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KPA100 SWR bridge adjustment 160m & 80m

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Koert Wilmink

KPA100 SWR bridge adjustment 160m & 80m

Dear all,

Please find my emails to Elecraft support below. Is there maybe somebody with additional tips?

Thanks!!

73 Koert PA1KW

-----------------------------------------------------
Hi Gary,

Sorry for my late response, but I had not the opportunity to work on
this earlier. I moved houses and that took longer than expected.

I did check the HV bias and that is 135volts. The diodes are ok and the
voltage on TP4 2mV on receive and 240mv on transmit (tune).
I did connect a good dummyload close to the KPA100.
I did the re-adjustment of the SWR bridge but this provided me not the
required results. The best results with adjusting C1 on 5 Watts is:

160m 1.8-1
80m 1.3-1
40m 1.0-1
30m 1.0-1
20m 1.0-1
18m 1.0-1
15m 1.0-1
12m 1.0-1
10m 1.0-1

Please help......

Best regards and 73,

Koert Wilmink
PA1KW

PS.
All the best for 2008!!

Gary Surrency wrote:

> > Did you check to see that the HV bias at the left side of R11 is at least
> > 100VDC, and the higher it is - the better? I like to see 120-150VDC
> > there.
> > Too little HV bias, or a damaged diode or error in the t-r switch, can
> > allow
> > too much RF to leak through the t-r switch when in transmit at high
> > power.
> > That can cause the KPA100 PA stage to self-oscillate and produce HI
> > CUR and
> > high SWR. The problem will be worse on the lower bands from 160m to 30m,
> > since the RF transistors have higher power gain at lower frequencies
> > and the
> > KPA100 typically will produce higher RF output on the lower bands.
> >
> > If damage has occurred to the MAX1406 RS232 IC, or to any of the parts in
> > the KPA100 t-r switch, it may load down the HV bias too much or allow too
> > much RF leakage through the t-r switch to provide the needed input to
> > output
> > isolation.
> >
> > You must see a low null voltage at TP4 of less than 10 millivolts (on
> > 40m or
> > 80m) when a good 50 ohm dummy load is connected as closely as possible to
> > the KPA100's antenna connectors. Do not have a SWR meter, wattmeter, coax
> > switch, or long cable inserted between the dummy load and the antenna
> > connector on the KPA100. Most rigs will null down to about 1-3 millivolts
> > with a very close and direct connection to the dummy load. Also, the
> > flange
> > of the SO-239 antenna connector should be installed so it is on the rear
> > panel, not on the inside.
> >
> >
> > If the antenna was left connected when there was a thunderstorm nearby,
> > lightning can damage the 1N5711 diodes in the KPA100 SWR sensors.
> >
> > If the diodes are OK, there should be about 98-100K ohms between their
> > cathodes and ground when tested with an ohmmeter. You can do this at the
> > side of the trimmer pots that connect to the diodes too. Connect the
> > postive ohmmeter lead to the cathode, and the negative lead to a ground
> > point on the tuner.
> >
> > Then, if your meter has a diode test function, there should be about
> > 0.38 to
> > 0.40VDC of diode drop across the 1N5711 diodes, with the + lead to
> > anode, -
> > lead to the cathode.
> >
> > The diodes are easier to replace if one lead is clipped. It can be
> > done from
> > the top of the PCB without removing the heat sink, if you are very
> > careful,
> > have good desoldering tools and skill, and you pretrim and form the
> > leads on
> > the new diodes. Leave a little extra lead length above the PCB for easier
> > soldering.
> >
> > Also check to see if the calibration of the null cap, and the FWD and
> > REFL
> > cal pots still check out OK.
> > While the PA shield is off of the KPA100, be sure to check the
> > tightness of
> > the screws in PA transistors Q1, Q2 as they do tend to loosen.
> >
> >
> >
> > --
> > 73, Gary AB7MY
> > =========
> > [hidden email]
> > Elecraft Technical Support
> >
> >
> >
> > -----Original Message-----
> > From: Koert Wilmink [mailto:[hidden email]] Sent: Tuesday, July 03,
> > 2007 1:26 PM
> > To: 'Gary Surrency'
> > Cc: 'CSinfo'
> > Subject: RE: PA1KW: RE: High SWR on 160M and 80M with dummyload
> >
> >
> > Hi Gary,
> >
> > Yes I have the new value of C31 installed. It is impossible to lower SWR
> > reading with C1. Other bands are perfectly working well. Any other
> > suggestions?
> >
> > 73 Koert PA1KW
> >
> > -----Oorspronkelijk bericht-----
> > Van: Gary Surrency [mailto:[hidden email]] Verzonden: Monday,
> > July 02, 2007 8:34 PM
> > Aan: 'Koert Wilmink'
> > CC: 'CSinfo'
> > Onderwerp: PA1KW: RE: High SWR on 160M and 80M with dummyload
> >
> > Koert,
> >
> > Is C31 in the KPA100 t-r switch the new value of 0.22uF, 63WVDC? This
> > change
> > is specifically to address oscillation and high SWR problems on the lower
> > bands of 160m-30m if there is too much RF feedback through the t-r
> > switch at
> > high power.
> >
> > The new value of C31 (0.22uF, 63WVDC) applies only to late model KPA100
> > kits, or to older ones that have had the KPA100UPKT modifications
> > installed.
> > Info on that mod kit is on our website at:
> >
> > http://www.elecraft.com/order_form_parts.htm
> > All parts requests should be sent to [hidden email] for the quickest
> > response. I'm not at the main office in Aptos, CA. I'm located near
> > Phoenix,
> > AZ and have no stock of parts here other than for doing repairs.
> >
> >
> > --
> > 73, Gary AB7MY
> > =========
> > [hidden email]
> > Elecraft Technical Support
> >
> > -----Original Message-----
> > From: Koert Wilmink [mailto:[hidden email]] Sent: Sunday, July 01,
> > 2007 2:08 AM
> > To: [hidden email]
> > Subject: High SWR on 160M and 80M with dummyload
> >
> >
> > Dear Elecraft,
> >
> > I've a high SWR on 160 and 80M with my K2/100 # 5618. With a dummyload I
> > cannot get the SWR meter adjusted right. I cannot get it lower than
> > 1.9-1 on
> > 160M and 1.7-1 on 8oM. On all other bands it's exactly 1.0-1. Any
> > suggestions?
> >
> > 73 Koert PA1KW
> >
> > No virus found in this outgoing message.
> > Checked by AVG. Version: 7.5.476 / Virus Database: 269.9.14/884 -
> > Release Date: 7/2/2007
> > 3:35 PM
> >
> >
> >
> >
> >
>

