On Jul 8, 6:11*pm, Engineer wrote:
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- Hide quoted text -

- Show quoted text -

Results, all good:
-- took off end caps, very clean inside, no rust
-- freed-up armature by hand, moved easily, not stuck
-- removed bearing end plates, bearing grease looked a bit congealed,
added some "electric motor" oil to thin it a bit
-- commutator and brushes in perfect shape - factory new, of course
(there are several new brushes in small metal cans actually stored in
each end of the dynamotor - great idea!)
-- powered it up unloaded on var. DC supply, it started to turn at
below 12 VDC (I did not record the exact value), ran it up to 28 VDC,
about 262 VDC open circuit on the B+ supply line
-- added 10 Kohm load to B+ line, 255 VDC loaded (~ 25 mA DC)
-- ran unit for 1/2 hour to mix the bearing lube and polish
commutators/brushes
-- unit now starts to turn at 2.6 VDC (unloaded)
-- reassembled and put back in chassis pending radio test.

Correction: title error. It's not "280 VDC", it's "230 VDC" (I read
the label wrongly)

Pretty good after 65 years on the shelf. Prognosis for band-switch
motor now seems good! But not looked at yet.
Cheers,
Roger