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KPA100 failure

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Paul Wilton-2

KPA100 failure

I was using my KPA100/KAT100 combination today to check out some changes to my OCFD antenna. Previously I have had problems with the balun heating up and the SWR increasing over an extended transmit on 7MHz (eg 50w on an FT8 over). I have made an improvement to my OCFD antenna by adding another choke to reduce the common mode current. So in order to test it out, I planned to run 100w CW for 15-30seconds watching the SWR meter on the KAT100 to see if I was going to get the SWR increase. This was done with a “cold” PA.

So I started the test and unlikely previously, the SWR remained good. However at about the 30s mark, I saw some smoke coming from the KPA100 and then it went dead. It also melted my fuseholder in the PA supply (although didn’t actually blow the fuse).

I have opened up the KPA100 only to find that the two 22R power resistors around the two final transistors (R38 and R39) have both unsoldered themselves from the board and were lying in the bottom of the case!

Clearly it is an easy job just to put them in again but before doing so I want to try and understand what is happening

So a few questions:
a) I am hoping that my two finals are still OK. Is there a simple test I can do perform applying power? Using a DMM in resistance mode, I measure approx 2M between collector and emitter and 11.2R between base and emitter (other than the missing 22R resistors I haven’t disconnected anything else).
b) Any idea what could have caused that much heat that the two resistors unsoldered themselves? It can’t be a DC failure since there is a capacitor blocking the path in the event of a PA transistor shorting to ground. I am concerned that there is some sort of oscillation going on ie the PA is starting to hoot.
c) Any advice on how to proceed?

73

Paul
M1CNK

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Don Wilhelm

Re: KPA100 failure

Paul,

I have three possible guesses.
First is that the KPA100 was developing the full power output that it is
capable of (often 180 watts) because the wattmeter diodes D16 and D17
have been damaged by static (wind, rain, snow, or nearby lightning).
If those diodes are damaged the MCU thinks there is no power output and
drives to it full power capability. If left to continue, the PA
transistors in the base K2 will eventually destroy themselves. The PA
transistors in the KPA100 are quite hardy and seldom need to be replaced
(I have only replaced 1 set in the last 14 years of repairing K2s).

The 2nd possibility is that there is some oscillation. That can happen
more frequently on 40 meters, and the KPA100 tries to transmit at about
8 MHz. Be certain your KPA100 is at the latest level. If you have blue
toroid cores at RFC1 and L16 it is the latest. If not the latest,
upgrade it with the KPA100UPKT.

Thirdly, were the PA transistors tight? If not, the PA transistors will
heat the board around the transistors and not be thermally coupled to
the heatsink. Are the thermal pads properly in place?
After initial assembly (and anytime afterwards if the thermal pads are
changed) you must re-tighten the screws after about 10 hours of
operation at high power.

A 4th possibility is that you have a problem in the T/R switch. After
you get it up and running, do the Diode Voltage checks in the manual.
If you have an older manual but have the latest upgrade installed, use
the chart in the KPA100UPKT instructions rather than what is in your manual.

Note well, the KPA100 is rated for only 10 seconds of keydown power at
100 watts, so you did exceed its maximum rating.

If you melted the fuseholder without blowing the fuse, you may have had
bad contact from the wire to the fuse. That would also result in a low
voltage to the KPA100 and cause it to draw greater than the normal
amount of current because the K2 controls try to keep the power constant
- less voltage requires more current for the same power.

Hopefully some of that information will answer your question.
Do all initial testing into a dummy load and not an antenna and bypass
the KAT100 by connecting the dummy load (and perhaps a wattmeter)
directly to the KPA100 - remove the coax between the KPA100 RF out and
the KAT100 RF IN jacks.

73,,
Don W3FPR

On 1/5/2019 2:34 PM, Paul Wilton wrote:

