[Boatanchors] Re: [Hammarlund] SP-200 Help Needed

[rrkrr]

Joe,
Where is the bypass cap in the circuit?
By touching the probe or antenna to the 1st RF plate, you're just 
bypassing the first RF amp and coupling signal to the input of the next 
stage.
Are you sure the new bypass cap is good and that it is connected 
correctly in the circuit? (I ask because I've inadvertently installed 
bad components or good components in the wrong place myself in the 
past.....) Is there a possibility the new bypass cap is bypassing the 
input signal to ground?  Is the AVC line still properly connected to the 
1st RF grid (through a resistor or inductor)?

You might try to establish what the bias conditions are on the 1st RF 
amp tube and compare them to those for the 2nd RF, since the latter 
seems to be working.  You indicated that the DC voltages on the 1st RF 
checked okay.  Is there a resistor in series with the cathode or plate 
where you can measure DC voltages on both ends to determine the bias 
current in the plate circuit?  The DC plate circuit bias currents in 
both RF stages ought to be about the same.  If the bias current in the 
1st RF stage is cut off, it won't amplify.

Bob K4ERR


Carole White-Connor wrote:
> Guys, I'm working on an SP-200 (BC-794). It was performing well with good
> sensitivity.
>
> Tonight, I changed out a leaky .01 bypass cap on the 1st RF (6K7). After I did
> so, the sensitivity went south. I checked the voltages on the 1st RF and all are
> normal.
>
> However, I did notice something odd. When I touched the probe to the plate of
> the 1st RF, the signal improved. I touched the antenna (through a cap, of
> course) to the plate of the 1st RF and sensitivity returned to normal. I touched
> the antenna to the grid cap of the 1st RF and sensitivity was poor. The 6K7
> tests good.
>
> The problem seems to be before the plate of the 1st RF. I have replaced all the
> caps under the chassis but have not yet gone into the RF deck.
>
> Where should I be looking and what should I be looking for? Any troubleshooting
> tips would be gratefully appreciated.
>
> As always, thanks for your help.
>
>
> Joe Connor
>
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