This page is for those interested in repairing US/C-3 Infrared Signaling Telescopes. If you have not already
seen the material about this telescope on the Image Tubes and Night Vision page of this site, take a look there
for history and basic information.
I will show examples of MI-2558(110V AC), MI-2558A(battery), and MI-2558F(battery). The battery units run
on 2 D-cells. I put batteries in and turned the units on. The MI-2558F unit did absolutely nothing when powered
on. The MI-2558A unit starting buzzing as it should, but only made an unfocused green glow in the eyepiece.
|MI-2558F power supply removed from telescope
In the picture above, you can see the image tube socket on the left, and the vibrator and transformer on the right.
At the top is a 0.5uF 200V capacitor that goes across the primary of the transformer, and at the bottom is the 1654 rectifier
It is not necessary in all cases to remove the entire power supply as I have done here. The capacitor can be replaced
and the vibrator points cleaned while the supply is left attached to the telescope. The vibrator connections are plainly visible
so it is easy to check to ensure that they are closed with no appreciable resistance when the power supply is switched off.
If they do not show .1ohm or less, cleaning is in order. I use a piece of paper soaked with De-Oxit, crocus cloth,
and then the paper again. Check with a magnifier to be sure you have not left any fibers behind.
Replacement of the .5uF capacitor is highly recommended as wax-encapsulated paper capacitors of this vintage are almost
|MI-2558A Vibrator and Transformer
In the photo above, you can see that the vibrator contacts are not as easily accessible on the A model as on the F model. I
have not undertaken to disassemble this unit as the vibrator is working. The connections to the vibrator are accessible
|MI-2558F with power supply removed
The transformer has a 1.5V winding for the 1654 filament, and the high-voltage winding. The high-voltage winding
measures about 2700 ohms from the plate of the 1654 to ground. The filter capacitor is a 2000pF, 2500V mica capacitor,
which is unlikely to fail.
If the vibrator is working but you get no green glow in the image tube, you probably have no high voltage. Check
the 1654 rectifier filament, which reads about 10 ohms between pins 1 and 7.
VOLTAGE DIVIDER AND FOCUS CONTROL:
Around the inside perimeter of the tube socket is a string of four 10Meghom resistors inside an insulating sleeve.
These tend to go higher in resistance as time goes by. I have found them to measure between 11Megohms to completely
open. Replacement of these four resistors brought my MI-2558A unit from a condition of unfocusing green glow, to where
it could be brought into electric focus and used.
With the power supply out, you can see the back of the image tube, and the terminals for the focus potentiometer and
power. The terminals are upper right=battery positive, upper left=potentiometer high, lower left=potentiometer low,
lower right=no connection.
The potentiometer should read 0 to 3 meghoms. The fixed resistor is 150k from the low end of the potentiometer
I replaced the 1P25 image tube in one telescope because it had objectionable flaws in the image. I wonder
if someone decided to see why there is a warning against pointing the telescope at the sun, and damaged the cathode.
There is a line parallel to the axis of the tube at the front end that aligns with the copper strip inside the tube mount.
When replacing the socket, see that the gap in the socket fits around the sealed evacuation tube on the image tube.
If you don't and you push too hard, you may break the tube. You will also have to make sure that when you install the
string of four resistors, that there is room between them and the tube for the tube retainer to fit in.
|1P25 Infrared Image Tube
|DC Adapter to power telescope from an external 3 Volt source
Once you have electrical focus, you may want to focus the optics. The eyepiece is straight-forward.
The primary focus is done by removing the cover plate and inserting a tool through the hole in the corrector lens. The
ideal tool would be a rod with a pin through it, but I didn't have the right size rod on hand when I wanted to do the project,
so I filed a piece of copper and soldered it to a brass tube.
I think that most of these units were focused on infinity for long-range signaling use. If you want to use it for
wildlife spotting or close-range demonstrations, refocusing may be worthwhile. I focused one for 50 feet.