Protective zener diode failure

[David Olean]

I have a K3 and for the second time, the protective zener diode has shorted out for no apparent reason. The first time, I sent it back under warranty, but I was charged for the repair. It died again today with the voltage set at 12.4 vdc at the power supply, (an Acopian commercial rack mount unit)  In each case the radio was not being used. It was just sitting there with power applied. Does this happen often?  Anyone else suffer similar events? I had another K3 hooked up at the same time that did not die.  I am at a loss as to why it happens to the same rig. There were no thunderstorms in the area. The power supply still is delivering 12.4 volts.

73
Dave K1WHS
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[Julian, G4ILO]

David Olean wrote

I have a K3 and for the second time, the protective zener diode has shorted out for no apparent reason. The first time, I sent it back under warranty, but I was charged for the repair. It died again today with the voltage set at 12.4 vdc at the power supply, (an Acopian commercial rack mount unit)  In each case the radio was not being used. It was just sitting there with power applied. Does this happen often?  Anyone else suffer similar events? I had another K3 hooked up at the same time that did not die.  I am at a loss as to why it happens to the same rig. There were no thunderstorms in the area. The power supply still is delivering 12.4 volts.

I don't think this happens often. I am running my K3 with 14.0V showing on the internal display. Several people in the past have recommended running the K3 on a higher voltage (below 15V) to get improved linearity from the PA. So it is not a problem if the voltage remains within the recommended limits.

I wonder if something is causing switching transients which are not being filtered out by your supply? I would try using one of those protected extension cables sold for use with computers on the input to your power supply.

[David Olean]

Hello Matt and Julian,
It is D-28, the big fat zener.  I had two K3's running off the same supply.
I have not had any failures of this type with other gear in the past. I
think I will put some logging software on the output of the supply and look
at the results. Both failures happened when I was not there to see it.. I
have to set the logging program sample rate fast enough to see transients
though. The recording might look like several miles long on my screen!!
    Acopian makes commercial supplies. This one is linear with a number of
pass transistors. It seems to be working just fine. Stable wiith great
regulation. However my nose has wrinkled too. I suspect an internal weird
problem with the PS.  I will use a DATAQ data logger on it, so I can watch
the ouitput on the computer at my leisure. I better substitute a big
resistor rather than the K3 this time.

Dave K1WHS


----- Original Message -----
From: "Matt Zilmer" <[hidden email]>
To: "David Olean" <[hidden email]>
Sent: Sunday, November 22, 2009 3:16 AM
Subject: Re: [Elecraft] Protective zener diode failure


Hi Dave -

Does this zener have a designator on the board?  Just curious.  Looks
like it might be D28.

I haven't had this problem, but heard of one K3 user off list that had
this problem - stated same as yours.  He replaced the device and has
had no further problems of this nature.  From over a year ago.


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[allenrw1955]

Dave,
Does the PS have a high voltage protective "crowbar" circuit?
I have never examined the reactive speed of the circuit  (I have them in
Astron linear supplies I own)  so I don't have any idea if they would be
fast enough to protect you but you would think  it would trip  on
overvoltage  if you had a transient high enough to blow the diode.
On the Astron supplies once this trips it stays at O volts output until
the power is recycled. This circuit is troublesome on some Astron
supplies due to tripping for no reason, there are fixes for it.
Bob
KB1FRW

David Olean wrote:

> Hello Matt and Julian,
> It is D-28, the big fat zener.  I had two K3's running off the same supply.
> I have not had any failures of this type with other gear in the past. I
> think I will put some logging software on the output of the supply and look
> at the results. Both failures happened when I was not there to see it.. I
> have to set the logging program sample rate fast enough to see transients
> though. The recording might look like several miles long on my screen!!
>     Acopian makes commercial supplies. This one is linear with a number of
> pass transistors. It seems to be working just fine. Stable wiith great
> regulation. However my nose has wrinkled too. I suspect an internal weird
> problem with the PS.  I will use a DATAQ data logger on it, so I can watch
> the ouitput on the computer at my leisure. I better substitute a big
> resistor rather than the K3 this time.
>
> Dave K1WHS
>
>
>  

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