[Boatanchors] Eico 720- 40 Meter Parasitic

[Roger Ruszkowski]

David,
 
I so hate old mechanical ground connections.
Get out the screw drivers and wrenches.
You want to loosen and retighten every tube socket and ground lug bolt / screw in the
oscillator and buffer section.
 
Over time things just get crud growing in them.
Loosen it all up, tighten it back down and often wonder what happened to the problem.
Or why was their even a problem to begin with.
Watch for a cold solder joint as you work through the mechanical stuff.

There are good reasons we consider RF power magic.
 
Roger AI4NI
 
 
-----Original Message-----
From: David Stinson <arc5 at ix.netcom.com>
To: boatanchors <boatanchors at mailman.qth.net>
Sent: Fri, Jan 9, 2015 3:03 pm
Subject: [Boatanchors] Eico 720- 40 Meter Parasitic


Working on an old Eico 720 "novice" transmitter for a 
friend and it's got me pulling what's left of my hair out.
It has a parasitic oscillation in the Buffer (6AQ5) only 
on 40 meters.  Works OK on other bands.
With the rig in "TUNE" and key-up, no xtal, the stage
oscillates in the high 6 MHZ area.  Tuneable by watching
the grid current meter for freq and amplitude 
with the GRID TUNING control.  GRID DRIVE,
which adjust the screen voltage on the stage,
controls amplitude. PA Plate circuit has no 
effect on the oscillation.  
I've subbed across all the bypasses.  
Resistance to ground in the stage is normal.

Has anyone seen this in an Eico 720?

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