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List:       elecraft
Subject:    Re: [Elecraft] K3+KPA500+KAT500 RFI Revisited
From:       Jack Brindle via Elecraft <elecraft () mailman ! qth ! net>
Date:       2024-01-13 20:11:52
Message-ID: 1F85BFD2-C2AA-48D9-9526-BADC2F52D174 () me ! com
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Please describe the aux cables you have in use between the equipment. It is highly \
unusual for the BCD lines to change state during transmission, on any other time \
without actually changing the band on the K3. If you are using very long Aux cables, \
or have other equipment connected to the cable (that uses the Band lines), it could \
account for issues. There is at least one band decoder kit on the market that uses \
poorly designed receivers that will adversely affect the band lines in a K3 or K4 \
system to the point that the KPA receives incorrect band indication.

Are you directly monitoring the band lines with an external indicator? If so, please \
describe the indicator. The KPA _will_ change bands due to a signal on another band \
(such as a harmonic). We have not seen that to this point with K3 or K4, but it is \
certainly possible. Not that the RF signal count always overrides the band signals - \
it counts the strongest signal at its input.

This situation may be due to RFI, but I am suspecting something else at present, such \
as very long AuxBus cables or an issue in the system. Any further info you can \
provide would be very helpful - is there another transmitter near by, or anything \
else that may be injecting alternate signals into the amplifier (usually on the xcvr \
input side, but if strong enough it could come in through the antenna). Note that a \
very strong signal coming into the antenna port would also affect the SWR \
calculations and display.

Something is going on, but it may surprise you when you discover it.

73,
Jack, W6FB


> On Jan 12, 2024, at 2:18 PM, Jeff Wandling <jdw@w7brs.com> wrote:
> 
> Dear Elecraft aficionados:
> 
> The symptom is KPA switching bands during Tx.
> 
> Seems like a while ago I was troubleshooting the RFI problem between the equipment.
> 
> K3 + KPA500 + KAT500
> 
> Using the "enhanced" configuration where serial cable between the K3 to KAT to KPA \
> assisted in communicating band changes as well as key-down event and so on. 
> I own two KPA-500's. Both are less than 6mo old.
> 
> Let me refer to them as KPA-1 and KPA-2. KPA-1 is in the hands of Elecraft for \
> repair. It worked flawlessly (until needing repair), but the RFI issue I will write \
> about did not exist when that KPA-1 was in the chain. 
> KPA-2 was purchased in December as a backup. Since putting KPA-2 in-line, the RFI \
> problems I will describe have gone unresolved. 
> Scenario is with KPA-2 in-line.
> 
> Even running tiny power (< 10W, <15 W) I can routinely and 100% reproduce the \
> problem where the BCD data in the AUX line that carries the information about which \
> band the KPA should be set on will change during Tx. 
> For example I can put KAT into MAN, put KPA in bypass. Tune the antenna via K3 and \
> witness the search resolve to a 1:1 on the SWR. That sometimes will trigger the \
> effect -- I can see in the KPA, even in bypass that the band the KPA is set on is \
> changed from the band the K3 is set on. Even more insidious is that if the KPA \
> winds up on a different band after the tune (or after "VVV" CW testing after tune), \
> that pressing the band button on the KPA to bring it back does not bring the KPA \
> back to the right band. It is as if the KPA has two states of band-awareness.. The \
> awareness of the band displayed in the LCD and the band that the input is set for. 
> Here's an example:
> K3 on 40m
> Tune per usual (KAT MANUAL, KPA Bypass)
> KPA remains on 40m.
> K3 power output 14W
> Then put the KPA into Operate
> Tap "VVV" out on the key.
> Witness the KPA operate but then switch band to 30m or 17m or 20m, etc.. (not \
> expected) Then on the KPA tap (Band) 7 (for the 40m band) and nothing happens (not \
> expected) Tap on the KPA 14 then the whole chain switches to 20m (expected)
> 
> Suppose the TUNE did not yield a band-switch during Tx, repeating the test but this \
> time putting the KPA into Operate after the tune is sure-fire going to cause the \
> bandswitch. 
> (I configured the KPA to go to Bypass in the event of a band-change event in order \
> to have some sense of protection while the key-down test Tx occurs). 
> That's the scenario where after Tx the KPA is out of sync with the K3. Whether or \
> not the KPA is attempting to frequency-count is not evident, the KPA:RADIO setting \
> is "K3" which means leverage the BCD data from the K3 vis-a-vis the KAT path. 
> I can replicate this band-inconsistency at will. I don't think I can safely operate \
> the equipment if the BCD encoded band data is fluctuating during Tx. What would \
> happen if I switched bands (N1MM) and hit the keyer and the KPA got confused on the \
> last Tx? Magic smoke I imagine. 
> Only happened with KPA-2 not KPA-1. But I'm wondering if KPA-1 suffered a \
> catastrophe caused by this scenario and I didn't notice the clicking band-switch \
> because I had not expected it to switch bands during Tx. 
> What have I done to remedy? I think I've tried everything but obviously I have a \
> gap -- otherwise I'd have solved this problem. 
> - New coax between everything.
> - Shorter or longer coax, different coax (RG-213, 8X, LMR-240, etc..)
> - Tighter loops (6 -8") when there is excess coax or lazy big loops where there is \
>                 excess coax.
> - New connectors/adapters (Yes, Amphenol)
> - Re-route coax different ways. Avoid this, that and the other, etc.. Permutations \
>                 of routing.
> - Move and adjust placement of equipment. KAT near the KPA, KAT away from the KPA, \
>                 both near or away from the K3, etc..
> - Chokes between things. K3 to KPA, KPA to KAT, KAT to antenna system
> - Non-bleeding chokes, bleeding chokes.
> - Ferrite on serial cables, more ferrite, insane amounts of ferrite, or zero \
>                 ferrite
> - Move the position of serial cables above, below, up, down, left, right.
> 
> I haven't got to the point of putting little tangerines on altars and burning \
> incense. I'm trying to keep a positive mood while part of me wants to take up the \
> art of Bonsai and raking the sand. 
> Obviously RFI is a likely culprit, but I'll be damned if I can figure out where it \
> is -- or rather why it would be -- affecting this given all of the mitigations \
> attempted. 
> But I'm always a student and looking for things that were/are not obvious at first.
> 
> I'm almost to the point where I will take away the "enhanced" mode serial data path \
> between the equipment and rely on the analog key-line between them and RF \
> frequency-counter capabilities so that the KAT and the KPA are on the right band \
> when I key-down. I just need to review the docs to make sure I have the KPA and the \
> KAT setup to go "analog" rather than "KPA:RADIO = K3" mode. 
> But the 'enhanced' mode is much more preferred since it is a K-line.
> 
> Thoughts and suggestions welcome. Ping directly if you wish.
> 
> Thanks!
> 
> Jeff
> 
> ---
> 73, Jeff W7BRS
> https://blog.w7brs.com
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