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K3 Power Reading/SWR/ALC Problem

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P.B. Christensen

K3 Power Reading/SWR/ALC Problem

I have a K3 at a remote site about an hour north of my home so
unfortunately, I cannot quickly investigate an issue that's recently
developed. After reviewing the K3 schematics, the root cause is likely
isolated to the following parts on the PA board: D36, D37 (BAS70 - Schottky
directional coupler sampling diodes), and/or U16 (LM358 dual op-amp used as
a voltage-follower).

My question: are these three parts leaded or SMT/SMD? Perhaps I'm not
looking hard enough on the Elecraft website but I don't see a detailed
B.O.M. for the K3/K3s. I want to be ready for replacement without wasting a
two-hour trip to/from the site. Thanks.

Paul, W9AC

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Don Wilhelm

Re: K3 Power Reading/SWR/ALC Problem

Paul,

I have not looked to be certain, but there is a 90% probability that
they are SMD.

Contact [hidden email] to obtain the part numbers. Those are not
normally Field Replacable parts.

73,
Don W3FPR

On 1/21/2018 11:02 AM, Paul Christensen wrote:
> I have a K3 at a remote site about an hour north of my home so
> unfortunately, I cannot quickly investigate an issue that's recently
> developed. After reviewing the K3 schematics, the root cause is likely
> isolated to the following parts on the PA board: D36, D37 (BAS70 - Schottky
> directional coupler sampling diodes), and/or U16 (LM358 dual op-amp used as
> a voltage-follower).
>
> My question: are these three parts leaded or SMT/SMD? Perhaps I'm not
> looking hard enough on the Elecraft website
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P.B. Christensen

Re: K3 Power Reading/SWR/ALC Problem

In reply to this post by P.B. Christensen

Dick & Don,

Thanks for confirming. Although unlikely to be the source of trouble, It
also appears U6 and U8 are in the VFWD and VREF circuit paths. I'll order
up everything and have it on hand when I replace this week. It could also
be a passive problem like a marginally-corroded connector, or failed
resistor.

By the way, the problem manifests with high SWR and power set readings that
no longer match anything close to actual output power into known good
loads. The SWR reading fluctuates excessively with change in the PWR
control and slight changes in the control quickly lead to 120W power spikes
when the K3 display indicates 15-20W. This tells me the ALC circuit is not
getting a good forward power sample. So, it appears the SWR bridge or
surrounding circuitry has partially failed.

Paul

-----Original Message-----
From: [hidden email] [mailto:[hidden email]]
Sent: Sunday, January 21, 2018 1:18 PM
To: 'Paul Christensen' <[hidden email]>
Subject: RE: [Elecraft] K3 Power Reading/SWR/ALC Problem

I don't know where purchasing actually buys the parts, but...

D36 and D37 are BAS70INCT Diode, 70V, SOT23, like Digi-Key BAS70INTR-ND
U16 is LM358, DUAL OP AMP, SOIC8, SMD, like Digi-Key LM358DR2GOSTR-ND

73 de Dick, K6KR

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Paul Christensen
Sent: Sunday, January 21, 2018 08:02
To: [hidden email]
Subject: [Elecraft] K3 Power Reading/SWR/ALC Problem

I have a K3 at a remote site about an hour north of my home so
unfortunately, I cannot quickly investigate an issue that's recently
developed. After reviewing the K3 schematics, the root cause is likely
isolated to the following parts on the PA board: D36, D37 (BAS70 - Schottky
directional coupler sampling diodes), and/or U16 (LM358 dual op-amp used as
a voltage-follower).

My question: are these three parts leaded or SMT/SMD? Perhaps I'm not
looking hard enough on the Elecraft website but I don't see a detailed
B.O.M. for the K3/K3s. I want to be ready for replacement without wasting a
two-hour trip to/from the site. Thanks.

Paul, W9AC

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Don Wilhelm

Re: K3 Power Reading/SWR/ALC Problem

Paul,

The diodes are the most likely suspect, so replace them first and re-check.

You are correct that the problem is that the wattmeter is not feeding
the power back to the MCU and the power becomes uncontrolled.

