[ale] OT help answering machine keeps failing - faulty power supply?

[Ron Frazier (ALE)]

Hi guys,

I need help with a problem where my answering machine keeps failing.  I 
have the ATT 1740 digital answering machine.  I have a couple of them.  
I really like the features and don't particularly want to change to a 
different model.  However, it appears that they have a design defect in 
the power supply and they keep failing when I have even very minimal 
power glitches.  I've had 3 fail, and just finished getting the last one 
swapped out.  The 1st failure was apparently when lightning caused my 
power to drop off for about 1/2 a second, very brief.  The answering 
machine had a heart attack and froze up. It was plugged into a surge 
protector.  Replaced it.  The 2nd time, the machine was actually plugged 
into an APC UPS.  I also have some routers plugged into the UPS.   I 
pushed the button on the UPS to turn everything off, waited 10 seconds, 
then pushed the button on the UPS to turn everything back on.  The 
answering machine died.  This should have been a very orderly power up 
procedure.  I'm not sure about the 3rd failure.  I suspect that it was 
when I used a table saw to cut a piece of wood.  The answering machine 
was, again, attached to the UPS.  For a second or so when the saw motor 
is turned on, the lights dim substantially.  Later, I found that the 
answering machine had again died.

None of my other equipment has this type of problem.  The answering 
machine is powered by a tiny 6 VAC 350 ma wall transformer, so they must 
be rectifying the power inside the unit.  I can only guess at the cause 
of the problem.  I suspect that the transformer is passing through or 
generating high frequency high voltage transients when the power is 
first applied; and that there are insufficient dc filtering capacitors 
inside the unit.  So, perhaps, circuitry that expects 5 VDC at power up 
gets 10 VDC, or whatever, and has a heart attack.

I want to protect the power circuit before installing this 3rd 
replacement unit, but pretty much have to do it externally.  I thought 
about powering it with a bigger 6 VAC transformer, which might have more 
inductance and might power up more slowly and stably.  I also thought 
about putting an 18V or so varistor across the power circuit, the theory 
being that if the peak to peak voltage exceeds what should be coming out 
of a 6 VAC transformer, it clamps it.  Note that this transformer is a 2 
prong ungrounded device.  I also thought of powering it with a 6 VAC 
regulated transformer, if such a thing exists.  The other thought I had 
was to introduce some sort of time delay.  So, perhaps the 6 VAC 
transformer activates a relay, and that relay then allows the 6 VAC to 
reach the answering machine.  But, the relay couldn't trip until the 
power had stabilized a bit.

I don't want to spend a huge amount of money, since the machine is only 
worth $ 20.  I also don't want to open the case, as I do have an 
extended warranty on one of my two remaining units.

PS, it will be hard to test any theories as I don't intend to 
intentionally try to pulse the power and break another machine.  
However, if a solution sounds plausible and cheap, I'll probably try it.

Any ideas?  Any help is greatly appreciated.

Sincerely,

Ron

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Ron Frazier
770-205-9422 (O)   Leave a message.
linuxdude AT techstarship.com