> I was using my KPA100/KAT100 combination today to check out some changes to my OCFD antenna. Previously I have had problems with the balun heating up and the SWR increasing over an extended transmit on 7MHz (eg 50w on an FT8 over). I have made an improvement to my OCFD antenna by adding another choke to reduce the common mode current. So in order to test it out, I planned to run 100w CW for 15-30seconds watching the SWR meter on the KAT100 to see if I was going to get the SWR increase. This was done with a “cold” PA.
>
> So I started the test and unlikely previously, the SWR remained good. However at about the 30s mark, I saw some smoke coming from the KPA100 and then it went dead. It also melted my fuseholder in the PA supply (although didn’t actually blow the fuse).
>
> I have opened up the KPA100 only to find that the two 22R power resistors around the two final transistors (R38 and R39) have both unsoldered themselves from the board and were lying in the bottom of the case!
>
> Clearly it is an easy job just to put them in again but before doing so I want to try and understand what is happening
>
> So a few questions:
> a) I am hoping that my two finals are still OK. Is there a simple test I can do perform applying power? Using a DMM in resistance mode, I measure approx 2M between collector and emitter and 11.2R between base and emitter (other than the missing 22R resistors I haven’t disconnected anything else).
> b) Any idea what could have caused that much heat that the two resistors unsoldered themselves? It can’t be a DC failure since there is a capacitor blocking the path in the event of a PA transistor shorting to ground. I am concerned that there is some sort of oscillation going on ie the PA is starting to hoot.
> c) Any advice on how to proceed?
>
>

Paul Wilton-2

Re: KPA100 failure

Don
Thanks for the helpful pointers.

Although I didn’t build the KPA100, I did have to fix a few items before it worked. It has got all the updates.

I have checked D16 and D17 - they are OK.

I’ll check for oscillations - should be easy to spot at 8MHz on an SDR

The PA transistors do appear to be tight. However, I have a spare pair of pads so I’ll replace them since I’ll take to undo the heatsink to put the resistors back. I’ll also check out the T/R switch when I recommission everything - I was aware of the changes to the manual thanks.

Hopefully I will be able to do these over the next couple of evenings. And I’ll pay attention to the 10s max power spec!

Thanks again

73

Paul
M1CNK

> On 5 Jan 2019, at 20:12, Don Wilhelm <[hidden email]> wrote:
>
> Paul,
>
> I have three possible guesses.
> First is that the KPA100 was developing the full power output that it is capable of (often 180 watts) because the wattmeter diodes D16 and D17 have been damaged by static (wind, rain, snow, or nearby lightning).
> If those diodes are damaged the MCU thinks there is no power output and drives to it full power capability. If left to continue, the PA transistors in the base K2 will eventually destroy themselves. The PA transistors in the KPA100 are quite hardy and seldom need to be replaced (I have only replaced 1 set in the last 14 years of repairing K2s).
>
> The 2nd possibility is that there is some oscillation. That can happen more frequently on 40 meters, and the KPA100 tries to transmit at about 8 MHz. Be certain your KPA100 is at the latest level. If you have blue toroid cores at RFC1 and L16 it is the latest. If not the latest, upgrade it with the KPA100UPKT.
>
> Thirdly, were the PA transistors tight? If not, the PA transistors will heat the board around the transistors and not be thermally coupled to the heatsink. Are the thermal pads properly in place?
> After initial assembly (and anytime afterwards if the thermal pads are changed) you must re-tighten the screws after about 10 hours of operation at high power.
>
> A 4th possibility is that you have a problem in the T/R switch. After you get it up and running, do the Diode Voltage checks in the manual. If you have an older manual but have the latest upgrade installed, use the chart in the KPA100UPKT instructions rather than what is in your manual.
>
> Note well, the KPA100 is rated for only 10 seconds of keydown power at 100 watts, so you did exceed its maximum rating.
>
> If you melted the fuseholder without blowing the fuse, you may have had bad contact from the wire to the fuse. That would also result in a low voltage to the KPA100 and cause it to draw greater than the normal amount of current because the K2 controls try to keep the power constant - less voltage requires more current for the same power.
>
> Hopefully some of that information will answer your question.
> Do all initial testing into a dummy load and not an antenna and bypass the KAT100 by connecting the dummy load (and perhaps a wattmeter) directly to the KPA100 - remove the coax between the KPA100 RF out and the KAT100 RF IN jacks.
>
> 73,,
> Don W3FPR
>
>
>
> On 1/5/2019 2:34 PM, Paul Wilton wrote:
>> I was using my KPA100/KAT100 combination today to check out some changes to my OCFD antenna. Previously I have had problems with the balun heating up and the SWR increasing over an extended transmit on 7MHz (eg 50w on an FT8 over). I have made an improvement to my OCFD antenna by adding another choke to reduce the common mode current. So in order to test it out, I planned to run 100w CW for 15-30seconds watching the SWR meter on the KAT100 to see if I was going to get the SWR increase. This was done with a “cold” PA.
>> So I started the test and unlikely previously, the SWR remained good. However at about the 30s mark, I saw some smoke coming from the KPA100 and then it went dead. It also melted my fuseholder in the PA supply (although didn’t actually blow the fuse).
>> I have opened up the KPA100 only to find that the two 22R power resistors around the two final transistors (R38 and R39) have both unsoldered themselves from the board and were lying in the bottom of the case!
>> Clearly it is an easy job just to put them in again but before doing so I want to try and understand what is happening
>> So a few questions:
>> a) I am hoping that my two finals are still OK. Is there a simple test I can do perform applying power? Using a DMM in resistance mode, I measure approx 2M between collector and emitter and 11.2R between base and emitter (other than the missing 22R resistors I haven’t disconnected anything else).
>> b) Any idea what could have caused that much heat that the two resistors unsoldered themselves? It can’t be a DC failure since there is a capacitor blocking the path in the event of a PA transistor shorting to ground. I am concerned that there is some sort of oscillation going on ie the PA is starting to hoot.
>> c) Any advice on how to proceed?

