[Milsurplus] Vibrator failure... even more

[J. Forster]

Subject:    Re: [armyradios] Vibrator failure... more
    Date:    Mon, 24 Nov 2003 21:15:32 -0500
   From:    Bruce Gentry <bwgent@dreamscape.com>
Reply-To:  armyradios@yahoogroups.com

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In my earlier years servicing automobile entertainment radios, I had
more than a few come across the bench with a 12 volt 4 pin vibrator in a
6 volt radio. It would indeed smoke the vibrator contacts and sometimes
the power transformer. One common reason for this mistake was the use of
8 volt batteries in older cars to encourage starting in cold weather. 8
volts was too much for many 6 volt vibrators and way too little for a 12
volt vibrator. Remember that many military radio vibrators have a
separate interrupter contact for the  coil. They will buzz even if the
main contacts are not carrying any load. Be sure that the voltage to the
coil is correct. Many 12 or 24 volt sets use a vibrator with a 6 volt
coil and feed the coil through a resistor. Using a vibrator that
requires more current than the original could mean that the coil is
underpowered. If you must use a substitute, change  the resistor until
the voltage is correct.

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The reed must swing over it's full travel to
cause the contacts to touch and then wipe. If they simply touch, they
will burn and any dirt that gets between them will cause poor switching.

I highlighted this part of Bruce's post. I am now convinced that contact
pressure and contact opening speed, both of which depend on proper drive
voltage, are critical to a vibrator's survival.

*************************
AND- be sure the buffer condensers are good. Military sets often have
them on both the primary and secondary of the power transformer. Don't
substitute their value. They actually tune the transformer to resonate
with the chopped DC pulses. Likewise, be sure the vibrator is the
correct frequency. Military sets often use a 100 cycle vibrator, car
radios usually used a 115 cycle. Catch a buzz without smoking
anything.... Bruce

IMO, Bruce is exactly on point here also. Vibrator circuits are carefully
optimized and nowhere near as simple as they look. An accurate
electro-mechanical model of the vibrator is clearly non-linear, but can be
broken into two linear models.

-John

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