Wattmeter damage is normally a result of static coming in on the antenna
feedline. If you do not have an means of disconnecting the antenna when
not in use, it would be wise to add that - switching the K3 to a dummy
load would be a good addition too.

73,
Don W3FPR

On 1/21/2018 1:39 PM, Paul Christensen wrote:

> Dick & Don,
>
> Thanks for confirming. Although unlikely to be the source of trouble, It
> also appears U6 and U8 are in the VFWD and VREF circuit paths. I'll order
> up everything and have it on hand when I replace this week. It could also
> be a passive problem like a marginally-corroded connector, or failed
> resistor.
>
> By the way, the problem manifests with high SWR and power set readings that
> no longer match anything close to actual output power into known good
> loads. The SWR reading fluctuates excessively with change in the PWR
> control and slight changes in the control quickly lead to 120W power spikes
> when the K3 display indicates 15-20W. This tells me the ALC circuit is not
> getting a good forward power sample. So, it appears the SWR bridge or
> surrounding circuitry has partially failed.

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ab2tc

Re: K3 Power Reading/SWR/ALC Problem

Hi Don and all,

I really don't understand why static build up on the antenna should be a
problem with the K3. Due to the (good) design of the SWR bridge (a two
transformer design with a voltage transformer right across the output) the
antenna port presents a dead short circuit at DC both in receive and
transmit. I just verified this with an ohm-meter (in receive only - no need
to check in transmit since the SWR bridge is surely present then - don't
want to kill my DMM).

Maybe we are talking semantics here but it appears to me that the cause of
diode damage in the SWR bridge circuit is more likely caused by transients
induced into the antenna system by nearby lightning strikes than static
buildup.

Knut - AB2TC

Don Wilhelm wrote

> Paul,
>
> The diodes are the most likely suspect, so replace them first and
> re-check.
>
> You are correct that the problem is that the wattmeter is not feeding
> the power back to the MCU and the power becomes uncontrolled.
>
> Wattmeter damage is normally a result of static coming in on the antenna
> feedline. If you do not have an means of disconnecting the antenna when
> not in use, it would be wise to add that - switching the K3 to a dummy
> load would be a good addition too.
>
> 73,
> Don W3FPR
> <snip>

--
Sent from: http://elecraft.365791.n2.nabble.com/
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Bob McGraw - K4TAX

Re: K3 Power Reading/SWR/ALC Problem

From experience, I'd say nearby lightning strikes would be the
culprit.   I strongly urge everyone to use protection devices on all
coax and balanced lines at the point where the feed lines and rotor
lines enter the house. I prefer Polyphaser devices.   Don't buy cheap
or ham fest bargain table crap! Your radio and your house depends on it.

I don't disconnect antennas nor do I ground antennas. I protect for
lightning outside and prefer to choose methods where it will not be
allowed in the house.    And I'm on a hill and have two towers which
are 500 ft above average terrain out to some 35 miles. I understand
lightning and what it can do. As to lightning, mitigation is the
approach, elimination is not physically possible.

73

Bob, K4TAX

On 1/21/2018 3:42 PM, ab2tc wrote:

> Hi Don and all,
>
> I really don't understand why static build up on the antenna should be a
> problem with the K3. Due to the (good) design of the SWR bridge (a two
> transformer design with a voltage transformer right across the output) the
> antenna port presents a dead short circuit at DC both in receive and
> transmit. I just verified this with an ohm-meter (in receive only - no need
> to check in transmit since the SWR bridge is surely present then - don't
> want to kill my DMM).
>
> Maybe we are talking semantics here but it appears to me that the cause of
> diode damage in the SWR bridge circuit is more likely caused by transients
> induced into the antenna system by nearby lightning strikes than static
> buildup.
>
> Knut - AB2TC
>
>
> Don Wilhelm wrote
>> Paul,
>>
>> The diodes are the most likely suspect, so replace them first and
>> re-check.
>>
>> You are correct that the problem is that the wattmeter is not feeding
>> the power back to the MCU and the power becomes uncontrolled.
>>
>> Wattmeter damage is normally a result of static coming in on the antenna
>> feedline. If you do not have an means of disconnecting the antenna when
>> not in use, it would be wise to add that - switching the K3 to a dummy
>> load would be a good addition too.
>>
>> 73,
>> Don W3FPR
>> <snip>
>
>
>
>
> --
> Sent from: http://elecraft.365791.n2.nabble.com/
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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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]
>

