Posted by
Leigh L. Klotz Jr WA5ZNU on
Jun 14, 2005; 5:49am
URL: http://elecraft.85.s1.nabble.com/KAT1-can-t-null-tp378946p378947.html
Thank you to all who helped. It turns out there was nothing wrong with
the SWR bridge -- it was right!
I spent too much time looking at the bridge and not enough time on the
rest of the board.
The problem with the SWR failure to null was an unsoldered pin 1 on one
of the inductor relays that left its L in circuit all the time.
I guess I just had the luck that the relay was stuck in the "in" position.
Apparently I didn't perform the "click" test properly, or I would have
caught the problem then.
Leigh / WA5ZNU
Leigh L. Klotz, Jr. wrote:
> I have built a KAT1 for a K1, which I acquired already built. The K1
> functioned before, and still seems to function now. (I put in the R36
> switch.)
> I cannot get the KAT1 SWR null setup to work, and I've run out of
> ideas as to what could be wrong with the SWR circuit.
> I'm looking for suggestions about what could be wrong.
>
> The minimum voltage I get during the null test is about 0.55 V.
> The forward voltage, measured off R1 at U2 pin 5, is about 1.55 V.
> The SWR reading is about 2.1:1 into an Elecraft DL-1, connected via a
> double-female BNC.
> With the DL-1 in place and the KAT1 removed, I get 50 ohms read with
> an ohmmeter from the pin on the new header of the KFL2 board to ground
> inside the rig (within a fraction -- closer than the accuracy of the
> meter).
>
> I rewound the transformers in the K1, one of which had the wrong
> number of turns, but I don't think the problem is in the rig itself.
>
> At the lowest Vref value, C9 has the screw slot pointing up and down,
> (parallel to the CPU).
> I assume this is one of the end points of its travel.
> I read that a Vref null at one of the end points of C9 indicates a
> problem with the SWR measuring components, and that seems likely.
>
> I rewound T1 on the KAT1, and have the pass-through wire in pins 5 and
> 6 and the bifilar wire in 1-4. I've compared the colors, tested for
> shorts and opens, and made sure to have the wires come from the
> correct side during winding.
> I tested D2 and D1 in circuit and my meter says they are still
> diodes. R3 and R4 in circuit seem rational as well.
> I touched up the joints on the K5, which is the network switch, and on
> the other components in the SWR bridge circuit.
> All the relays click when I put them through their tests.
> R1 and R2 are both at about 70%, and the power reading on the LCD
> matches the power determined from the DC measurement on the built-in
> RF probe of the DL-1.
>
> I have previously built a KX1 and K2, with all QRP options, and while
> it's possible, I don't think I've overlooked a missed pin or bridge.
>
> Does an SWR of a hair over 2:1 for a 50 ohm load suggest any component
> to examine or replace?
> I don't know if the failure is it seeing R=25 Ohms, R=100 Ohms, some
> reactive component induced internally in the bridge, or if the failure
> to achieve a sufficiently low Vref is unrelated to the load impedance
> and it is just a voltage offset induced somehow else.
>
> I have some extra 1N5711's on hand, if it's possible that they can
> develop a non-obvious failure mode (i.e, in circuit the conduct one
> way and not the other).
> Both have the cathodes down, near their D1 and D2 labels.
> I don't have an extra variable capacitor for C9 to swap out.
>
> Leigh / WA5ZNU
> _
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