[Heathkit] DX-60B Function Switch damage.

[Kenneth G. Gordon]

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