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KPA500 receive loss

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KPA500 receive loss

Just noticed this evening that on 160 I lose about 6 dB of rx level (noise and signal ) when I go from STDBY to OPER. On 40 maybe a dB or so , on 20 no change -- On160 almost always am using RX antenna so am not going thru the KPA but this evening had Tx ant on RX .

Maybe has always been that way and just never noticed ???

Hank K7HP

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ANDY DURBIN

KPA500 receive loss

There was a similar report fairly recently. I copied this to my KPA500 notes:

"Reported problem(s):
Receive signal is attenuated when in operate. Receive signal is normal when
in standby.

Problem(s) found:
Confirmed problem. Found that D8, D9 and R7 in the T/R circuit had failed.

Repair(s) performed:
Replaced D8, D9, and R7 on the LPF board."

I didn't record who posted this but I expect you will find this and similar reports in the archive.

Here is another that may be related:

"Elecraft list May 1 2021:

I'm repairing a KPA500 for a friend that has the RX loss in standby mode.
After a time, he also lost receive in operate mode. After doing a teardown,
I found the problem.

With the PA board removed, I checked K23A relay. The normally closed
contact on the antenna side of the relay was open. K23B was OK. With relay
on the board, there was no safe way to energize the relay so I didn't check
the contacts on the normally open side. I also noticed that resistor R7 was
un-populate and pieces of it was stuck to bottom cover.

Looking at the PA schematic I came to the conclusion that the relay K23A was
also having problems on the operate position. With no or a poor connection
to the antenna, the 500-watt output voltage can go sky high. It can get
rectified by D9 and feed a high DC voltage through L4 and damage resistor R7
or even un-solder itself.

I replaced the relay and R7. I remove the cover on the old relay and using
a 12v power supply, I set up bench test. With the relay energized, I
connected a Fluke meter in diode mode and it proved that the antenna
connection in operate mode was intermittent.

-----
Jack WA9FVP"

73,
Andy, k3wyc
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Dr. William J. Schmidt, II

Re: KPA500 receive loss

This happened with my backup amp at the contest station on J6. I left it
connected by accident overnight (too much rum) and we had some static
storms. Took out the pin diodes (well, more accurately the diodes that act
like PIN diodes). When put in operate, the receive drops by 30+ dB but
still transmits fine. A simple replacement of the four diodes and three (?)
transistors solved the problem. Dirt cheap. You can make these
measurements at the test-points to confirm:

TP2= RX 13.1, TX 0.8
TP4= RX 3.9, TX 13

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

email: [hidden email]

-----Original Message-----
From: [hidden email] <[hidden email]> On
Behalf Of Andy Durbin
Sent: Saturday, June 5, 2021 8:20 AM
To: [hidden email]
Subject: [Elecraft] KPA500 receive loss

There was a similar report fairly recently. I copied this to my KPA500
notes:

"Reported problem(s):
Receive signal is attenuated when in operate. Receive signal is normal when
in standby.

Problem(s) found:
Confirmed problem. Found that D8, D9 and R7 in the T/R circuit had failed.

Repair(s) performed:
Replaced D8, D9, and R7 on the LPF board."

I didn't record who posted this but I expect you will find this and similar
reports in the archive.

Here is another that may be related:

"Elecraft list May 1 2021:

I'm repairing a KPA500 for a friend that has the RX loss in standby mode.
After a time, he also lost receive in operate mode. After doing a teardown,
I found the problem.

With the PA board removed, I checked K23A relay. The normally closed
contact on the antenna side of the relay was open. K23B was OK. With relay
on the board, there was no safe way to energize the relay so I didn't check
the contacts on the normally open side. I also noticed that resistor R7 was
un-populate and pieces of it was stuck to bottom cover.

Looking at the PA schematic I came to the conclusion that the relay K23A was
also having problems on the operate position. With no or a poor connection
to the antenna, the 500-watt output voltage can go sky high. It can get
rectified by D9 and feed a high DC voltage through L4 and damage resistor R7
or even un-solder itself.

I replaced the relay and R7. I remove the cover on the old relay and using
a 12v power supply, I set up bench test. With the relay energized, I
connected a Fluke meter in diode mode and it proved that the antenna
connection in operate mode was intermittent.

-----
Jack WA9FVP"

73,
Andy, k3wyc
______________________________________________________________
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Elecraft mailing list

Re: KPA500 receive loss

Would a gas discharge tube save the sensitive parts?

