[AMRadio] FW: Homebrew receiver

[Donald Chester]

>It does not have the same looks as a 75A4, but those are going
>for about $1000.00, then you have to hack it up to install
>other filters and audio


Not really.  My outboard filter box runs miniature coax to a 9-pin plug, 
that plugs into one of the filter slots on the rx.  The only mod I had to 
make was to clip one lead on the 100 pf resonating cap on the "hot" side.  I 
just pushed it aside, so if I ever wanted to put the rx back to stock condx, 
it would be very simple to re-solder that lead.  The filters in the box use 
their own variable resonating caps, so each filter can be peaked for optimum 
performance.  As I recall, I had to run an AGC lead so that the AGC 
functions normally with the outboard filters.  I could return the rx to 
stock in less than hour if I so desired.

As for the audio mods, I simply bridged .1 caps across the existing .01 
caps, without removing them.  It would be a simple matter to remove the 
.1's, since I didn't follow the Collins practice of "making a firm 
mechanical connection before applying solder".  I swear they must have tied 
some of the component leads into square knots on the solder lugs before 
applying solder.  But, I don't know why anyone would want to un-do the audio 
modification that makes the rx sound decent on AM.  I pick audio off the 
grid lead to the final audio amp before the 6AQ5.  I recall it is a 12AT7 
(or is it a 12AU7?).  I use another 9-pin plug that replaces the tube.  I 
have an outboard cathode follower in a small box to couple the audio to the 
external amp and avoid loss of high frequency response because of cable 
capacitance.  The cathode follower gets its power entirely from the 
circuitry that originally fed power to the audio stage.  This mod can be 
reversed simply by unplugging the coupling device and plugging the tube back 
in.

I bought my 75A4's back in the early 1980's when they were dirt cheap.  I 
paid $100 for one (with all 3 stock filters) and $200 for the other (with 
only the SSB filter).  With a little fixing up, both receivers work FB OM.

Don K4KYV

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