I replaced my regulator several years ago after a hot trip back from Shasta,
didn't occur to me that the ambient temp might be near the operating temp of
the stock instrument voltage regulator. As Tom said I used an LM317 and the
std application circuit from the Motorola data book, I didn't try to fit it
in the orignal can though, I kept it as a back-up. I calibrated mine to
provide best indication of engine temperature, I can live with the bias in
the fuel level reading. Seems about 8vdc worked best, in fact if I do it
again I'll probably use a 7808 for more compact package. Both devices can
supply ~1.5A so no problem for these old gauges. BTW, I'm really not much of
an electronic type, I found out if you hook up tantalum capacitors backwards
the explode -- thought I had a serious circuit problem but a simple fix. I
learned everything about this mod. including the fact that the roadster has
an instrument voltage regulator from this list. I recommend this mod. for
anyone willing to fiddle with electronics a little, works great and the
gauges are rock steady. The message below is from my old files and provides
useful info for calibrating the regulator.
Todd (still on the list, but so busy I mostly lurk...)Osborn
San Jose, CA
'68 2L DATSFUN
***********************************************************************
Date: 05-Jun-1996 10:15:54
Subject: Re: Instruments and Voltage Regulator
I've been running a homemade solid state instrument voltage regulator in my
'66 for three years now. Its output voltage (which is constant) is 7.3
volts. I adjusted the output as described in Jerome Gallagher's posting
yesterday. That method worked fine for me except MY temp gage reads a bit
high and my gas gage reads a bit low! These gages aren't calibrated and
it's a wonder they work as well as they do.
The regulator consists of an LM317 three terminal voltage regula-tor IC, a
couple of resistors, a capacitor, and a "trimpot" (adjustable resistor for
setting the output voltage as required). All the parts except the trimpot
fit inside the case for the original mechanical "on-off" bimetal strip
regulator, and I glued the trimpot on the bottom of the case with superglue.
It doesn't look that different from the original part and it works a hell of
a lot better. The bimetal strip regulator depends on the thermal "inertia"
of the gages to integrate out the pulsing of the supply voltage. The solid
state regulator just supplies the constant voltage the gages need to do
their job.
If anybody wants a schematic and parts list for this thing, let me know.
As Jerome pointed out, grounds can be a problem. I had to wire up an
external ground to the temp sensor on my 2 liter motor, even though it was
screwed into the metal thermostat housing which was bolted to the head.
Also, you need a good electrical connection between the motor and the frame,
and the gas tank and the frame.
I measured the resistance of the temp sensor by heating it up in a pot of
water along with a thermometer and measuring resistance as a function of
temperature.
Temp (F) R (ohms)
60 400
90 240
120 130
150 75
180 42
210 (208) 27
Water starts to boil around here at 208 degrees so I couldn't go any higher.
However, most roadsters seem to run at about 190 degrees if the cooling
system is right. You could use a 33 ohm 5 watt power resistor from Radio
Shack in place of the temp sensor to "calibrate" your temp gage this way, I
guess.
Barrie
'66 1600
San Diego
Date: 29-Mar-1996 23:01:57
Subject: Voltage requlators - more from Australia
"I worked in a TeleCommunications Research Lab for 8 years. I also observed
the crude voltage regulator (Bi-Metal Strip) type used on the instruments.
By trial & error and measurement, I found the average DC voltage to obtain
the same reading on the gauges with a fixed input resistance to be 7.94
volts. I converted the standard regulator into an electronic type with a
7808 Regulator (+ve type) and 2 x 0.1uf capacitors. The regulated voltage
was 7.92 volts, when complete and its given the SR311 owner no trouble
since. Its a constant voltage device. Well within 7808 specifications.
Lou Mondello
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