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K3 power switching.

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Alan. G4GNX

K3 power switching.

Following a mishap with a PSU, our local club’s K3 is in need of repair and I don’t want anything similar to happen to mine.

The PSU is a linear device with a voltage control potentiometer and a meter. The operator had an issue with the PSU which was showing an output of 15 volts; the K3 was connected and working. He switched off the PSU to ‘exercise’ the potentiometer and unfortunately turned it back on with the K3 still connected and switched on. The reading was still 15 volts, so he touched the potentiometer again and the K3 started to emit smoke and died. At this time, we’re assuming that the PSU grossly over-volted.

My question: is the power switch on the K3 mechanical? If it is, then my fears will not be realized because I ALWAYS turn on my PSU AND check the output voltage on its meter before turning on the K3.

It will be interesting to find out what died in the K3. I hope it’s not too expensive.

73,

Alan. G4GNX
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Alan. G4GNX

Re: K3 power switching.

Ah yes, I should have thought of that.

So presumably the status is stored in NVM, which would keep it switched on during a power outage.

I’ll now check my PSU and (if none is present) add a crowbar circuit to the output, so that it kills the PSU if it goes above 14.5 volts.

73,

Alan. G4GNX

From: Rick Tavan N6XI
Sent: Sunday, May 10, 2015 7:48 PM
To: G4GNX
Subject: Re: [Elecraft] K3 power switching.

No, it's not mechanical. Since the K3 can also be powered on remotely, the front-panel switch is logical, not hard on/off.

/Rick N6XI

On Sun, May 10, 2015 at 8:42 AM, G4GNX <[hidden email]> wrote:

Following a mishap with a PSU, our local club’s K3 is in need of repair and I don’t want anything similar to happen to mine.

The PSU is a linear device with a voltage control potentiometer and a meter. The operator had an issue with the PSU which was showing an output of 15 volts; the K3 was connected and working. He switched off the PSU to ‘exercise’ the potentiometer and unfortunately turned it back on with the K3 still connected and switched on. The reading was still 15 volts, so he touched the potentiometer again and the K3 started to emit smoke and died. At this time, we’re assuming that the PSU grossly over-volted.

My question: is the power switch on the K3 mechanical? If it is, then my fears will not be realized because I ALWAYS turn on my PSU AND check the output voltage on its meter before turning on the K3.

It will be interesting to find out what died in the K3. I hope it’s not too expensive.

73,

Alan. G4GNX
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Joe Subich, W4TV-4

Re: K3 power switching.

In reply to this post by Alan. G4GNX

> My question: is the power switch on the K3 mechanical?

No. See the schematics available on the Elecraft Download
pages. Power switching is controlled by a FET (Q2 on the
RF board). There is a +13.8 "always on" line via a 1K
resistor for power control and unswitched +13.8V is supplied
to the KPAIO3/KPA3.

73,

... Joe, W4TV

On 2015-05-10 2:42 PM, G4GNX wrote:

> Following a mishap with a PSU, our local club’s K3 is in need of repair and I don’t want anything similar to happen to mine.
>
> The PSU is a linear device with a voltage control potentiometer and a meter. The operator had an issue with the PSU which was showing an output of 15 volts; the K3 was connected and working. He switched off the PSU to ‘exercise’ the potentiometer and unfortunately turned it back on with the K3 still connected and switched on. The reading was still 15 volts, so he touched the potentiometer again and the K3 started to emit smoke and died. At this time, we’re assuming that the PSU grossly over-volted.
>
> My question: is the power switch on the K3 mechanical? If it is, then my fears will not be realized because I ALWAYS turn on my PSU AND check the output voltage on its meter before turning on the K3.
>
> It will be interesting to find out what died in the K3. I hope it’s not too expensive.
>
> 73,
>
> Alan. G4GNX
> ______________________________________________________________
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Alan. G4GNX

Re: K3 power switching.

In reply to this post by Alan. G4GNX

I'm going to bring this up at the next discussion evening. We should be able
to disable the voltage control pot, or replace it with a fixed resistor.

The guy in question is actually very knowledgeable, so it's just bad luck
this time. Even if we'd 'fixed' the PSU, it may have developed the fault
anyway.

73,

Alan. G4GNX

-----Original Message-----
From: Robert Wood
Sent: Sunday, May 10, 2015 9:22 PM
To: 'G4GNX'
Subject: RE: [Elecraft] K3 power switching.

after the fact this is clear but we don't allow variable voltage power
supplies at club
too many fumble fingers with membership over 100 and no telling who shows up
at Field Day!!

73 W5AJ

-----Original Message-----
From: Elecraft [mailto:[hidden email]] On Behalf Of G4GNX
Sent: Sunday, May 10, 2015 1:43 PM
To: Elecraft Reflector
Subject: [Elecraft] K3 power switching.

