[Elecraft] K2 - signal loss in band-pass filter

[Don Wilhelm]

Are,

The 'wierd' symptoms you report could be normal depending on the output 
characteristics of your signal generator.  The capacitors C7 and C8 in 
the 40 meter bandpass form a voltage divider that is intended to match 
the high impedance of the filter parallel tuned circuits down to a lower 
impedance level. If a signal is arbitrary injected at the top end of C7, 
there can (and likely will) be more voltage developed across the filter 
than if the same signal voltage is injected at the junction of C7 and C8 
(a similar argument applies to the other bands).  Put RFC7 and W6 back 
in place to give the proper termination for the filter.

A valid comparison is to compare the signal at the junction of C7 and C8 
with the signal at the junction of C4 and C5.  That would give an 
indication of the loss through the bandpass filter.  That comparison is 
more easily done in transmit than receive (the filter is bi-lateral).  
Compare the RF voltage at the junction of D6 and D7 with the RF voltage 
at W6 using a transmit power level of 1 to 2 watts.

If the bandpass filter is not indicating a substantial loss, then you 
may have some other problem in the receive path.

If your bandpass filter does show a substantial loss, try re-peaking it 
first, and if that does not produce adequate results, then look for an 
impedance mismatch at the filter terminations - check R36 and R5 first 
for proper values and good soldering.

There are many other places other than the bandpass filter that can 
cause weak receive.  If your K2 develops full output power on all bands, 
then the bandpass filter is likely not the problem area.

73,
Don W3FPR


Are LB3SA wrote:
> I recently completed my K2 and everything seemed to work OK. All tests are ok
> with exception of a slightly low 8V regulator output (7.59V).
>
> The sensitivity seems low however (my KX1 is much better at this point).
> Tracing the receiver revealed that sensitivity is top notch at certain
> points. This is where it gets weird...
>
> When I inject a signal on the north side (according to schematic) of C7
> (40m), C15 (80M), C24 (20M) and the same on all other bands, the sensitivity
> is great. But when I inject the signal on the other side of these capacitors
> (pin 8 on the relays), the level drops  (at least 20db) to the same as what
> it is at antenna, W6, W1,D2/D3, D3/D4 and all other points along the signal
> path.
>   
>