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w9cf

Re: KPA100 SWR bridge adjustment 160m & 80m

Dear Koert,

It probably isn't the problem, but you might check that the core
used for T4 is really an FT50-43. The wrong core might not have enough
permeability at the lower frequencies.

73 Kevin w9cf

>Dear all,
>
>Please find my emails to Elecraft support below. Is there maybe somebody with
>additional tips?
>
>Thanks!!
>
>73 Koert PA1KW
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Geoffrey Mackenzie-Kennedy

Re: KPA100 SWR bridge adjustment 160m & 80m

In reply to this post by Koert Wilmink

Hi Koert,

1) Do you have an external RF power meter?

2) If you do, does your K2/100 produce 100 watts minimum, CW mode, on all
bands 160m - 10m when connected to a dummy load and the RF power output
measured by an external meter?

3) All RF power measurements should use an external meter only.

4) If the K2/100 does not produce 100 watts CW on all bands, on which bands
is the maximum output LESS than 100 watts CW?

5) How much DC current is being drawn by the PA (or the K2/100) on each band
if you adjust the power output to 100 watts CW? If you cannot get 100 watts
on some bands, what is the maximum power output produced on each 'low output
band' and how much DC current is drawn in each low output case.

These tests should provide some indication of what is causing your problem
without having to heat up your iron. If you wish we can continue off List.

73,
Geoff
GM4ESD

----- Original Message -----
From: "Koert Wilmink" <[hidden email]>
To: <[hidden email]>
Sent: Saturday, December 29, 2007 9:03 PM
Subject: [Elecraft] KPA100 SWR bridge adjustment 160m & 80m

> Dear all,
>
> Please find my emails to Elecraft support below. Is there maybe somebody
> with additional tips?
>
> Thanks!!
>
> 73 Koert PA1KW

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
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Don Wilhelm-4

Re: KPA100 SWR bridge adjustment 160m & 80m

In reply to this post by Koert Wilmink

Koert,

The absolute minimum voltage found in the C1 null (as observed at TP4)
is not as important as the depth of the null.

