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

[Are LB3SA]

Thanks to both for info and assistance. You were right Don about the
'weirdness' when injecting signals by C7.

It turns out that the sensitivity issue was caused by a tiny solder bridge
on the SMD MC1350 (IF amp) which I found by injecting a signal @ IF freq.
before and after the IF Amp.

The receiver is now working very nice with an IMD of -136db. The filter/BFO
set-up went without a hitch and RX sounds like a million bucks with
absolutely no signs of ringing with tight filtering.

Are - LB3SA

KX1# 1864
K2# 6498
K3: Delivery late July


Don Wilhelm wrote:
> 
> 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
> 

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