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