[Heathkit] DX-60B Function Switch damage.

[Richard Solomon]

I tried searching the QST Archive for the original article but came up empty.

Perhaps someone else may have better luck locating it.

73, Dick, W1KSZ
________________________________
From: heathkit-bounces at mailman.qth.net <heathkit-bounces at mailman.qth.net> on behalf of Kenneth G. Gordon <kgordon2006 at frontier.com>
Sent: Saturday, May 22, 2021 11:13 AM
To: Heathkit List QTH <heathkit at mailman.qth.net>
Subject: Re: [Heathkit] DX-60B Function Switch damage.

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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