[Boatanchors] Mica compression trimmers?

[Carl]

I stabilized my 129X with Johnson subminiature variables for the upper 
bands.
This was ages ago so I dont remember the part # but they were a simple mod.

My WW2 Super Pros and an early SP-400 used the later 400 TC cap value plus a 
bit extra of NPO on the highest bands.
SSN often carries a nice selection of those.

Carl

----- Original Message ----- 
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
To: <boatanchors at mailman.qth.net>
Sent: Monday, October 26, 2020 4:01 PM
Subject: Re: [Boatanchors] Mica compression trimmers?


> I have a Hammarlund HQ-129-X, it uses mica compression trimmers but is 
> quite stable for what it is. That sounds like a left handed compliment but 
> really isn't. I am not sure how the 129 is stabilized but its earlier 
> version, the HQ-120-X used a bimetalic strip for TC. OTOH, the Hammarlund 
> Super-Pro did not have any TC until the very last of the SP-400 series, 
> which used a ceramic compensating cap. The Super-Pro uses air dielectric 
> caps. It has substantial temperature drift until run for a couple of days 
> continuously, then is very stable. Hammarlund seems to have had trouble 
> with drift in its later receivers. Perhaps the most stable of the 1940's 
> vintage receivers was the RCA AR-88. This used air dielectric trimmers but 
> had separate ceramic TC caps for each band. It takes perhaps an hour to 
> stabilize and then just doesn't move.
>  I have not seen a commercial receiver with separate trimmers for 
> frequency and TC. TC is a compromise, especially if a single TC cap is 
> used on the main tuning cap for all bands. Can be a delicate adjustment. 
> Perhaps that is why Hammarlund didn't use TC in the Super-Pro until very 
> late in the game. OTOH, the SP-600 has only a single TC cap and is pretty 
> stable. However, I think a lot of attention was paid to the quality of the 
> components in the oscillator (also true of the AR-88). Collins got very 
> high stability by using crystal controlled oscillators in the high 
> frequency band oscillators where the drift is worst.
>  The demand for stability in ham gear came with single sideband. While 
> stability was desirable before it was not as critical. That said, the 
> collins 75A-1, which introduced the tunable IF concept, pre-dated SSB, but 
> not by much.
>
> On 10/26/2020 12:35 PM, McDonald, J Douglas wrote:
>> Its all a matter of degree.
>>
>> I've seen such things with two adjustments: one for capacity,
>> one for temperature coefficient, which goes through zero. That was done 
>> with
>> a bimetal strip.
>>
>> Doug McDonald
>>
>> ________________________________
>> From: boatanchors-bounces at mailman.qth.net 
>> <boatanchors-bounces at mailman.qth.net> on behalf of arc5 at ix.netcom.com 
>> <arc5 at ix.netcom.com>
>> Sent: Monday, October 26, 2020 10:20 AM
>> To: boatanchors <boatanchors at mailman.qth.net>; boatanchors at theporch.com 
>> <boatanchors at theporch.com>; Milsurplus <Milsurplus at mailman.qth.net>
>> Subject: [Boatanchors] Mica compression trimmers?
>>
>> Resolved: For any receiver local oscillator which uses Mica compression 
>> trimmers, thermal drift is inescapable. Debate?
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>
> -- 
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
>
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