If you cannot achieve a deep and rather sharp null, there is something
wrong with either T4 and its associated components or with your dummy
load. The actual voltage at the null will depend on the power level used.

Assuming your dummy load is good (you can check it with an antenna
analyzer), you should see a 1.0:1 SWR on all bands.

Since you indicate a higher SWR on the low frequency bands, that
suggests that T4 does not have adequate inductance for those bands.
Check that you have used the correct core for T4 - it should be one of
the gray (ferrite) cores and not one of the black (powdered iron)
cores. Be certain that T4 has well stripped leads and is properly
soldered. If you observe toroid solder connections that look a bit like
volcanos, that lead was not well stripped - a good solder connection
will look like a mountain with concave sides - if it has a convex shape,
too much solder was applied and that fact can mask a poor solder
connection - wick away the excess solder and resolder properly.

73,
Don W3FPR

Koert Wilmink wrote:

> Dear all,
>
> Please find my emails to Elecraft support below. Is there maybe somebody with additional tips?
>
> Thanks!!
>
> 73 Koert PA1KW
>
>
>
> -----------------------------------------------------
> Hi Gary,
>
> Sorry for my late response, but I had not the opportunity to work on
> this earlier. I moved houses and that took longer than expected.
>
> I did check the HV bias and that is 135volts. The diodes are ok and the
> voltage on TP4 2mV on receive and 240mv on transmit (tune).
> I did connect a good dummyload close to the KPA100.
> I did the re-adjustment of the SWR bridge but this provided me not the
> required results. The best results with adjusting C1 on 5 Watts is:
>
> 160m 1.8-1
> 80m 1.3-1
> 40m 1.0-1
> 30m 1.0-1
> 20m 1.0-1
> 18m 1.0-1
> 15m 1.0-1
> 12m 1.0-1
> 10m 1.0-1
>
> Please help......
>
> Best regards and 73,
>
> Koert Wilmink
> PA1KW
>
>
>

Koert Wilmink

Re: KPA100 SWR bridge adjustment 160m & 80m

Dear Don,

Thanks for your support!! there is a deep and sharp null for all bands
except 80 &160m, cannot get lower than 1.3 for 80 and 1.8 for 160, I did
check the core and it is the large dark gray core. I also removed T4 and
did check the solder connections. No difference in results.
Maybe asking Elecraft for a new FT50-43 core? I also will ask my
colleague amateurs for another dummyload, just to be 100% sure. The
current one I'm using is from Annecke and the freq it is capable of
handling is 2 Ghz.
73 Koert PA1KW

Don Wilhelm wrote:

> Koert,
>
> The absolute minimum voltage found in the C1 null (as observed at TP4)
> is not as important as the depth of the null.
>
> If you cannot achieve a deep and rather sharp null, there is something
> wrong with either T4 and its associated components or with your dummy
> load. The actual voltage at the null will depend on the power level
> used.
>
> Assuming your dummy load is good (you can check it with an antenna
> analyzer), you should see a 1.0:1 SWR on all bands.
>
> Since you indicate a higher SWR on the low frequency bands, that
> suggests that T4 does not have adequate inductance for those bands.
> Check that you have used the correct core for T4 - it should be one of
> the gray (ferrite) cores and not one of the black (powdered iron)
> cores. Be certain that T4 has well stripped leads and is properly
> soldered. If you observe toroid solder connections that look a bit
> like volcanos, that lead was not well stripped - a good solder
> connection will look like a mountain with concave sides - if it has a
> convex shape, too much solder was applied and that fact can mask a
> poor solder connection - wick away the excess solder and resolder
> properly.
>
> 73,
> Don W3FPR
>
> Koert Wilmink wrote:
>> Dear all,
>>
>> Please find my emails to Elecraft support below. Is there maybe
>> somebody with additional tips?
>>
>> Thanks!!
>>
>> 73 Koert PA1KW
>>
>>
>>
>> -----------------------------------------------------
>> Hi Gary,
>>
>> Sorry for my late response, but I had not the opportunity to work on
>> this earlier. I moved houses and that took longer than expected.
>>
>> I did check the HV bias and that is 135volts. The diodes are ok and the
>> voltage on TP4 2mV on receive and 240mv on transmit (tune).
>> I did connect a good dummyload close to the KPA100.
>> I did the re-adjustment of the SWR bridge but this provided me not the
>> required results. The best results with adjusting C1 on 5 Watts is:
>>
>> 160m 1.8-1
>> 80m 1.3-1
>> 40m 1.0-1
>> 30m 1.0-1
>> 20m 1.0-1
>> 18m 1.0-1
>> 15m 1.0-1
>> 12m 1.0-1
>> 10m 1.0-1
>>
>> Please help......
>>
>> Best regards and 73,
>>
>> Koert Wilmink
>> PA1KW
>>
>>
>

