The electroluminescent dash instruments
were powered by this device which is a transistor oscillator that
converts the 12V D.C. battery input voltage to around 230 to 300V
A.C. output voltage. The following circuit diagram shows that the
circuitry is fairly simple. The orange wire receives the 12 volts
D.C. from the battery when the headlight switch is pulled out. The
white wire is the 230+ volts A.C. output to power the instruments.
The power pack consists of a capacitor, 3 resistors, a transformer,
and the power transistor.
When diagnosing electroluminescent dash
lighting maladies, it is easy to determine whether the power pack or head light switch is at fault. The 60-62 Chrysler lighting has a bulb in the heater and shifter buttons. If you have no lights at the heater and shifter buttons, it’s probably in the
head light switch at fault.
If you have heater buttons lit but no dash lighting then very likely it is caused by a fault or short in one of the
electroluminescent panels, which will kill all of the lighting
(gauges, clock and radio). The gauge dial pointers
might not light up as well, and this is very typical because the
extremely fine “litz” wire that connects to the gauge
pointer breaks over time. That said, a failed gauge pointer will not
kill the rest of the lighting.
If the dash lighting works, but is dim
or fades out quickly, then the problem is with the power pack.
Typically, the capacitor goes bad from age and replacing the
capacitor alone might restore full electroluminescent functionality.
That is good news because we DIY types can easily fix this problem
for only a couple dollars and avoid the need for sending out the unit
for a $100+ overhaul. All you really need is a good soldering iron,
some rosin core solder, and a good digital volt-ohm meter (I use a
Fluke Automotive meter).
So, let's examine the electronic
components inside the power pack.
The transistor will be one of a couple
types of Bendix PNP transistors that are no longer produced. In this
example, it is a PS-26. There are modern replacements that will work
but might require one of the resistor values to be changed. The
transformer can be tested by checking the impedance as indicated in
the schematic. Fortunately, both of these components are often still
operational. In fact, the transformer almost never goes bad. That
leaves the capacitor and three resistors. Always assume the capacitor
is bad. The resistors can also go out of spec with age and heat, and
so need to be checked with an ohmmeter as well. All of these parts
are inexpensive and easy to find online at retailers such as Mouser
or DigiKey. I like the Mouser web site and the $4.99 economy
shipping. The shipping will cost more than the parts.
The capacitor (C1) in this unit is a
.068uf (microfarad) unit rated at 330 volts A.C. Other power packs
might have a .047uf or .05uf capacitor, but a value within 10% to 20%
will be OK. What is critical is the voltage rating. Most capacitors
are rated for D.C. voltage. Make sure to order a capacitor rated for
at least 330V A.C. and preferably more such as 400V A.C. or higher.
Do not use an electrolytic capacitor due to the nature of the
oscillator circuit.
R1 is a 2 ohm 5 watt wirewound resistor
with 10% tolerance. R2 is a 50 ohm 5 watt wirewound resistor without
a tolerance spec (therefore, assume it is 20% tolerance). R3 is a
1500 ohm ½ watt carbon resistor with 20% tolerance. If these
resistors test out within the tolerance spec, you can leave them
alone, or just replace them anyway since you have the unit on the
bench. To test R3, you will need to unsolder one of the two leads to
get a valid reading. The other two resistors will test reasonably
accurate within the circuit. In our example, we replaced the
capacitor and two of the resistors as shown in the photo.
Each power pack will give a slightly
different output reading in A.C. volts. A precise value isn't
important. Anything between 230v and 300V A.C. is probably OK.
To be honest, it will be a lot more
work removing and replacing the power pack in the car than it will be
to repair the unit.
After reinstalling the refurbished
power pack in the Imperial, we have beautiful, full brightness
electroluminescent lighting, for a total repair cost under $2 plus
shipping.