[Collins] 51J4 Crystal Filter

[Carl]

Those caps are rated at only 50V so dont do so well in the primary circuit 
of IF transformers.

Carl
KM1H


----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at weather.net>
To: <collins at mailman.qth.net>
Sent: Saturday, September 24, 2011 12:47 PM
Subject: Re: [Collins] 51J4 Crystal Filter


> 75 to 100 pf would be expected values, though in 75S IF cans the tuning
> caps are as much as 500 pf. The classic reference of receiver design,
> Radiotron Tube Designer's Handbook suggests 100 pf is a nice value so
> the tuning doesn't change too much when changing tubes that contribute 3
> to 8 pf to the circuit capacitance and may vary a couple pf from one
> tube to the next (especially if different brands).
>
> Depending on the construction, IF transformer capacitors have been known
> to go open. And coils too, often from the corrosion of an activated
> solder flux on wire smaller than hair. An ohmmeter can show the coil
> conductivity, and there is a simple test for an open capacitor.
> Temporarily wire in a capacitor, ideally a 120 pf compression mica
> trimmer, and tune it for a peak. If it peaks other than minimum C the
> original tuning cap is open. Then unhook it, check the value and solder
> in a fixed capacitor that value. Or for $1.14 from Mouser buy several
> 120 pf trimmers and leave them in the circuit if they work.
> http://www.mouser.com/ProductDetail/Murata/TZ03R121N169B00/?qs=sGAEpiMZZMvkLkkMVBW%252bEMgRGSj6TTuq83rzMXqvshc%3d
> 81-TZ03R121N169B00
>
> The switch, the variable capacitor (especially the shaft wiper), and the
> crystal are other possible places to lose signal in that circuit.
>
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association
>
>
> On 9/24/2011 8:18 AM, Jon L. Turner wrote:
>> Hello All,
>>
>> I am going through T101 and T102 on a 51J4 that I purchased. I am 
>> wondering if anyone knows the value of the capacitors inside the cans. My 
>> tester shows 75 pf on the one in T101. I am not seeing a value on them in 
>> the schematic. The receiver seems to be low on sensitivity and it is 
>> looking like it is in this area of the receiver.
>>
>> Thank you,
>>
>> JT
>>
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