[Heathkit] DX-60B Function Switch damage.

[Mark K3MSB]

Hi Ken

Thanks for posting that.   I had not considered that failure mechanism -- I
was fixated on inrush current.

I had to install a snubber across the HV relay contacts in my BC-456
modulator a few years ago.   Years of use had caused significant damage to
the relay contacts,  and it got to the point where the relay contacts would
arc and not disengage.     I put in the snubber, as recommended by Dave
AB5S, when I installed the new relay.

So, perhaps a snubber and inrush limiting resistor may be a good fix.

>>This past month, Electric Radio Magazine published a nomograph....

I immediately picked up the May issue off the table as I couldn't remember
seeing such a graph,  and couldn't find it.   Looked twice.   With my
reading specs on....

I then ran out to the mailbox in hopes the June issue was here......    No
luck.

What issue Ken ?

73 Mark K3MSB


On Sat, May 22, 2021 at 2:13 PM Kenneth G. Gordon <kgordon2006 at frontier.com>
wrote:

> On 21 May 2021 at 22:30, Mark K3MSB wrote:
>
> > For the 260V secondary that gets switched by the function switch.....
> >
> > A relay is the best solution,
>
> Well, not necessarily the best solution, but a good one, nonetheless.
>
> IMHO, what is ruining that switch is a "switch-off transient". This is a
> very high-voltage arc,
> (measured in one case at over 10 kV in a small power supply) caused by the
> sudden
> collapse of the magnetizing field at the transformer when the load is
> suddenly removed. This
> arc is bigger when the transformer is of better quality, i.e. exhibits
> lower resistance in its
> windings.
>
> This issue was brought up and discussed many years ago in both a GE Ham
> News, and in
> an RSGB handbook I have here.
>
> Both sources provided formulae which would permit one to calculate the
> values of C and R
> for an "RC Snubber" to eliminate this problem.
>
> This past month, Electric Radio Magazine published a nomograph which had
> been published
> in a QST magazine long ago which allowed us to arrive at the correct
> combination of R and C
> values for such a snubber without doing any calculation.
>
> What an "RC-Snubber" consists of is a series-connection of a capacitor and
> a resistor, the
> values of such being determined, broadly, by the voltage in the circuit
> and the current being
> switched, this combination being installed directly across the switch
> contacts involved.
>
> When the R and C values are chosen correctly, this device completely
> elminates the arc that
> occurs at switch-off.
>
> An RC-Snubber is far simpler, cheaper, and far easier to install than a
> relay. Besides, in
> order for that arc to not eat the relay contacts, an RC-Snubber should be
> installed across
> those contacts anyway.
>
> Also, it has become pretty clear of late that this "switch-off transient"
> is what causes power
> transformers to eventually fail when that method of switching a
> transmitter to standby by
> opening the center-tap connection to ground is used. The "switch-off
> transient"-caused arc
> eventually penetrates the insulation of the HV transformer winding
> somewhere and thus
> shorts it out. An RC-Snubber installed at the swtich contacts should
> elminate this issue.
>
> I believe this is what has caused the power transformers in the DX-35 and
> DX-40 to fail so
> soon. In 1956, and 1957, I lost two power transformers in my DX-35 within
> the first year of
> operation.
>
> vy 73 for now,
>
> Ken W7EKB
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