[Boatanchors] Mica compression trimmers?

[Richard Knoppow]

    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