CW filter weirdness

[Bob Burns]

Hi Eddy
 
Thanks for your email. If the dip in the centre is too large then the  
impedance of the centre tapped tuned circuit may be a touch too low. Used to  make 
reasonable SSB crystal filters at 455KHz so AM filters certainly possible  at 
3.395MHz.
 
I have long been pondering how to add FM to my Heath units which requires  
filter passbands from 7.5 - 15KHz - not easy to do with DIY crystal filters  at 
3.395MHz. Could use ceramic resonators if only they were available at that  
frequency although there may be a bit of temperature sensitivity.
 
I rebuild and also redesign quite a lot of Heath SB series units and there  
are some references on my web site to these projects. One SB301 was completely  
rebuilt commencing with a new chassis and a 7360 in each mixer and includes 
the  WARC bands. Performance is much improved on the original and it does full  
electronic breakin on CW with a matching but somewhat modified SB401.
 
For a new HF TxRx currently being built in a refurbished SB101 case I have  a 
first IF of 9MHz for all modes including FM (8 filters in all) and a  second 
IF of 3.395MHz with filters for AM, SSB and CW but but no crystal  filter for 
FM. I had to select the widest AM filter in my Heath collection to  get as 
close as possible to a 6KHz passband. I use individual matching  networks for 
each filter which gives me a little more control on ripple etc  as the Heath 
filters will have aged a long way from their original  specs.
 
You will also find a 1.8 - 146MHz Z-Match in the Heath style on my site  
which is built in another SB101 case.
 
Do many forum users actually redesign Heath units as opposed to just  
renovating them?
 
73

Bob 

G3OOU
G-QRP  6907
_http://members.aol.com/rfcburns_ (http://members.aol.com/rfcburns) 



   

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