David G3UNA

> On 05 June 2021 at 16:20 "Dr. William J. Schmidt" <[hidden email]> wrote:
>
>
> This happened with my backup amp at the contest station on J6. I left it
> connected by accident overnight (too much rum) and we had some static
> storms. Took out the pin diodes (well, more accurately the diodes that act
> like PIN diodes). When put in operate, the receive drops by 30+ dB but
> still transmits fine. A simple replacement of the four diodes and three (?)
> transistors solved the problem. Dirt cheap. You can make these
> measurements at the test-points to confirm:
>
>
> TP2= RX 13.1, TX 0.8
> TP4= RX 3.9, TX 13
>
>
> Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ
>
> email: [hidden email]
>

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Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

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Dr. William J. Schmidt, II

Re: KPA500 receive loss

I'm sure you could invent many ways to protect these parts (gas tubes, sacrificial TVS diodes, fusing, etc.). The best way, of course, is to disconnect it (which of course I did not do). These are low voltage parts on the receive side path so I don't know if a gas tube would be appropriate here... Someone with more experience with them can chime in here.

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

email: [hidden email]

-----Original Message-----
From: CUTTER DAVID <[hidden email]>
Sent: Saturday, June 5, 2021 10:46 AM
To: [hidden email]; Elecraft Mail List <[hidden email]>
Subject: Re: [Elecraft] KPA500 receive loss

Would a gas discharge tube save the sensitive parts?

David G3UNA

> On 05 June 2021 at 16:20 "Dr. William J. Schmidt" <[hidden email]> wrote:
>
>
> This happened with my backup amp at the contest station on J6. I left
> it connected by accident overnight (too much rum) and we had some
> static storms. Took out the pin diodes (well, more accurately the
> diodes that act like PIN diodes). When put in operate, the receive
> drops by 30+ dB but still transmits fine. A simple replacement of the
> four diodes and three (?) transistors solved the problem. Dirt cheap.
> You can make these measurements at the test-points to confirm:
>
>
> TP2= RX 13.1, TX 0.8
> TP4= RX 3.9, TX 13
>
>
> Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ
> PJ2/K9HZ
>
> email: [hidden email]
>

NK7Z

Re: KPA500 receive loss

In reply to this post by Dr. William J. Schmidt, II

Does that confirm correctness, or brokenness?

73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 6/5/21 8:20 AM, Dr. William J. Schmidt wrote:

> This happened with my backup amp at the contest station on J6. I left it
> connected by accident overnight (too much rum) and we had some static
> storms. Took out the pin diodes (well, more accurately the diodes that act
> like PIN diodes). When put in operate, the receive drops by 30+ dB but
> still transmits fine. A simple replacement of the four diodes and three (?)
> transistors solved the problem. Dirt cheap. You can make these
> measurements at the test-points to confirm:
>
>
> TP2= RX 13.1, TX 0.8
> TP4= RX 3.9, TX 13
>
>
> Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ
>
> email: [hidden email]
>
>
> -----Original Message-----
> From: [hidden email] <[hidden email]> On
> Behalf Of Andy Durbin
> Sent: Saturday, June 5, 2021 8:20 AM
> To: [hidden email]
> Subject: [Elecraft] KPA500 receive loss
>
> There was a similar report fairly recently. I copied this to my KPA500
> notes:
>
> "Reported problem(s):
> Receive signal is attenuated when in operate. Receive signal is normal when
> in standby.
>
> Problem(s) found:
> Confirmed problem. Found that D8, D9 and R7 in the T/R circuit had failed.
>
> Repair(s) performed:
> Replaced D8, D9, and R7 on the LPF board."
>
> I didn't record who posted this but I expect you will find this and similar
> reports in the archive.
>
> Here is another that may be related:
>
> "Elecraft list May 1 2021:
>
> I'm repairing a KPA500 for a friend that has the RX loss in standby mode.
> After a time, he also lost receive in operate mode. After doing a teardown,
> I found the problem.
>
> With the PA board removed, I checked K23A relay. The normally closed
> contact on the antenna side of the relay was open. K23B was OK. With relay
> on the board, there was no safe way to energize the relay so I didn't check
> the contacts on the normally open side. I also noticed that resistor R7 was
> un-populate and pieces of it was stuck to bottom cover.
>
> Looking at the PA schematic I came to the conclusion that the relay K23A was
> also having problems on the operate position. With no or a poor connection
> to the antenna, the 500-watt output voltage can go sky high. It can get
> rectified by D9 and feed a high DC voltage through L4 and damage resistor R7
> or even un-solder itself.
>
> I replaced the relay and R7. I remove the cover on the old relay and using
> a 12v power supply, I set up bench test. With the relay energized, I
> connected a Fluke meter in diode mode and it proved that the antenna
> connection in operate mode was intermittent.
>
>
> -----
> Jack WA9FVP"
>
>
>
> 73,
> Andy, k3wyc
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> 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]
>
>

Elecraft mailing list

Re: KPA500 receive loss

In reply to this post by Elecraft mailing list

This doesn’t really have the signature of pin-diode failure. When the pin diodes fail you generally get major signal loss across all bands. The description simply does not match that diagnosis.
I would ask for further information:

* Is there any change in 160m transmit operation (change in drive requirement)?
* Was any retuning of the 160m antenna required?
* How is the 160m antenna connected? Might it be split to another load somehow?
* Any other changes you see?