Don Wilhelm

Re: KPA100 failure

Paul,

Sometimes it is just better to replace D16 and D17 than to trust them to
an ohmmeter reading.
They can be "half bad" - OK at DC, but not at RF. They are 1N5711 diodes.

If you have the latest updates, the oscillation is not a concern unless
you have a T/R switch problem.

If your thermal pads look OK, reuse them. they already have conformed
to the slight contours of the heatsink and PA transistor surfaces. Just
be sure to put then back the same way they were - they usually stick to
the heatsink, so re=positioning them is not a problem.

Measure those 22 ohm resistors to make certain they have not changed
value due to overheating.

73,
Don W3FPR

On 1/5/2019 3:55 PM, Paul Wilton wrote:

> Don
> Thanks for the helpful pointers.
>
> Although I didn’t build the KPA100, I did have to fix a few items
> before it worked. It has got all the updates.
>
> I have checked D16 and D17 - they are OK.
>
> I’ll check for oscillations - should be easy to spot at 8MHz on an SDR
>
> The PA transistors do appear to be tight. However, I have a spare
> pair of pads so I’ll replace them since I’ll take to undo the heatsink
> to put the resistors back. I’ll also check out the T/R switch when I
> recommission everything - I was aware of the changes to the manual thanks.
>
> Hopefully I will be able to do these over the next couple of evenings.
> And I’ll pay attention to the 10s max power spec!
>
> Thanks again
>
> 73
>
> Paul
> M1CNK
>
>
>
>
>
>> On 5 Jan 2019, at 20:12, Don Wilhelm <[hidden email]
>> <mailto:[hidden email]>> wrote:
>>
>> Paul,
>>
>> I have three possible guesses.
>> First is that the KPA100 was developing the full power output that it
>> is capable of (often 180 watts) because the wattmeter diodes D16 and
>> D17 have been damaged by static (wind, rain, snow, or nearby lightning).
>> If those diodes are damaged the MCU thinks there is no power output
>> and drives to it full power capability. If left to continue, the PA
>> transistors in the base K2 will eventually destroy themselves. The
>> PA transistors in the KPA100 are quite hardy and seldom need to be
>> replaced (I have only replaced 1 set in the last 14 years of
>> repairing K2s).
>>
>> The 2nd possibility is that there is some oscillation. That can
>> happen more frequently on 40 meters, and the KPA100 tries to transmit
>> at about 8 MHz. Be certain your KPA100 is at the latest level. If
>> you have blue toroid cores at RFC1 and L16 it is the latest. If not
>> the latest, upgrade it with the KPA100UPKT.
>>
>> Thirdly, were the PA transistors tight? If not, the PA transistors
>> will heat the board around the transistors and not be thermally
>> coupled to the heatsink. Are the thermal pads properly in place?
>> After initial assembly (and anytime afterwards if the thermal pads
>> are changed) you must re-tighten the screws after about 10 hours of
>> operation at high power.
>>
>> A 4th possibility is that you have a problem in the T/R switch.
>> After you get it up and running, do the Diode Voltage checks in the
>> manual. If you have an older manual but have the latest upgrade
>> installed, use the chart in the KPA100UPKT instructions rather than
>> what is in your manual.
>>
>> Note well, the KPA100 is rated for only 10 seconds of keydown power
>> at 100 watts, so you did exceed its maximum rating.
>>
>> If you melted the fuseholder without blowing the fuse, you may have
>> had bad contact from the wire to the fuse. That would also result in
>> a low voltage to the KPA100 and cause it to draw greater than the
>> normal amount of current because the K2 controls try to keep the
>> power constant - less voltage requires more current for the same power.
>>
>> Hopefully some of that information will answer your question.
>> Do all initial testing into a dummy load and not an antenna and
>> bypass the KAT100 by connecting the dummy load (and perhaps a
>> wattmeter) directly to the KPA100 - remove the coax between the
>> KPA100 RF out and the KAT100 RF IN jacks.
>>
>> 73,,
>> Don W3FPR
>>
>>
>>
>> On 1/5/2019 2:34 PM, Paul Wilton wrote:
>>> I was using my KPA100/KAT100 combination today to check out some
>>> changes to my OCFD antenna. Previously I have had problems with the
>>> balun heating up and the SWR increasing over an extended transmit on
>>> 7MHz (eg 50w on an FT8 over). I have made an improvement to my
>>> OCFD antenna by adding another choke to reduce the common mode
>>> current. So in order to test it out, I planned to run 100w CW for
>>> 15-30seconds watching the SWR meter on the KAT100 to see if I was
>>> going to get the SWR increase. This was done with a “cold” PA.
>>> So I started the test and unlikely previously, the SWR remained
>>> good. However at about the 30s mark, I saw some smoke coming from
>>> the KPA100 and then it went dead. It also melted my fuseholder in
>>> the PA supply (although didn’t actually blow the fuse).
>>> I have opened up the KPA100 only to find that the two 22R power
>>> resistors around the two final transistors (R38 and R39) have both
>>> unsoldered themselves from the board and were lying in the bottom of
>>> the case!
>>> Clearly it is an easy job just to put them in again but before doing
>>> so I want to try and understand what is happening
>>> So a few questions:
>>> a) I am hoping that my two finals are still OK. Is there a simple
>>> test I can do perform applying power? Using a DMM in resistance
>>> mode, I measure approx 2M between collector and emitter and 11.2R
>>> between base and emitter (other than the missing 22R resistors I
>>> haven’t disconnected anything else).
>>> b) Any idea what could have caused that much heat that the two
>>> resistors unsoldered themselves? It can’t be a DC failure since
>>> there is a capacitor blocking the path in the event of a PA
>>> transistor shorting to ground. I am concerned that there is some
>>> sort of oscillation going on ie the PA is starting to hoot.
>>> c) Any advice on how to proceed?
>

