: check the simple stuff first! 

- WX2S

[KF7BBJ]

Hi Steve,

Would the mod. affect the functionality of the detector?  Have you tested it with a real fire/heat source?  Thanks.

73,
Jonathan W6GX

[WX2S]

Hi Jonathan,

The mod would affect signaling between detectors only, not the ability of the detectors to detect smoke. I tested the signaling using the test button on the detectors, and it works fine. In fact it works a little bit too well; I accidentally set off the detectors by holding down the signaling button while I was putting them back in place.

My question to myself now is whether the mod was really necessary at all. If I had done the simple thing to start, and removed the detectors one by one until they stopped alarming when I Keyed the amp, I might've found that there was just one bad detector.

By the way, smoke detectors come in two types, photoelectric and thermal. (There also used to be ionization detectors, but they are becoming uncommon now.) To test a photoelectric detector you would use some test gas. If you want to do this, I'll give you a source for the Gas. To test a thermal detector, use a heat gun or hairdryer.

73,

Wx2s.

[KF7BBJ]

Thanks Steve.  I need to take a look at what kind of detector I have.  My home is about a year old.

The odd thing is that it only goes off on certain band/mode combination.  I need to document the combinations and power levels before I do the mod.  Otherwise I have no way to test the mod.

73,
Jonathan W6GX

[WX2S]

Jonathan,

Good idea -- my guess is that the wiring is resonating at specific frequencies, and maybe some modes (like PSK31 or RTTY) just emit more energy than others. Please let us know what you find.

But before you take the trouble to do the mod, I'd strongly suggest that you eliminate a single (or multiple) bad detectors as the cause of the problem.

73, -WX2S

[KF7BBJ]

Thanks.  I haven't had the time to perform the test.  Hopefully I could get it done in the next couple of days.

73,
Jonathan W6GX