Let’s try to diagnose the whole situation. This might not be in the KPA at all, or might be something completely different.

73,
Jack, W6FB

> On Jun 5, 2021, at 12:05 AM, HP via Elecraft <[hidden email]> wrote:
>
> Just noticed this evening that on 160 I lose about 6 dB of rx level (noise and signal ) when I go from STDBY to OPER. On 40 maybe a dB or so , on 20 no change -- On160 almost always am using RX antenna so am not going thru the KPA but this evening had Tx ant on RX .
>
> Maybe has always been that way and just never noticed ???
>
> Hank K7HP
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> 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]

hb9cvq

Re: KPA500 receive loss

In reply to this post by Dr. William J. Schmidt, II

Even (sub) nano second (ns) fast acting gas tubes are primarily not a safe solution to RX front end burnout protection.

Semiconductors:
Issues: Energy Protection level, RX Intermodulation prevention.
There are however some fast semiconductor diodes (ESD protection) available with low capacitance.
They have safe enough voltage protection level even for sensitive RX electronics, but limited energy absorption.

Spark gaps:
A several kV, ns-fast, transient (ESD, EMP, high dV/dt) voltage RX input spike may be better limited in energy but more voltage amplitude; still around 800 to 1000V left over for a short time.
Suitable spark gap can trigger quickly (dV/dt dependent).
There is always some time delay in gap breakdown. Sensitive RX components need to survive that safely, without being pre-damaged .

This low pressure gas ionization process is also governed by HV breakdown Paschen law https://en.wikipedia.org/wiki/Paschen%27s_law .
The number of first available free gas electrons, needed for a quick avalanche breakdown, are important.
By gap fast overvolting (field emission process from electrodes-> source of free electrons, without radioactive , ionizing material ) the spark gap is triggered.
The gas tube would fire at much lower DC voltage levels ( e.g. <100.. 200V+ )

Fast pulse triggering creates a very broadband spectrum of the residual onset pulse (Time -Frequency Domain Fourier transformation ) .
This triggering is different from a finally, time retarded, development of low resistance breakdown plasma channel.
Here the residual voltage may be 30 to 50V.

Too bad, the first spike may have already killed the RX.

Therefore , if capacitance is not an issue (no HF RX) , one uses low pass filters after the spark gap.
To control the energy reduction one can use additionally multi-stage protection modules ( 1.) Gap tube-full energy ,2.) elevated energy semiconductor, 3.) low energy semiconductor )
The triggering of the stages must be well time coordinated. If AC voltages are continuously present in PS Networks , the spark gap (low R Plasma) network follow on currents must be limited to lead to well controlled extinguishing of the gap.

PS:
One other trick to avoid high capacitances (high VSWR) is to arrange protection diodes in a bridge configuration -cancelling out C
One other trick to avoid intermodulation is to DC bias diodes appropriately.

Experience furthermore teaches: Even 4 unbiased Schottky Diodes ( ca. 0.3V each ) in series/anti parallel at RX front end are often critical regarding INTERMOD , e.g.in multi TX contest site scenarios )

End of lecture ...if considered too long and irrelevant just ignore it Hi Hi 😉

-----Original Message-----
From: [hidden email] <[hidden email]> On Behalf Of Dr. William J. Schmidt
Sent: Samstag, 5. Juni 2021 18:02
To: 'Elecraft Mail List' <[hidden email]>
Subject: Re: [Elecraft] KPA500 receive loss

I'm sure you could invent many ways to protect these parts (gas tubes, sacrificial TVS diodes, fusing, etc.). The best way, of course, is to disconnect it (which of course I did not do). These are low voltage parts on the receive side path so I don't know if a gas tube would be appropriate here... Someone with more experience with them can chime in here.

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

email: [hidden email]

-----Original Message-----
From: CUTTER DAVID <[hidden email]>
Sent: Saturday, June 5, 2021 10:46 AM
To: [hidden email]; Elecraft Mail List <[hidden email]>
Subject: Re: [Elecraft] KPA500 receive loss

Would a gas discharge tube save the sensitive parts?

David G3UNA

--
This email has been checked for viruses by AVG.
https://www.avg.com

______________________________________________________________
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Post: mailto:[hidden email]

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Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email]

______________________________________________________________
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Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

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