N2TK

Re: KPA100 failure

Don,
You are correct about Schottky diodes such as the 1N5711. Don't just trust using an ohmmeter. They are not typical PN bipolar devices like diodes such as the 1N5811. We actually do some tests as if they are a FET. They are so cheap, that if you suspect an issue, just replace them.
73,
N2TK, Tony

-----Original Message-----
From: [hidden email] <[hidden email]> On Behalf Of Don Wilhelm
Sent: Saturday, January 05, 2019 4:12 PM
To: Paul Wilton <[hidden email]>
Cc: Elecraft Discussion List <[hidden email]>
Subject: Re: [Elecraft] KPA100 failure

Paul,

Sometimes it is just better to replace D16 and D17 than to trust them to an ohmmeter reading.
They can be "half bad" - OK at DC, but not at RF. They are 1N5711 diodes.

If you have the latest updates, the oscillation is not a concern unless you have a T/R switch problem.

If your thermal pads look OK, reuse them. they already have conformed to the slight contours of the heatsink and PA transistor surfaces. Just be sure to put then back the same way they were - they usually stick to the heatsink, so re=positioning them is not a problem.

Measure those 22 ohm resistors to make certain they have not changed value due to overheating.

73,
Don W3FPR

On 1/5/2019 3:55 PM, Paul Wilton wrote:

