[AMRadio] Re: [T-368_BC-610] BC-610 terminating impedance


Rbethman rbethman at comcast.net
Sat Oct 29 12:49:18 EDT 2005


Robert & others,

Both, my BC-610(I) AND my T-213 have had the porcelain insulators 
replaced with an SO-239.  Upon loading into the ME-165G, my SWR into the 
50 ohm dummy load "barely" flickers the needle off the stop on 
reflected, it at all.

Both of the transmitters came with movable link coils.  The only coils I 
have that DON'T have movable links are the 160 meter ones.

In testing the ME-165G with an antenna impedance test bridge shows it 
indeed to be 50 ohm impedance.

With all this in consideration, I must therefore conclude that the 
output of BOTH transmitters to be 50 ohm output.

The 3 terminal input insulators on the side of the BC-939 are 
misleading.  Internal inspection reveals one terminal to be for a ground 
connection.  The other two are wired internally with 50 ohm coaxial 
cable.  The reason for the space between those two terminals is the 
plate voltage issue.  In the CW position, the plate voltage is 2500VDC.  
In AM, the plate voltage is 2000VDC.  were it NOT for the spacing of the 
terminals, there would be constant issues with possible arcing - 
ESPECIALLY while you are tuning the BC-939 to the tuning tables/cards.

There are also TWO different models of the BC-939.  There is an A model 
and a B model.  I haven't noted any obvious differences, but there 
apparently are some somewhere.

Bob N0DGN

WA5CAB at cs.com wrote:

> Bob & Group,
>
> As originally issued, the BC-610-A through -E had plug-in final 
> amplifier tank coils with fixed links.  Conversion kits were issued 
> during the war that included the rotary link tank coils.  Those issued 
> for SCR-299 also included tuning units above 8 MC (originally the 
> upper limit of the SCR-299-A through -D).  Some of the kits, including 
> one for SCR-399-A and SCR-499-A (BC-610-E), came with a doublet 
> (dipole) antenna which eventually (post-war) became AN/GRA-4.  The 
> 1.0-2.0 MC kits also included a doublet.
>
> The BC-939-A originally came with a 3-terminal feed-through insulator 
> on the side for input.  It and the earlier BC-729 were connected to 
> the BC-610-(*) with a short length of shielded cable.
>
> The ME-165/G was built for the T-368/GRT, which was set up for coaxial 
> cable from the git-go.
>
> But anyway, the output impedance of a BC-610 depends on the number of 
> turns in the output coupling link on the plug-in final amplifier plate 
> tank coil, not whether it has the original ceramic feed-through posts 
> or not.
>
> In a message dated 10/28/2005 12:53:43 PM Central Daylight Time, 
> rbethman at comcast.net writes:
>
>> There is NO question that the output of the BC-610 series, my first one
>> happens to be an (I) model, is indeed 50 ohms impedance.  It is
>> unfortunate that some hams make the ASSUMPTION that since it has
>> porcelain insulators for the output, that it is a balanced output of
>> OTHER than 50 ohms.
>>
>> I cannot stress the importance of obtaining the manual, carefully
>> studying it, and pay close attention to the details.  The ME-165G is
>> unquestionably a 50 ohm input SWR/Wattmeter/dummy load.  It comes with
>> "N" connectors for input and output.
>

-- 
Bob - NØDGN
                       
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