[ARC5] SCR-274 Quick Refurb and Checkout Part 1- Dynos.

[David Stinson]

(Sorry this has taken awhile- Life interferes...)

(Copied to the list for general information...)
(I'll do this in several parts)

> ......  The biggest question that I have
> is how do I trouble shoot each receiver, transmitter, dynamotor, etc
before
> assembly? Without hooking them up I have no power or headset or microphone
> access???  Do you mean check all tubes, condensers and etc with test
> equipment with no power on? I need some guidance here.

I advise against assembling the set and powering it
without doing considerable refurbishing first.
One would not fly an aircraft that was sitting in a barn
for 60 years without doing a lot of work first.
If one flies the aircraft or runs the radio
without doing the work, the results for each will be the same.

Here are some procedures that will get your sets ready to power-up.
Some are labor intensive; if it was easy, it wouldn't be special, right?
Before you power up any set, do these steps:

Put a large towel or blanket on your work bench;
one that doesn't give off a lot of lint.  This catches dropped screws
and small parts so they don't bounce off the table and into oblivion.
You'll need a power supply that can handle the starting surge
of the transmitter dynamotor.  Just about anything will start the
receiver dynos; they don't have that much mass/inertia.
For the transmit dyno, you will need about 25 amps capacity
or, if you have a more common 15 amp supply, hang some of those
computer-grade big capacitors across the output to supply the
starting surge- I use about 200,000 uFd on mine and it works well.

Dynamotors:
Open the end bells.
Examine the commutaters for damage.
Discoloration is normal.
If they're noticeably worn down
(shaved down to the mica spacers between the bars)
or obviously damaged, set this one aside;
that's more than the average collector can fix.
The group can find you a replacement.
Ditto for heavy water corrosion or rust.

Examine the brushes *one at a time.*
Make a pencil mark on the end
so you can put it back "right side up."
If they're worn down, they need replacing.
I was told that if the length of the brush was
less than 1.5 times it's thickness, it should be replaced.
(Comments on that "rule of thumb?")
I have spares; just ask.
Check the wire solder connections carefully.
The noise suppression capacitor is often bad.
Check it at full operating voltage plus 20% (spikes)
or summarily replace.

See if the armature will turn by hand.
Sticky is OK.  Stuck is not.  Don't force it.
Check for junk between the armature and field.
If, after checking the bearings, it's still stuck,
put it aside and talk to the group for suggestions.

Place a pencil mark on the round bearing covers to the frame
so you can put them back as they came out.
Use the correct size screwdriver to remove the screws.
If a screw won't come out easy, place the dyno on its side,
put the screwdriver in the screw head slot and
gently tap the screwdriver to break the screw loose.

Once you get it open you will find a set of shims.
Place the cover and the shims on your bench
in the order they came out, so you can put them back
the same way.
If there is dried grease on the bearing cover, clean it off.
If you use mineral spirits or some other compound,
be sure it's all gone before reassembly.

Examine the grease.  Turn the armature (don't use the
commutater- push the windings).  If the grease looks
"wet" and serviceable, work 3-4 drops of small electric
motor oil into the bearing.  "3-N-1" makes one in a
blue bottle that is commonly available.
Don't use common lubricating oil.
If the grease looks oxidized or dried,
pick out as much as possible.  If it's really bad,
or if it has the whiteish cast that means water contamination,
you'll need to pull the armature and clean it with mineral spirits.
If that's the case, ask about it and we'll turn to the experts.
Get some good hi-speed bearing grease.
Mike Hanz recommends one made by Mobil (Mike?).
If you can't find that, find your local electric motor/sewing machine
repair shop and ask them for a dollop.
Put some on your finger and press into the bearing.
Rotate and press some more.  Don't put in so much
that it mushes out between the back of the bearing
and the grease slinger disc.  If that happens, you'll
need to clean it out or it will foul the commutater.

Reassemble the bearing shims and covers and
put the power to it.  It should spin up freely.
A couple of seconds of stiffness and/or
over-sparking of the brushes is normal at first,
but if they continue to "throw fire" more than a few
seconds, look for brush or commutater problems.
If the motor "lugs down," disconnect immediately;
every second of overheating in a stalled/lugged motor
is doing damage.  Find the reason before proceeding.


REMEMBER-
There is HIGH VOLTAGE present.
250 volts and 650 volts
will knock your dinky in the dirt.
Respect it, or we'll be paying our respects.

Let the motor run.  Look and listen.
Put your hand on it regularly to check for overheating
in both the windings and the bearings.
If you can't keep your hand/finger on it for a count of
ten in this unloaded condition, something's getting too hot.
If you hear rattling/vibrating bearings, don't ignore it.
Shut down and talk to the group.
Check for output voltage.
If you can run the motor for an hour without excessive heating
or bearing problems, you're ready to go.
Put the end bells back on it.  Use a pencil to make
a notation on the bottom of the dyno of the
date you lubricated it.

There are many list members who are smarter
than me and will have valuable suggestions
about this.  They are your best resource.

More later- Gotta go to work.

73 Dave S. AB5S