Koert Wilmink

Re: Re: KPA100 SWR bridge adjustment 160m & 80m

In reply to this post by w9cf

> Dear Kevin,
>
> Thanks for your reply! I did check the core and it is the large dark
> gray core. I also removed T4 and did check the solder connections. No
> difference in results.
> Maybe asking Elecraft for a new FT50-43?
>
> 73 Koert PA1KW
>
> Kevin Schmidt wrote:
>> Dear Koert,
>>
>> It probably isn't the problem, but you might check that the core
>> used for T4 is really an FT50-43. The wrong core might not have enough
>> permeability at the lower frequencies.
>>
>> 73 Kevin w9cf
>>
>>
>>> Dear all,
>>>
>>> Please find my emails to Elecraft support below. Is there maybe somebody with
>>> additional tips?
>>>
>>> Thanks!!
>>>
>>> 73 Koert PA1KW
>>>
>> _______________________________________________
>> Elecraft mailing list
>> Post to: [hidden email]
>> You must be a subscriber to post to the list.
>> Subscriber Info (Addr. Change, sub, unsub etc.):
>> http://mailman.qth.net/mailman/listinfo/elecraft
>>
>> Help: http://mailman.qth.net/subscribers.htm
>> Elecraft web page: http://www.elecraft.com
>>
>>

Koert Wilmink

Re: KPA100 SWR bridge adjustment 160m & 80m

In reply to this post by Geoffrey Mackenzie-Kennedy

Dear Geoff,

I did check the power with an external wattmeter, on all bands I'm
measuring 100watts on a dummyload.

73 Koert PA1KW

Geoffrey Mackenzie-Kennedy wrote:

> Hi Koert,
>
> 1) Do you have an external RF power meter?
>
> 2) If you do, does your K2/100 produce 100 watts minimum, CW mode, on
> all bands 160m - 10m when connected to a dummy load and the RF power
> output measured by an external meter?
>
> 3) All RF power measurements should use an external meter only.
>
> 4) If the K2/100 does not produce 100 watts CW on all bands, on which
> bands is the maximum output LESS than 100 watts CW?
>
> 5) How much DC current is being drawn by the PA (or the K2/100) on
> each band if you adjust the power output to 100 watts CW? If you
> cannot get 100 watts on some bands, what is the maximum power output
> produced on each 'low output band' and how much DC current is drawn in
> each low output case.
>
> These tests should provide some indication of what is causing your
> problem without having to heat up your iron. If you wish we can
> continue off List.
>
> 73,
> Geoff
> GM4ESD
>
>
> ----- Original Message ----- From: "Koert Wilmink" <[hidden email]>
> To: <[hidden email]>
> Sent: Saturday, December 29, 2007 9:03 PM
> Subject: [Elecraft] KPA100 SWR bridge adjustment 160m & 80m
>
>
>> Dear all,
>>
>> Please find my emails to Elecraft support below. Is there maybe
>> somebody with additional tips?
>>
>> Thanks!!
>>
>> 73 Koert PA1KW
>
>

Koert Wilmink

Re: KPA100 SWR bridge adjustment 160m & 80m

In reply to this post by Koert Wilmink

Dear Don,

Just did check the dummyload with an MFJ analyzer and is working fine on
160m and higher.

