[Drake] T4X sticking relay

[Garey Barrell]

Rick -

I have never seen or heard of a T-4XC using a 15k ohm relay.  My understanding
was that the 15k version was used in the TR-3 and early TR-4's.  The problem was
the very fine wire used in a 15k coil was broken when the coil form expanded
from moisture absorption.  They switched to the 2500 ohm relay sometime in the
life of the T-4X.  Tube manual typical use for the 6EV7 specifies a 250V supply
and a 2500 ohm relay coil.

You may be seeing the result of a gassy 6EV7 rather than a relay problem.  A
little gas, and the tube doesn't cut off as quickly for a given negative bias.

All DC relays suffer from the core magnetization problem sooner or later.
Usually it doesn't cause a problem as long as the armature isn't allowed to
"touch" the pole piece.  Many relays had a copper button on top of the pole
piece for this reason, but I guess some "value engineer" nixed that idea and
decided that all you needed was a "down stop" to keep them from touching.   I
have seen cases where the down stop, (usually just a small arm bent out from the
frame,) after years of pounding bends down enough to allow them to touch, and
stick!  Bending the stop back up "fixes" the problem.

Barry's magnetization idea may be another scenario, although I would think it
wouldn't take 30 years to build up enough magnetization to stick!   Maybe if you
don't transmit very often?!?  :-)

73, Garey - K4OAH
Atlanta

----- Original Message ----- 
From: "Rick Wells (W2RW)" <W2RW@frontiernet.net>

> The coil in my T4XC relay went open some years but I have never been able to
> find a replacement.  It had a 15k ohm coil.  Had to replace it with a unit
> with somewhere around a 2500 ohm coil and it has since never been reliable
> with pull in and drop out.  I believe this is due to the higher current it
> draws through the triode switching circuit.  6EV7 maybe if I recall?
>
> 73,
> Rick, W2RW
>
>
> ----- Original Message -----
> From: "Barry L. Ornitz" <ornitz@tricon.net>
>
> > Jim Barrie, WA8SDF, asked:
> >
> > > Need the group's advice. The relay in my very early model
> > > T4X has started to stick... after transmitting, I have to
> > > hit the PTT switch a couple of times to get my R4C  back on.
> > > I suspect the relay for the antenna isn't releasing...  I
> > > can hear signals, but they are very weak until I cycle he
> > > PTT switch, then I get full audio.
> >
> > You are correct that the problem lies with the relay.  This is
> > a relatively common problem with high-impedance DC-coil relays
> > like this.  One approach is to purchase a new relay.  I think
> > Mouser stocks them.
> >
> > But in most cases, you can cure the problem by simply
> > reversing the leads to the relay's coil.
> >
> > What happens is that over time the DC current magnetizes the
> > relay's iron core, making it stick.  Naturally this does not
> > happen with AC-coil relays.  If you have a very old,
> > powerful,  tape-head demagnetizer, you can use this on the
> > core.  However, since there is more than adequate "pull" from
> > energizing the relay, reversing the leads will tend to
> > magnetize the core in the opposite direction and let it work
> > properly again.  In another 20 to 30 years, you may have to do
> > this once again!    :-)
> >
> > Actually I first saw this problem with a friend's T-4x in
> > 1972.  I have never seen this problem with my unit which I
> > have had since early 1966, but then again I am not too active
> > in using the radio.
> >
> >         73, Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net