KAT1 can't null

[Leigh L. Klotz Jr WA5ZNU]

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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[Leigh L. Klotz Jr WA5ZNU]

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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