[MRCA] [MMRCG] SCR-284/BC-654 receiver

[Tim]

That's some good sleuthing Ray - Thanks!
Tim
N6CC

On Mon, Mar 10, 2025 at 12:42 PM Ray Fantini via groups.io <RAFANTINI=
salisbury.edu at groups.io> wrote:

> The SCR-284/BC-654 has been a fun project. Just about finished with the
> two sets that have been in the shop for the last couple months. Changed a
> bunch of capacitors, built solid state replacements for the vibrators and
> converted them to have a more useful coverage for modern CW Sub bands.
>
> One big issue with the radio is the week BFO injection 2-C-46 is too
> small. K4CHE conversion information identifies this and his modification
> was an easy fix. Another issue I discovered is that this radio has poor AVC
> and issues with strong signals. So much so that beyond a certain level the
> receiver will start distorting and will motor boat. Spent a bunch of time
> replacing all the capacitors in the AVC circuit, that’s 2-C-4, 2-C-10,
> 2-C-14 and 2-C-47 with 2-C-10 (0.25uf) being the one that has the biggest
> effect on the AVC hang time but still the high level AVC action sucks.
> Changing the value of 2-C-10 will change the frequency of the motorboating
> but no good value makes it go away. The problem turns out to be the B+
> filtering network for the RF amplifier (2-R-6, 2-C-45 and 2-R-2) has way
> too small a capacitor for 2-C-45 (0.1) and that creates the problems. I
> spent maybe a week or two trying to resolve this and then looking at the
> manual discovered that there are different schematics dependent on the sets
> serial number.
>
> All sets under serial # 17,691 use a 0.1 capacitor for 2-C-45 and have
> poor high level AVC action. above that serial number they changed 2-C-45 to
> a 12uf electrolytic capacitor and swapping that small capacitor for a big
> capacitor removes the motor boating and distortion.
>
> It makes a huge difference in the receiver’s ability to work with large
> signals so thought I would put something up about it if you have one oldest
> series of that radios changing that capacitor will improve its performance.
>
> Attached is a picture of the new capacitor.
>
>
>
> Ray F/KA3EKH
>
>
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