For no really good reason, I've been working on a solid state replacement
for the electro-mechanical voltage stabilizer that is used on the TR4-6,
Spitfire and some MGs to provide constant voltage to the dash gauges. The
original design does the regulation by using a bi-metal strip to interrupt
the current flow to the gauges at about a 1 Hz rate. Since the gauges are
very slow time constant, thermal devices this is not apparent to the driver
and results in a roughly constant 10V to the gauges.
Since the 1960's, better ways have been developed to regulate voltage. A
friend of mine and I found a solid state regulator that is set at 10V and
have been able to fit the appropriate parts into the original voltage
stabilizer "can". Externally, there is no visible difference between the
original part and the solid-state modification.
I won't go into all the backgroudn on why the this stabilizer is necessary,
but in brief, if the voltage to the gauges varies, the gauge reading will
vary independently of the sensor input. 10 volts is well below the operating
voltage of the battery/generator, so by regulating the voltage at 10V the
gauge reading should be constant regardless of the generator output.
However, the data I've collected from my small sample of stabilizers shows
that they're not very good at regulating (maybe +/- 10%) and the solid state
regulator provides a rock solid 10V (<1% variation) all the time. With the
solid state reguatlor, you can be sure that any gauge variation is not due
to the regulator.
I have tested this on the bench and it works fine. Unfortunately, my TR4A
is not roadworthy and I'd like to have some "field test" before I offer this
design to others as an alternative to the original device. All the major
suppliers offer replacements at about $15-20, but I haven't seen them. I'd
be interested to hear from anyone who has bought a new one recently to see
if they are still the original design or just a functional equivalent.
I'm willing to provide some sample "new" stabilizers to a few people who
will be driving their cars over the next few months. I've got enough parts
to make five, but only two "cans" to put them in so I need old stabilizer
units to modify. If anyone has some old ones I'd be happy to pay postage to
get them to me. If it works out well I plan to write an article for one of
the club magazines about the modification.
The part number is 128484, which is superceded by 148876A. Moss's Part
number is 131-540. The part is marked "Smiths Motor Accessories Ltd
England" and the terminals are marked B, I and E (Earth). Green wires are
attached to the "B" terminal and green with black stripe to the I terminal.
Its mounted under the dash, on the right side, with one screw and is about
1.5" x 3" x 1".
If anyone is having trouble with their gauges, its important the the case of
the stabilizer have good electrical contact to the body for the ground
return. The stabilizer won't work without a good ground.
Please contact me with any questions or comments. Thanks!
Ben Zwissler
bjzwissler@comcast.net
Columbus, IN
66 TR4A IRS OD
80 TR8
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