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KAT3 Mistuning

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John G3PQA

KAT3 Mistuning

Hi Everyone,
I would appreciate some advice on what might be causing an ATU tuning fault which my K3 developed, possibly after tuning by mistake on an open load.
The KAT3 now mistunes on all LF bands below 18MHz, progressively worse when going lower in frequency.
It tries to tune, sometimes hunting and the display ending in dashes, and other times partially succeeding.
For example, on 1825 kHz it turns a 1:1 swr into 1.5:1, and when going up 100kHz it turns a 4:1 swr into 5:1.
On 80m it turns a 1.8 to 1 swr into 1.5:1, etc. etc.. up to 18MHz where it tunes more or less well, and appears normal on 28MHz.
The fault is still present when using a dummy load, and when using a 100 ohm dummy load the results are similar (turns 2:1 into 1.5:1 on 160m, and 2:1 into 1.3:1 on 7MHz)
The swr bridge readings of power and swr on the K3 screen appear accurate as previously calibrated.
The LPA and KAT3 screws are properly tightened, and RF was not a problem before the fault occurred.
The KAT3 itself is not the problem as swapping it with another module gives the same fault.
Has anyone seen this problem before, or can give suggestions as to cause?
73
John

John G3PQA

Re: KAT3 Mistuning

Update on fault.
As recommended by Gary (K3 Support) I looked at the TX spectral purity in case of stuck relay or filter fault.
CW signal appeared clean on all bands (using VNA in spectrum analyser mode).
Also visually checked for LPF or stop filter damage or coils touching etc., and all ok.
However, whilst testing earlier I noticed an anomoly in the swr reading - 1.5 to 1 instead of 1 to 1 on antenna on 3.5MHz compared with external swr meter, I had not seen this difference before.
So replaced swr bridge diodes D36, 37 with 1N5711 radial diodes, recalibrated power and tested ATU.
Fault at least partly fixed, now tunes 80m and above ok, and has improved 1.8MHz..
On 1.8MHz band it matches a 10:1 reactive load ok, but will not reduce a 1.5:1 swr antenna better than 1.4:1, as indicated when pressing "tune" button. This does not occur on a dummy load, results correct with low swr either with ATU in circuit or bypassed.
Will check again when proper diodes fitted.
John

ZL3IO

Re: KAT3 Mistuning

In reply to this post by John G3PQA

Hi John
I have a similar problem on all bands. The antenna tuner worked nicely for many years and now tunes everything worse. Antennas that are 1.5:1 get tuned into 4 : 1 no matter what band.

Got the advice to clean contacts which I did many time but nothing changed.
I'm not using the tuner anymore.

73 Holger

John G3PQA

Re: KAT3 Mistuning

Hi Holger,

Firstly, nice to work you on 80m cw recently and on 160m from ZL2IO!

My KAT3 mistuning symptoms were similar to those you describe, except it was worse on the LF bands, improving and almost normal on higher HF bands.

My main wattmeter and swr functions appeared to be working, so the usual suspects (swr bridge diodes D36, D37) did not seem to be the cause.

I would say that those diodes and associated components, and L10 C10 on the KAT3 are the first things to check.

I was given very comprehensive (and as it turned out, good advice) from Gary Surrency over what to look for (summarised below), but it did not fix the fault initially in my case (perhaps because of multiple causes), I was chasing around for ages.

1. To replace the swr bridge diodes D36, D37 (I used 1N5711 but the swr bridge worked just the same so refitted the original smd diodes)

2. To re-fix all earthing screws and connectors around the LPA (3 screws underneath), KAT3 and the SO-259 antenna sockets.

3. To check for dry joints or touching wires on the LPA HFP filters and KAT3.

4. To check filter L10 C10 on the KAT3 (8.215MHz IF trap, can be temporarily shorted, only a parallel tuned circuit)

5. To check relay function if a specific band or bands (which does not appear to be your fault)

5. After these checks and changes every time do a re-calibration (Wattmeter and TX gain)

I did all this (but without re-cal every time), found no definite fault even though it improved slightly, without a complete cure. It was still hunting. At this stage I was thinking of sending the motherboard back to UK or USA but reluctant to do this because of the trouble involved and I wanted to fix it myself!

On checking further I looked at the swr toroids and found one (T3) where the centre insulation was missing partly and the copper wire may have been touching the ferrite – if so this could have caused an intermittent. I replaced the small “U” wire with another using PTFE insulation.

Also I found correspondence relating to KAT3 earthing. (thanks to GM3SEK, W3FPR, G3TJP and others). The KAT3 is earthed via a screw holding its pcb to a hex pillar and 2D fastener to the side panel connected to the back panel, what appears to be rather an indirect route. If any of these dis-similar metal fixings go high resistance, or there is higher than normal RF induced in the K3, then this may cause a problem. I fitted an extra 1mm flex wire on tags between the bottom SO-259 bolt and the back of the KAT3 pcb. I cut the side of the tag behind the KAT3 pcb to avoid a track which ran close, and used a new nickel plated screw with very small flat washer to fix the pcb. Surfaces cleaned and with thin layer of professional contact lubricant.

After re-calibration again the fault had gone!

Unfortunately because of its intermittent and non-precise nature, it was difficult to pinpoint the exact cause. Possibly general deterioration via several factors listed above ?

Am sure Elecraft support or others can either correct any errors above or suggest other things to try in your case.

73 gl

John G3PQA

(K3 s/n 1123)

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