[Collins] 51J4 Crystal Filter

[Dr. Gerald N. Johnson]

But they see only signal voltage, not DC. If they saw DC or PA grid 
drive, they would not survive.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association

On 9/24/2011 3:57 PM, Carl wrote:
> 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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