Jim Brown-10

Re: K3 Power Reading/SWR/ALC Problem

I strongly agree with Bob's advice, EXCEPT that I've gotten really tired
of replacing Polyphasers on my high wire dipoles. I've recently switched
to the arrestors that Array Solutions sells, both because I like the
circuit better and because the gas discharge tubes are user replaceable.

The Polyphasers are feed-through types mounted to the entry panel. The
Array Solutions are mounted in a big Hoffman box, for which W6GJB built
some custom mounting brackets. It's mounted to the wall next to the
plate with the Polyphasers, with two 2-in PVC conduit stubs going
through the wall and holes drilled at the bottom of the enclosure for
coax entry.

73, Jim K9YC

On 1/21/2018 2:14 PM, Bob McGraw K4TAX wrote:

> From experience, I'd say nearby lightning strikes would be the
> culprit.   I strongly urge everyone to use protection devices on all
> coax and balanced lines at the point where the feed lines and rotor
> lines enter the house. I prefer Polyphaser devices.   Don't buy cheap
> or ham fest bargain table crap! Your radio and your house depends on it.
>
> I don't disconnect antennas nor do I ground antennas. I protect for
> lightning outside and prefer to choose methods where it will not be
> allowed in the house.    And I'm on a hill and have two towers which
> are 500 ft above average terrain out to some 35 miles. I understand
> lightning and what it can do. As to lightning, mitigation is the
> approach, elimination is not physically possible.

Don Wilhelm

Re: K3 Power Reading/SWR/ALC Problem

In reply to this post by ab2tc

Knut and all,

Yes, BUT...
Lightning is a static charge which *will* produce transients in the
antenna system. As will wind static, rain static and snow static.
Large voltages can build up on the antenna and feedline, and will remain
until dissipated. Sort of like a charged capacitor that holds that
charge until dissipated. Some of those events can produce very large
voltages.

So yes, I believe it is a matter of semantics. Charges of any kind on
the antenna feedline can cause damage. The K3 has more protection than
the K2/KPA100, but that does not make it immune to all events.

Have you ever touched across the terminals of a feedline after a nearby
lightning event and felt a 'tickle' or even worse? I have, and it is
not nice. A bleeder resistor of 2500 ohms to 50k across the feedline
will bleed off a static charge in short order.

Even with a lightning suppression device in the feedline, there is still
a threshold voltage that it responds to. At voltages less than that
threshold, the protection device will not be of any assistance in
bleeding off the charge.

73,
Don W3FPR

On 1/21/2018 4:42 PM, ab2tc wrote:

P.B. Christensen

Re: K3 Power Reading/SWR/ALC Problem

In reply to this post by Don Wilhelm

>"From experience, I'd say nearby lightning strikes would be the
culprit."

Highly unlikely. Clear skies for days. No lightning activity here in a long time here. Two years ago, I had a similar problem with static-damaged Schottky diodes in an Alpha 9500. Those diodes are used to sample RF for the amp's EBS circuit.

>"I strongly urge everyone to use protection devices on all
coax and balanced lines at the point where the feed lines and rotor
lines enter the house. I prefer Polyphaser devices."

Already installed and we never disconnect at the remote site. The site conforms to Motorola R56. No lightning-induced damage in almost 3 years of operation in a very lightning-intense location near the Okefenokee Swamp. High power Alpha-Delta arc-plug devices are located at the master ground bus where all coaxial and control lines enter the station. Low power (200W) Alpha-Delta devices are installed between the K3 and amplifiers. The Alpha-Delta devices use field-replaceable arc-plugs. Like Polyphaser, these devices are intended to mitigate lightning-induced charge, not necessarily antenna static charge. Still, it's possible that one or more of the Alpha-Delta arc plugs has failed, leaving the K3 vulnerable to static damage.

