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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