[ARC5] Parasitic suppressors on ARC-5 VFO?

[Neil Barnett]

Ian
Is a calibration crystal fitted?  Did you try removing it?
Did you try swapping the 1626 osc tube?
Look for wiring mistakes made by a previous owner.
In particular, look for swapped tube heater connections
to the osc coil.  If the heater/cathode and crystal taps
got swapped, you may have either inadequate or far too
much positive feedback.

73 de Neil ZL1ANM
in Auckland.


----- Original Message ----- 
From: "Ian Wilson" <ianmwilson at earthlink.net>
To: <ARC5 at mailman.qth.net>
Sent: Wednesday, April 20, 2005 12:28 PM
Subject: [ARC5] Parasitic suppressors on ARC-5 VFO?


> Having a tough time getting a 7MHz VFO to be stable. The effect is a
> random 'warble' of a few 100Hz, with occasional 'pops' of a few 100Hz.
> The VFO is fed from a stabilized supply.
> 
> I believe the problem to be a parasitic VHF oscillation (see later for
> stuff that I've already tried). If I insert a 1k grid stopper resistor
> the problem goes away (but chirp and drift are made much worse). Is
> there arcane wisdom on parasitic stoppers for plate & grid of the 1626
> for this type of problem?
> 
> Thanks for any tips, ian K3IMW
> 
> Things that I've tried already:
> - removing the 1625's
> - using a different power supply
> - replacing all the decoupling capacitors
> - replacing grid leak capacitor
> - replacing the dogbone ceramic cap in the tank enclosure
> - replacing the resistors around the 1626
> - cleaning the VFO variable capacitor
> - cleaning between the tube socket pins
> - disconnecting/replacing the 1629 (really desparate)



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