In looking at the K3 SWR bridge circuit, both sides of the Schottky diodes have a reasonably low impedance to chassis ground. One side is slightly less than 22K-ohm. However, it may not be low enough. To supplement protection, I may add a pi-section 1 mH choke between T3 and the chassis. It's the same choke used as a static drain in the Alpha 89 and 87A amplifiers to prevent static-induced PIN diode damage, and the type used at the output of many vacuum tube amps to function as a safety device in the event a plate blocking capacitor fails in a shorted condition that would otherwise allow high-voltage on the output network, transmission line, and antenna.

Although fun to discuss and contemplate fixes, it's conjecture until validated and the root-cause may not be related to any of this.

Paul, W9AC

ly failed.

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Wes Stewart-2

Re: K3 Power Reading/SWR/ALC Problem

I bought a couple of these (TT3G50) from DX Engineering. Unscrewed the gas
discharge plug, took a look and some photos of the inside construction, and sent
them back. Simply awful, (Alpha-Delta, not DX Engineering) BTW, the only
"protection" is a 1000V breakdown gas discharge tube with questionable contact
to the coax center conductor.

Wes N7WS

On 1/21/2018 5:12 PM, Paul Christensen wrote:
> [snip]
High power Alpha-Delta arc-plug devices are located at the master ground bus
where all coaxial and control lines enter the station. Low power (200W)
Alpha-Delta devices are installed between the K3 and amplifiers. The Alpha-Delta
devices use field-replaceable arc-plugs. Like Polyphaser, these devices are
intended to mitigate lightning-induced charge, not necessarily antenna static
charge. Still, it's possible that one or more of the Alpha-Delta arc plugs has
failed, leaving the K3 vulnerable to static damage.
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Edward A. Dauer

K3 Power Reading/SWR/ALC Problem

In reply to this post by P.B. Christensen

I missed the original post that began this thread, so the following may or may not be relevant. I gather the question is about voltage spikes on the antenna or feedline that damaged something nearer the transmitter end. If that is so, I would ask the original submitter if they are using a remote antenna switch that couples the relay DC through the feedline. If so, you might examine the switch to see if the proper diode protections are present on the remote relays. Some time ago I had the PA transistors in a new K2 blown away three times, until Don Wilhelm put me onto an investigation in which I saw on a scope very high voltage short-time-rise spikes on the line when the relay coil fields collapsed, from an MFJ remote switch system that was sold before MFJ changed its design to include those protections. Just a thought from a bad experience.

Ted. KN1CBR

------------------------------

Message: 6
Date: Sun, 21 Jan 2018 16:14:55 -0600
From: Bob McGraw K4TAX <[hidden email]>
To: [hidden email]
Subject: Re: [Elecraft] K3 Power Reading/SWR/ALC Problem
Message-ID: <[hidden email]>
Content-Type: text/plain; charset=utf-8; format=flowed

From experience, I'd say nearby lightning strikes would be the
culprit.?? I strongly urge everyone to use protection devices on all
coax and balanced lines at the point where the feed lines and rotor
lines enter the house.? I prefer Polyphaser devices.?? Don't buy cheap
or ham fest bargain table crap!? Your radio and your house depends on it.

I don't disconnect antennas nor do I ground antennas.? I protect for
lightning outside and prefer to choose methods where it will not be
allowed in the house.??? And I'm on a hill and have two towers? which
are? 500 ft above average terrain out to some 35 miles.? I understand
lightning and what it can do.? As to lightning, mitigation is the
approach, elimination is not physically possible.