Following a mishap with a PSU, our local club's K3 is in need of repair and
I don't want anything similar to happen to mine.

The PSU is a linear device with a voltage control potentiometer and a meter.
The operator had an issue with the PSU which was showing an output of 15
volts; the K3 was connected and working. He switched off the PSU to
'exercise' the potentiometer and unfortunately turned it back on with the K3
still connected and switched on. The reading was still 15 volts, so he
touched the potentiometer again and the K3 started to emit smoke and died.
At this time, we're assuming that the PSU grossly over-volted.

My question: is the power switch on the K3 mechanical? If it is, then my
fears will not be realized because I ALWAYS turn on my PSU AND check the
output voltage on its meter before turning on the K3.

It will be interesting to find out what died in the K3. I hope it's not too
expensive.

73,

Alan. G4GNX
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Fred Townsend-2

Re: K3 power switching.

In reply to this post by Alan. G4GNX

Alan the power circuit on the K3 is very robust and is protected from over voltage. There is a 15 volt zener that should trip a self resetting 6A fuse. It may be that zener over heated and shorted but even so there should not be smoke.

I agree your psu should be set so it can not exceed 14.8 volts.

73
Fred, AE6QL

-----Original Message-----

>From: G4GNX <[hidden email]>
>Sent: May 11, 2015 3:36 AM
>To: Elecraft Reflector <[hidden email]>
>Subject: Re: [Elecraft] K3 power switching.
>
>I'm going to bring this up at the next discussion evening. We should be able
>to disable the voltage control pot, or replace it with a fixed resistor.
>
>The guy in question is actually very knowledgeable, so it's just bad luck
>this time. Even if we'd 'fixed' the PSU, it may have developed the fault
>anyway.
>
>73,
>
>Alan. G4GNX
>
>-----Original Message-----
>From: Robert Wood
>Sent: Sunday, May 10, 2015 9:22 PM
>To: 'G4GNX'
>Subject: RE: [Elecraft] K3 power switching.
>
>after the fact this is clear but we don't allow variable voltage power
>supplies at club
>too many fumble fingers with membership over 100 and no telling who shows up
>at Field Day!!
>
>73 W5AJ
>
>
>-----Original Message-----
>From: Elecraft [mailto:[hidden email]] On Behalf Of G4GNX
>Sent: Sunday, May 10, 2015 1:43 PM
>To: Elecraft Reflector
>Subject: [Elecraft] K3 power switching.
>
>Following a mishap with a PSU, our local club's K3 is in need of repair and
>I don't want anything similar to happen to mine.
>
>The PSU is a linear device with a voltage control potentiometer and a meter.
>The operator had an issue with the PSU which was showing an output of 15
>volts; the K3 was connected and working. He switched off the PSU to
>'exercise' the potentiometer and unfortunately turned it back on with the K3
>still connected and switched on. The reading was still 15 volts, so he
>touched the potentiometer again and the K3 started to emit smoke and died.
>At this time, we're assuming that the PSU grossly over-volted.
>
>My question: is the power switch on the K3 mechanical? If it is, then my
>fears will not be realized because I ALWAYS turn on my PSU AND check the
>output voltage on its meter before turning on the K3.
>
>It will be interesting to find out what died in the K3. I hope it's not too
>expensive.
>
>73,
>
>Alan. G4GNX
>______________________________________________________________
>Elecraft mailing list
>Home: http://mailman.qth.net/mailman/listinfo/elecraft
>Help: http://mailman.qth.net/mmfaq.htm
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>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html Message
>delivered to [hidden email]
>
>______________________________________________________________
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>
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Alan. G4GNX

Re: K3 power switching.

Forgive me Fred, but so far I've not been able to locate those components on
the schematics. Do you know which page they're on?

It's also possible that the zener had failed previously, so this time it
didn't cause the fuse to trip. On the other hand, what sort of fuse is it,
mechanical, thermal or electronic? As we all known, chips are the fastest
fuse on multiple legs and given a gross voltage overload, I doubt that a
mechanical or thermal fuse would be fast enough.

I'd prefer to see a full-blown crowbar circuit at the start of the power
circuit and of course the same in the PSU although I do admit to having
defeated a crowbar once. I'd built a 12V 10A variable PSU which would go up
to 15V before the crowbar tripped in and the regulator then saw the overload
and shut the output down. For some reason I had connected the PSU across a
car battery and then absent mindedly increased the PSU voltage past 15V at
which point the crowbar shorted the output, but of course car batteries
don't care about that, so considerable current hit the crowbar and destroyed
it! No other damage, so I replaced the thyristor and learned from the
experience. :-)

73,

Alan. G4GNX

-----Original Message-----
From: Fred Townsend
Sent: Wednesday, May 13, 2015 7:36 AM
To: G4GNX ; Elecraft Reflector
Subject: Re: [Elecraft] K3 power switching.

