[Boatanchors] Eico 720 Issues

[Bill W7KXB]

arc5 Davis:...Belay my last...I did not use a loop between the plate and grid....I soldered two short wires (insulated) and twisted them together, forming a capacitor.  That stopped the oscillation.  This was 55 years ago and my memory of the event has diminished somewhat.

Ciao

de kxBill
From: w7kxb at msn.com
To: boatanchors at mailman.qth.net
Subject: Eico 720 Issues
Date: Sat, 10 Jan 2015 22:49:43 -0800




Try a little capacity between the grid and plate. Or a small loop (several turns).
Ciao
de Bill-w7kxbFrom: 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?