[Elecraft] KAT1 can't null

[Leigh L. Klotz, Jr.]

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
> _