[ARC5] BC-221 Notes

[Richard Knoppow]

    I have had some private correspondence about the BC-221. I 
thought I posted some corrections but I think they did not go to 
the list, at least I did not see them.
    Correction 1, is that the values of the capacitors given in 
the schematic in the calibration handbook and pasted into the 
meter are _correct_. They are 0.05uF, probably 200V. In mine 
there were three Micamold and one Solar flat caps, all bad. 
Replaced them with modern plastic caps. Cured some of the 
problems but I found another, namely the detector howled when the 
gain was increased.  I figured that out while driving. My BC-221 
is equipped with an AC power supply, type RA- 133, in place of 
the batteries. It suddenly occurred to me that the filter 
capacitors might be bad. I tried putting a good cap across the 
supply and it fixed the howling. Bad filter caps can  cause not 
only hum but can cause feedback through the power supply. My cure 
was a makeshift, I now have to construct a replacement cap. The 
original is a three section can electrolytic, 2 X 10uF and a 20uf 
section. I might be able to get a replacement from Hayseed 
Hamfest but for the moment will just assemble one and fit it in.
    There is an instruction sheet for the supply in with the 
BC-221 stuff in the BAMA files.
    I suspect the power supply caps have been bad for a long time.
    To repeat the flat paper caps were so bad that they could not 
be measured using the capacitance function of a Tektronix 
multi-meter. On a bridge the ESR was very high and the Solar has 
very high leakage in addition. These caps may have had good RF 
performance when new but they are around 75 years old. Same for 
the electrolytic caps in the power supply.
     In any case, I correct my statement about the BC-221 
handbook and capacitor values in case I did not do so before.
    Also, my BC-221 is NOT an AU, it is an AN.
On 4/7/2022 5:44 PM, Richard Knoppow wrote:
>      I have a BC-221 AU. The modulator did not work on the VFO 
> although it did on the crystal oscillator. 
-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL