[Hallicrafters] Troubleshooting an SX-71

[rrkrr]

I think Rodney has it right.  The tuning capacitor is the common element 
for all bands.  I forget whether the tuning cap on the sx-71 has 
parallel trimmer caps mounted on the stator sections that would be in 
the circuit for all bands, but you might check for a short or open there 
and also check to see whether any of the blades in the RF or mixer 
tuning sections are bent enough to short circuit as you tune across the 
band.  Of course you would know immediately if the sections had DC high 
voltage on them, but they often have DC blocking caps in series.

Bob
WB4TGG


Rodney Bunt wrote:

>This behavior is indicative of a poorly tracking tuning capacitor.
> 
>I have seen this on a Hammarlund SP-200. Where there was a miss alignment between the mixer and RF
>
>stages, it would align only at one place on the band top or bottom or middle but never over all of
>
>the band. Use the "blades" on the tuning CAP to trim this.
> 
>Because there are three tuning caps....
> 
>1. adjust the Oscillator segment to track the dial scale for frequency. Set top, set bottom, and 
>adjust blades (bend in or out) to adjust linearity against the dial scale.
> 
>2. inject RF signal generator at grid of mixer, adjust "MIXER" cap segment to obtain good output 
>across the bands
> 
>3. Inject RF at grid of RF tube and ajust RF cap for same...( good output across band.
> 
>4. Adjust antenna matching for good broad response for each band segment....
> 
>Rodney
>VK2KTZ
>
>
>--- ARDUJENSKI at aol.com wrote:
>
>  
>
>>(SX-71) It is apparent that on every band, however, that there is more  
>>sensitivity at the upper end of the band than at the lower end.  This  
>>might be poor alignment, but because it is the same for all bands,  I 
>>was wondering if it may be part the receiver's design? Any good  references 
>>raddressing peculiarities with the SX-71? I did GOOGLE search and  checked the 
>>archives
>> 
>>Alan  KB7MBI
>>Woodinville, WA
>>Member ARRL / ARCI / ARS / FISTS /  NAQCC
>>
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