73

Bob, K4TAX

On 1/21/2018 3:42 PM, ab2tc wrote:
> Hi Don and all,
>
> I really don't understand why static build up on the antenna should be a
> problem with the K3. Due to the (good) design of the SWR bridge (a two
> transformer design with a voltage transformer right across the output) the
> antenna port presents a dead short circuit at DC both in receive and
> transmit. I just verified this with an ohm-meter (in receive only - no need
> to check in transmit since the SWR bridge is surely present then - don't
> want to kill my DMM).
>
> Maybe we are talking semantics here but it appears to me that the cause of
> diode damage in the SWR bridge circuit is more likely caused by transients
> induced into the antenna system by nearby lightning strikes than static
> buildup.
>
> Knut - AB2TC
>
>

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Karin Johnson

Re: K3 Power Reading/SWR/ALC Problem

In reply to this post by P.B. Christensen

Paul:
I had a problem with my K2 with the same identical symptoms you are
describing.

The fault was diagnosed to an intermittent ground/shield connection in the
RF output

Cable going to my amplifier. It was an old cable that was out in the
weather and when

I took it apart you could see the corrosion in the PL259 inside the holes
for grounding.

This cable worked for me for several years but recently failed.

Bring some extra jumper coax with you.

Karin Anne Johnson P.E. K3UU

Palm Harbor, FL

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P.B. Christensen

Re: K3 Power Reading/SWR/ALC Problem

In reply to this post by Don Wilhelm

Epilog: Today, I replaced both BAS70 Schottky diodes (D36/D37) and now the K3 SWR readings are stable and the internal ALC is working as expected. At this time, it appears diode failure was caused by antenna static.

A suggestion to those who may run into this problem in the future: it turns out that D36 and D37 are easily accessible by removing only the bottom rear plate. Access could not be easier. The diodes are SMT/SMD and so great care must be exercised in the rework process.

What should have taken me less than 30 minutes turned into several hours. I had convinced myself that the diodes were either on the PA module or KAT3 and so I had disassembled much of the K3. The K3 block diagram doesn’t show a module where the SWR bridge resides. The schematic indicates "K3 RF Board," but that's not the PA module. The RF Board is actually the flat motherboard that the PA module and KAT3 plug into. Once that detail was figured out, I reassembled the K3, leaving only the bottom plate open for the repair.

Paul, W9AC

-----Original Message-----
From: Paul Christensen [mailto:[hidden email]]
Sent: Sunday, January 21, 2018 7:12 PM
To: '[hidden email]' <[hidden email]>
Subject: RE: [Elecraft] K3 Power Reading/SWR/ALC Problem

>"From experience, I'd say nearby lightning strikes would be the
culprit."

Highly unlikely. Clear skies for days. No lightning activity here in a long time here. Two years ago, I had a similar problem with static-damaged Schottky diodes in an Alpha 9500. Those diodes are used to sample RF for the amp's EBS circuit.

>"I strongly urge everyone to use protection devices on all
coax and balanced lines at the point where the feed lines and rotor
lines enter the house. I prefer Polyphaser devices."

Already installed and we never disconnect at the remote site. The site conforms to Motorola R56. No lightning-induced damage in almost 3 years of operation in a very lightning-intense location near the Okefenokee Swamp. High power Alpha-Delta arc-plug devices are located at the master ground bus where all coaxial and control lines enter the station. Low power (200W) Alpha-Delta devices are installed between the K3 and amplifiers. The Alpha-Delta devices use field-replaceable arc-plugs. Like Polyphaser, these devices are intended to mitigate lightning-induced charge, not necessarily antenna static charge. Still, it's possible that one or more of the Alpha-Delta arc plugs has failed, leaving the K3 vulnerable to static damage.

In looking at the K3 SWR bridge circuit, both sides of the Schottky diodes have a reasonably low impedance to chassis ground. One side is slightly less than 22K-ohm. However, it may not be low enough. To supplement protection, I may add a pi-section 1 mH choke between T3 and the chassis. It's the same choke used as a static drain in the Alpha 89 and 87A amplifiers to prevent static-induced PIN diode damage, and the type used at the output of many vacuum tube amps to function as a safety device in the event a plate blocking capacitor fails in a shorted condition that would otherwise allow high-voltage on the output network, transmission line, and antenna.

Although fun to discuss and contemplate fixes, it's conjecture until validated and the root-cause may not be related to any of this.

Paul, W9AC

ly failed.

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