On Jul 8, 4:13*pm, (Scott Dorsey) wrote:
Engineer wrote:

We acquired this receiver "new in box", unissued, clean and complete,
with a test report dated "3/2/44" - a good start! *Unfortunately, the
"new" Bendix dynamotor type DA-TA is seized up (tried it on a variable
low voltage DC, the current spiked up, no motion, switched off fast!)
It last ran in 1944 and for the last 65 years has been in storage. *My
guess is the bearing grease has congealed and/or the brushes have
stuck firmly on the commutators. *I just hope there is no rust between
the armature and field core. *Is there any expertise, or any
instructions, out there for servicing this unit? *If so, I would
really appreciate a few pointers from knowlegeable people. *BTW, we'll
be running the radio off external power supplies (app. 250 VDC B+ and
28 VDC for the heaters) but we'd still like the (disconnected)
dynamotor to be servicable.

The big Bendix dynamotors have ball bearings, the little ones have sleeve
bearings. *Remove the two screws from each bell end and look inside. *You
will soon see what is up.

If they are sleeves or open ball bearings, squirt a little turbine oil
into them and rock the thing back and forth. *Check the color of the
oil coming out... if it's not rusty, don't worry about it. *Clean the
open ball bearings out with any reasonable polar solvent (even WD-40 is
probably okay) and repack with Mobil 1 Racing Grease or a high stability
instrument grease.

If they are sealed ball bearings, take 'em out and remove the shields
and look inside. *

Also, of course, open the brushes and check the brushes to make sure they
aren't stuck.

I have seen dynamotors that looks fine outside but were a solid clot of
rust inside. *You'll know when you pull the ends and look inside.

Another thing: the band switch is motor driven. *I've not checked this
as we have not yet wired in the tuning unit, *but it crossed my mind
that this motor, too, could be seized. *Can the band switch be
operated manually (case open, of course), or, if motor OK, by a simple
external switch (I can probably figure the latter out.)

Yup. *I'd also check all the paper capacitors for leakage before applying
actual power to the receiver.
--scott
--
"C'est un Nagra. *C'est suisse, et tres, tres precis."

Scott, many thanks for your reply. I'll pull the ends off the
dynamotor, take a look inside and proceed along the lines you suggest.
As for the band-switch motor, I think I'll pull that assy. right out
(carefully!) and free up the motor before working on the logic - this
may give me an obvious manual band switch change.
Re. paper caps: I've already put up to 200 VDC on the B+ (heaters not
powered, very slow voltage rise), and the current draw is:
-- at 100 VDC, 10 mA
-- at 200 VDC, 26 mA
I sensed that the current went down just a bit over the first hour of
so, suggesting a electrolytic or two is forming. I think C77 and/or
C78 near the B+ filter are electrolytic, plus another, but it's not
marked on our unit - just looks like one, i.e. a greyish can with 3
tabs. I'll see if I can diagnose any DC leakage in the paper/mica
caps in circuit using a DVM "creatively" - I don't want to pull any
out yet as they are very solidly wired in ("military wrap" very close
to component, plus solder.)
Thanks again.
Cheers,
Roger