73 Koert PA1KW

Koert Wilmink wrote:

> Dear Don,
>
> Thanks for your support!! there is a deep and sharp null for all bands
> except 80 &160m, cannot get lower than 1.3 for 80 and 1.8 for 160, I did
> check the core and it is the large dark gray core. I also removed T4 and
> did check the solder connections. No difference in results.
> Maybe asking Elecraft for a new FT50-43 core? I also will ask my
> colleague amateurs for another dummyload, just to be 100% sure. The
> current one I'm using is from Annecke and the freq it is capable of
> handling is 2 Ghz.
> 73 Koert PA1KW
>
>
> Don Wilhelm wrote:
>
>> Koert,
>>
>> The absolute minimum voltage found in the C1 null (as observed at TP4)
>> is not as important as the depth of the null.
>>
>> If you cannot achieve a deep and rather sharp null, there is something
>> wrong with either T4 and its associated components or with your dummy
>> load. The actual voltage at the null will depend on the power level
>> used.
>>
>> Assuming your dummy load is good (you can check it with an antenna
>> analyzer), you should see a 1.0:1 SWR on all bands.
>>
>> Since you indicate a higher SWR on the low frequency bands, that
>> suggests that T4 does not have adequate inductance for those bands.
>> Check that you have used the correct core for T4 - it should be one of
>> the gray (ferrite) cores and not one of the black (powdered iron)
>> cores. Be certain that T4 has well stripped leads and is properly
>> soldered. If you observe toroid solder connections that look a bit
>> like volcanos, that lead was not well stripped - a good solder
>> connection will look like a mountain with concave sides - if it has a
>> convex shape, too much solder was applied and that fact can mask a
>> poor solder connection - wick away the excess solder and resolder
>> properly.
>>
>> 73,
>> Don W3FPR
>>
>> Koert Wilmink wrote:
>>
>>> Dear all,
>>>
>>> Please find my emails to Elecraft support below. Is there maybe
>>> somebody with additional tips?
>>>
>>> Thanks!!
>>>
>>> 73 Koert PA1KW
>>>
>>>
>>>
>>> -----------------------------------------------------
>>> Hi Gary,
>>>
>>> Sorry for my late response, but I had not the opportunity to work on
>>> this earlier. I moved houses and that took longer than expected.
>>>
>>> I did check the HV bias and that is 135volts. The diodes are ok and the
>>> voltage on TP4 2mV on receive and 240mv on transmit (tune).
>>> I did connect a good dummyload close to the KPA100.
>>> I did the re-adjustment of the SWR bridge but this provided me not the
>>> required results. The best results with adjusting C1 on 5 Watts is:
>>>
>>> 160m 1.8-1
>>> 80m 1.3-1
>>> 40m 1.0-1
>>> 30m 1.0-1
>>> 20m 1.0-1
>>> 18m 1.0-1
>>> 15m 1.0-1
>>> 12m 1.0-1
>>> 10m 1.0-1
>>>
>>> Please help......
>>>
>>> Best regards and 73,
>>>
>>> Koert Wilmink
>>> PA1KW
>>>
>>>
>>>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>
>

Koert Wilmink

Re: KPA100 SWR bridge adjustment 160m & 80m

In reply to this post by Koert Wilmink

Dear Don, Geof and Gary,

I did receive the diodes and did replace them today. The problem is
solved!! SWR is ok on all bands now (with the dummyload).
I want to thank you all for the support!!! I'm in the US next week, see
what I can order from Elecraft :-)

73 Koert PA1KW

Don Wilhelm wrote:

