DX-100B Problems - Final Report?

[Kenneth G. Gordon]

On 10 Feb 2008 at 9:12, Mike Hardie wrote:

> I hope the choke addition has been described accurately.  It worked on
> this rig but whether or not it will on others is not known.  If anyone
> can shed some light on where the problem really is or why the choke
> has eliminated it please post to the list.

Good job, Mike.

I might advise the list members that I believe the problem was caused 
by a cold-solder joint in the vicinity of the two 12BY7 plate chokes.

Your description of the way the circuit was acting which you sent to me 
off-list just about convinces me that the 15 microhenry choke (at least) 
was not actively in the circuit, and that the seemingly correct plate and 
screen voltages you measured at the 12BY7 were arriving there via 
the 47K "Q" reducing resistor instead of through the coils.

I also believe that one of the purposes for the 27K decoupling resistor 
between the bias supply and the 5763 grid without a choke is to 
preclude the necessity for neutralization of that stage by swamping any 
parasitics that might occur there.

Although I have always believed that an RF choke is necessary 
between any source of DC and any element which has RF on it, and 
that your installing it was a good idea, one has to be prepared for the 
possible appearance of parasitics in original circuits without such a 
choke.

I have also always believed that all amps should be properly 
"neutralized", except, perhaps grounded-grid amps, and even those 
need that treatment sometimes.

In any case, I am firmly convinced that your original problem was 
caused by a cold-solder joint at the two buffer plate coils.

I heartily congratulate you on your work!

I might add that Heathkit design engineers were certainly no dummies. 
They carefully thought out practically everything, and sometimes we 
have to think very carefully about a circuit to really understand why 
they did certain things, ESPECIALLY before attempting any serious 
modifications.

Your slight modification was very worthwhile, and is what I would have 
done had I been in your position.

Again, congrats on a job well done.

Ken Gordon W7EKB 

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