Alan the power circuit on the K3 is very robust and is protected from over
voltage. There is a 15 volt zener that should trip a self resetting 6A fuse.
It may be that zener over heated and shorted but even so there should not be
smoke.

I agree your psu should be set so it can not exceed 14.8 volts.

73
Fred, AE6QL

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Bruce Beford-4

Re: K3 power switching.

In reply to this post by Alan. G4GNX

> Forgive me Fred, but so far I've not been able to locate those components
on
> the schematics. Do you know which page they're on?

The 12V power in circuitry that Fred referred to is on sheet 3 of 4 or the
K3 RF board, lower right quadrant.
GL,
Bruce N1RX

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Alan. G4GNX

Re: K3 power switching.

Thanks Bruce.

I was expecting it to be on the sheet marked "Power Supply" and skipped past
the RF boards. I have it now. :-)

It looks as if either the zener or the fuse or both have failed, so the rest
of the K3 didn't get protected. What caused the initial failure may never be
known. As for the smoke, all we can say is "it didn't oughta done that"!

This evening's club meeting may well reveal all.

73,

Alan. G4GNX

-----Original Message-----
From: Bruce Beford
Sent: Wednesday, May 13, 2015 12:44 PM
To: [hidden email]
Subject: Re: [Elecraft] K3 power switching.

> Forgive me Fred, but so far I've not been able to locate those components
on
> the schematics. Do you know which page they're on?

The 12V power in circuitry that Fred referred to is on sheet 3 of 4 or the
K3 RF board, lower right quadrant.
GL,
Bruce N1RX

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Fred Townsend-2

Re: K3 power switching.

In reply to this post by Alan. G4GNX

Alan by now you know the offending parts are located on page 3 of the RF Board. Perhaps taking their queue from QST Magazine they go against convention and flow power from right to left so look on the right side for the power entry. Also because these components are supplied as part of an assembly their part numbers are not individually shown.

As for what type parts I do not have the assembly before me. Check with Elecraft for verification of what I list. The part information is my best guess and is provided to assist you in locating the defective parts. Use at your own risk. Digikey will also lead you to the manufacture's data sheets and give you a picture of what you are looking for.

Fuse F1 similar to Digi-Key Part Number 20LR600SU-ND Thermal fuse
Zener D28 similar to Digi-Key Part Number 497-6686-3-ND Manufacturer's # BZW50-15

One more note. The schematic shows a zener diode. However the part number is actually a Transient Voltage Suppressor (TVS). These are slightly different parts.

73,
Fred, AE6QL
See more comments below:

-----Original Message-----

>From: G4GNX <[hidden email]>
>Sent: May 13, 2015 4:29 AM
>To: Elecraft Reflector <[hidden email]>
>Subject: Re: [Elecraft] K3 power switching.
>
>Forgive me Fred, but so far I've not been able to locate those components on
>the schematics. Do you know which page they're on?
>
>It's also possible that the zener had failed previously, so this time it
>didn't cause the fuse to trip. On the other hand, what sort of fuse is it,
>mechanical, thermal or electronic?

Thermal, self resetting

As we all known, chips are the fastest
>fuse on multiple legs and given a gross voltage overload, I doubt that a
>mechanical or thermal fuse would be fast enough.

The normal failure mode for the TVS is shorted. Therefore I doubt it was already failed. The fuse is another matter. They have a soft failure mode which is hard to detect.
>
>I'd prefer to see a full-blown crowbar circuit at the start of the power
>circuit and of course the same in the PSU although I do admit to having
>defeated a crowbar once.

This is pretty much a full blown crowbar circuit except it is self resetting and it does not disconnect the source. Also the rest of the circuit is very robust and has voltage regulators which protect the critical circuits.

I'd built a 12V 10A variable PSU which would go up

>to 15V before the crowbar tripped in and the regulator then saw the overload
>and shut the output down. For some reason I had connected the PSU across a
>car battery and then absent mindedly increased the PSU voltage past 15V at
>which point the crowbar shorted the output, but of course car batteries
>don't care about that, so considerable current hit the crowbar and destroyed
>it! No other damage, so I replaced the thyristor and learned from the
>experience. :-)
>
>73,
>
>Alan. G4GNX
>
>-----Original Message-----
>From: Fred Townsend
>Sent: Wednesday, May 13, 2015 7:36 AM
>To: G4GNX ; Elecraft Reflector
>Subject: Re: [Elecraft] K3 power switching.
>
>Alan the power circuit on the K3 is very robust and is protected from over
>voltage. There is a 15 volt zener that should trip a self resetting 6A fuse.
>It may be that zener over heated and shorted but even so there should not be
>smoke.
>
>I agree your psu should be set so it can not exceed 14.8 volts.
>
>73
>Fred, AE6QL
>
>______________________________________________________________
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