> Koert,
>
> A Great 2008 to you and yours as well.
>
> The only other suggestion at this time is that you make a practice of
> disconnecting antennas when not actually in use. A small amount of
> lightning surge or even 'snow or rain static' can produce enough peak
> voltage to zap those diodes.
>
> 73,
> Don W3FPR
>
> Koert Wilmink wrote:
>> Dear Don and Geoff,
>>
>> First of all I want to wish you a great and healthy 2008!!I did cut both
>> diodes D16 and D17 and I'm able to set R26 and R27 to 43K!! I will ask
>> Elecraft to send me 2 new diodes. Any other suggestions?
>>
>> 73 Koert PA1KW
>>
>> Don Wilhelm wrote:
>>
>>> Koert,
>>>
>>> Yes, that would be a good and informative step - do it that way.
>>> If the trimmers then adjust over the 0 to 100k range without the
>>> diodes in place, just set the trimmers at 43K and replace the diodes
>>> with new ones - then try the balance adjustment (C1) again before
>>> changing anything else - if it nulls, then all the rest is OK and you
>>> can move forward to the calibration of the forward power..
>>>
>>> 73,
>>> Don W3FPR
>>>
>>> Koert Wilmink wrote:
>>>
>>>> What do you both suggest? First take out the diodes and see if I can
>>>> adjust R26 and R27 to 43K? That will exclude the LM358 as the course of
>>>> failure?
>>>>
>>>> 73 Koert PA1KW
>>>>
>>>> Geoffrey Mackenzie-Kennedy wrote:
>>>>
>>>>
>>>>> Don and Koert.
>>>>>
>>>>> Please have patience with me while I get our 18 year old daughter
>>>>> safely to one of her friend's house in Northern 'darkest Scotland',
>>>>> the sun is setting as I write :-) I will be back in a few hours time.
>>>>>
>>>>> Koert, if you do start to replace components may I suggest that you
>>>>> change only one or two at a time, if you replace several at one
>>>>> time before checking the circuit's performance it might not be
>>>>> possible to determine which is the troublemaker unless a component is
>>>>> obviously faulty e.g a shorted diode.
>>>>>
>>>>> Don, yes I note the path through R29 which made me wonder about the
>>>>> type of meter Koert is using to measure resistance.
>>>>>
>>>>> In haste,
>>>>>
>>>>> 73,
>>>>> Geoff
>>>>> GM4ESD
>>>>>
>>>>> Geoff and Koert,
>>>>>
>>>>> I am not yet convinced that a valid null was measured during the
>>>>> initial setup. At a power level of 10 watts, I typically want to
>>>>> see a voltage of less than 20 mV at TP4. Something has been awry
>>>>> all the time based on the information that I have seen so far.
>>>>>
>>>>> Perhaps I am getting confused by all the pieces, but so far, the
>>>>> resistance readings at the R26 and R27 wipers are not correct and
>>>>> are an indication of a problem - that should be resolved before
>>>>> taking other steps. It may be that reversing the leads of the
>>>>> meter will give better readings, but if not, then the diodes are
>>>>> suspect. Notice that with shorted diodes, there is a path
>>>>> straight to ground through R29 - its 3.3k value is not
>>>>> significant compared to 25k.
>>>>>
>>>>> Koert, you are in the midst of a typical troubleshooting process -
>>>>> find a potential problem (it may not be the source), and then
>>>>> verify it is OK or determine the cause of the improper readings -
>>>>> after that is done, move on to the next situation. Bear with us
>>>>> while several possible causes are explored - and we need your help
>>>>> to take the measurements and report them so that we all understand
>>>>> the conditions under which those measurements were made. We will
>>>>> get to the end of this problem, but it may take some time but we
>>>>> will go down some pathways that lead to nothing fruitful during
>>>>> the process. Troubleshooting is as much a process of eliminating
>>>>> possibilities as it is in taking steps toward the problem source.
>>>>>
>>>>> 73,
>>>>> Don W3FPR
>>>>>
>>>>> Geoffrey Mackenzie-Kennedy wrote:
>>>>>
>>>>>
>>>>>> Don and Koert,
>>>>>>
>>>>>> Koert, what type of ohmeter are you using to measure the
>>>>>> resistance between each potentiometer wiper (R26 and R27) and
>>>>>> ground? If it is an Avo or something similar which
>>>>>> could cause a diode to conduct, the maximum value of resistance
>>>>>> between wiper and ground would be found when the wiper is close
>>>>>> to the mid point of its travel if the meter forces enough
>>>>>> current. If you are using a meter similar to an Avo swapping
>>>>>> leads would be worthwhile. However this type of meter could upset
>>>>>> the op amp's, adding further error to the resistance measurements
>>>>>>
>>>>>> Don, if the diodes were shorted I don't think that it would be
>>>>>> possible for Koert to find a null on the higher frequency bands,
>>>>>> or am I missing something?
>>>>>>
>>>>>> 73,
>>>>>> Geoff
>>>>>> GM4ESD
>>>>>>
>>>>>>
>>>>>> Don Wilhelm wrote:
>>>>>>
>>>>>> Koert,
>>>>>>
>>>>>> Doing a bit more thinking about your resistance measurements,
>>>>>> I believe your diodes D16 and D17 are shorted. There is no
>>>>>> other way that I can see for the maximum resistance of the
>>>>>> wipers of R26 and R27 to indicate 25k ohms to ground.
>>>>>>
>>>>>> 73,
>>>>>> Don W3FPR
>>>>>>
>>>>>>
>>>>>>
>>>>
>>>>
>>
>>
>>