> Don
> Thanks for the helpful pointers.
>
> Although I didn’t build the KPA100, I did have to fix a few items
> before it worked. It has got all the updates.
>
> I have checked D16 and D17 - they are OK.
>
> I’ll check for oscillations - should be easy to spot at 8MHz on an SDR
>
> The PA transistors do appear to be tight. However, I have a spare
> pair of pads so I’ll replace them since I’ll take to undo the heatsink
> to put the resistors back. I’ll also check out the T/R switch when I
> recommission everything - I was aware of the changes to the manual thanks.
>
> Hopefully I will be able to do these over the next couple of evenings.
> And I’ll pay attention to the 10s max power spec!
>
> Thanks again
>
> 73
>
> Paul
> M1CNK
>
>
>
>
>
>> On 5 Jan 2019, at 20:12, Don Wilhelm <[hidden email]
>> <mailto:[hidden email]>> wrote:
>>
>> Paul,
>>
>> I have three possible guesses.
>> First is that the KPA100 was developing the full power output that it
>> is capable of (often 180 watts) because the wattmeter diodes D16 and
>> D17 have been damaged by static (wind, rain, snow, or nearby lightning).
>> If those diodes are damaged the MCU thinks there is no power output
>> and drives to it full power capability. If left to continue, the PA
>> transistors in the base K2 will eventually destroy themselves. The
>> PA transistors in the KPA100 are quite hardy and seldom need to be
>> replaced (I have only replaced 1 set in the last 14 years of
>> repairing K2s).
>>
>> The 2nd possibility is that there is some oscillation. That can
>> happen more frequently on 40 meters, and the KPA100 tries to transmit
>> at about 8 MHz. Be certain your KPA100 is at the latest level. If
>> you have blue toroid cores at RFC1 and L16 it is the latest. If not
>> the latest, upgrade it with the KPA100UPKT.
>>
>> Thirdly, were the PA transistors tight? If not, the PA transistors
>> will heat the board around the transistors and not be thermally
>> coupled to the heatsink. Are the thermal pads properly in place?
>> After initial assembly (and anytime afterwards if the thermal pads
>> are changed) you must re-tighten the screws after about 10 hours of
>> operation at high power.
>>
>> A 4th possibility is that you have a problem in the T/R switch.
>> After you get it up and running, do the Diode Voltage checks in the
>> manual. If you have an older manual but have the latest upgrade
>> installed, use the chart in the KPA100UPKT instructions rather than
>> what is in your manual.
>>
>> Note well, the KPA100 is rated for only 10 seconds of keydown power
>> at 100 watts, so you did exceed its maximum rating.
>>
>> If you melted the fuseholder without blowing the fuse, you may have
>> had bad contact from the wire to the fuse. That would also result in
>> a low voltage to the KPA100 and cause it to draw greater than the
>> normal amount of current because the K2 controls try to keep the
>> power constant - less voltage requires more current for the same power.
>>
>> Hopefully some of that information will answer your question.
>> Do all initial testing into a dummy load and not an antenna and
>> bypass the KAT100 by connecting the dummy load (and perhaps a
>> wattmeter) directly to the KPA100 - remove the coax between the
>> KPA100 RF out and the KAT100 RF IN jacks.
>>
>> 73,,
>> Don W3FPR
>>
>>
>>
>> On 1/5/2019 2:34 PM, Paul Wilton wrote:
>>> I was using my KPA100/KAT100 combination today to check out some
>>> changes to my OCFD antenna. Previously I have had problems with the
>>> balun heating up and the SWR increasing over an extended transmit on
>>> 7MHz (eg 50w on an FT8 over). I have made an improvement to my
>>> OCFD antenna by adding another choke to reduce the common mode
>>> current. So in order to test it out, I planned to run 100w CW for
>>> 15-30seconds watching the SWR meter on the KAT100 to see if I was
>>> going to get the SWR increase. This was done with a “cold” PA.
>>> So I started the test and unlikely previously, the SWR remained
>>> good. However at about the 30s mark, I saw some smoke coming from
>>> the KPA100 and then it went dead. It also melted my fuseholder in
>>> the PA supply (although didn’t actually blow the fuse).
>>> I have opened up the KPA100 only to find that the two 22R power
>>> resistors around the two final transistors (R38 and R39) have both
>>> unsoldered themselves from the board and were lying in the bottom of
>>> the case!
>>> Clearly it is an easy job just to put them in again but before doing
>>> so I want to try and understand what is happening So a few
>>> questions:
>>> a) I am hoping that my two finals are still OK. Is there a simple
>>> test I can do perform applying power? Using a DMM in resistance
>>> mode, I measure approx 2M between collector and emitter and 11.2R
>>> between base and emitter (other than the missing 22R resistors I
>>> haven’t disconnected anything else).
>>> b) Any idea what could have caused that much heat that the two
>>> resistors unsoldered themselves? It can’t be a DC failure since
>>> there is a capacitor blocking the path in the event of a PA
>>> transistor shorting to ground. I am concerned that there is some
>>> sort of oscillation going on ie the PA is starting to hoot.
>>> c) Any advice on how to proceed?
>

______________________________________________________________
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Help: http://mailman.qth.net/mmfaq.htm
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]

______________________________________________________________
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]