[MRCA] SCR-284/BC-654 receiver

[scottjohnson1 at cox.net]

Aha-

Just looked in mine and that cap is inert, explains the motorboating!

I'll fix that, then it's on to the BC-474 restoration.  Thanks for the post,
saves a bunch of troubleshooting!

 

Scott W7SVJ

 

From: mrca-bounces at mailman.qth.net <mrca-bounces at mailman.qth.net> On Behalf
Of Ray Fantini via MRCA
Sent: Monday, March 10, 2025 12:42 PM
To: MMRCG at groups.io; mrca at mailman.qth.net
Subject: [MRCA] SCR-284/BC-654 receiver

 

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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