Analog Angle Article

My phone dialer dilemma

The phone-line dialer for my home alarm system died just two years after I installed it. That's not a very good MTTF (mean time to failure) for what is a fairly simple circuit in an unstressed environment. The failure mode is that the unit seizes the phone line and won't let go: it's in perpetual dial-out mode.

I opened the plastic box, which is about 5 x 7 inches (13 x 18 cm) to see if there were any obvious problems, such as a scorched component or visibly defective solder joint, but all looked well. I even unsoldered the small battery (or capacitor) which I assume is used to support the internal memory and circuits during power failure, in case the circuit had latched up due to some sort of transient or glitch (hey, it happens).

What bothers me is not only the failure itself, but the cost of the unit versus what it contains. It's has a single, simple circuit board, a single “on” LED, three RJ-11 receptacles, and not much else. It gets its regulated 12 Vdc power from the separate alarm controller unit, so its internal supply circuitry is minimal. Yet it retails for abut $100, despite the very modest BOM and assembly.

Compare this to the alarm unit I bought at the same time, which includes a controller PCB with lots of I/O, a substantial metal case, a separate user panel with various indicator LEDs, a gel cell backup battery, and a 120 Vac to 12 Vac transformer. This entire setup was also about $100, despite the fact that it is far more complex in terms of BOM and assemblies than the dialer, which I figure should retail fro $40 maximum.

Maybe these dialer companies use some extremely expensive components that I don't recognize or fully appreciate, such as the DAA (Data Access Arrangement) interface to the phone line, or surge protectors? Or maybe their vendors are a lot smarter than we know?

I have just about given up any repair fo the dialer, since I lack information and it's a discontinued model. Interestingly, I checked the web for other basic dialers, and they all sell for about $85 and up. There's something going on here I don't understand.

Meanwhile, if anyone out there has insight or information on the Model PD-2 from Linear Corp, (not to be confused with mixed-signal IC vendor Linear Technology Corp.), let me know. It's certainly frustrating to know that it is likely a single small part which will cost me another $75-100 for a replacement dialer.

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