[Heathkit] Oscillation in IT-12, I'm getting somewhere now

[Phil]

First, I want to thank all who responded.  I got to wondering if there 
was maybe a ground loop somewhere, or if some bypassing would 
eliminate/reduce it.

First, I noticed that the speaker is grounded at the top of the unit, 
one side of the output Xformer goes to the speaker switch, and from 
there to ground near the pre-amp section.  Tried lifting the ground 
connections and going directly from the speaker to the switch to 
eliminate possibility of a ground loop there.  Nope, no change.

So I took a .0022uF cap grounded one end of it with a test lead and 
started poking around, figuring I'd start at the output section and work 
my way towards the pre-amp.

Bypassing the output Xformer blue wire AT it's binding post totally 
eliminated the oscillation, tried moving back to pin 7 (theoretically 
same point, the plate), the oscillation was reduced, but NOT eliminated.

And finally, I tried bypassing the grid of the 12CA5 output tube with 
the same .0022uF to ground.  This too eliminated the oscillation, but 
also noticeably reduced normal tube hiss, telling me that it was 
affecting the high frequency response more (not surprising since it's a 
Higher impedance point).

So, I'll experiment with it a bit more, and if I don't find a better 
cure I'll bypass that one binding post to ground. It looks like that's a 
good cure, but I wonder why nobody else has apparently seen this problem 
(remember, the same problem existed BEFORE I did a complete rebuild with 
all new caps and resistors etc).

NOTE:  C9, the .001uF cap is new, but to cover all bases I tried 
bypassing it with the .0022uF cap and it didn't change anything.

-- 
73 de Phil,  KO6BB
http://www.qsl.net/ko6bb/  (Web Page)

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