[Heathkit] Re: CWF-2 Audio Filter and HW-16s.

[Tom NØJMY - AAR7FV]

Kenneth G. Gordon wrote:
> Trouble is, my HW-16 has a LOT of hum, especially with the volume 
> control all the way down, and so far, I can't figure out where it is coming 
> from.
>   
You and me both, Brother!  I've spent a lot of time trying to find that 
problem on about 90% of the HW-16's I've done over the years.  Finally, 
I said the heck with it and did the following:  Replace C87 with a .01 
uf disk.  From the factory , this cap is a .001 uf. disk and it bypasses 
the control grid of V6B, the audio power amp half of the 6HF8. 

I experimented with various values using a capacitor substitution box, 
(although it and the attendant test jumpers exacerbate the hum problem a 
little bit themselves).  Generally, the larger the value of the cap the 
greater the hum reduction (through about .047 uf. where it was 
imperceptible).  However, the gain of the amp started to take a hit 
above about .0033 uf..  I settled on .01 as a good compromise, as the 
decrease in gain was barely noticeable, and once the jumper wires were 
removed and the installation was made permanent, the hum virtually 
disappeared.  Maybe a .0068 would have sufficed.

As an aside, any reduction in gain affects the higher audio frequencies 
first -- actually improving selectivity a minute amount.

> I have taken it down the the shop at work and intend to follow the hum 
> around with a scope until I find it. I know it is not coming from any place 
> ahead of the audio amp stages. If I yank the 12AX7 product 
> detector/bfo tube, there is no change.
>   
I figure it's got to be a ground-loop somewhere, but life's too short to 
worry about it.  It's possible that this fix might be an uncouth "brute 
force" approach to a problem whose origins are elsewhere in the rig, but 
hey, God must have created us uncouth brutes for *some* reason.
> I also have a problem with my HW-16 in that there are terribly loud 
> clicks in the headphones or speaker, but not on the air, when keying 
> the darned thing.
>
> I've tried several different methods for minimizing those, but have not 
> yet succeeded in finding a proper cure.
>
> I'm not even sure where it is coming from yet. I know at least one fellow 
> attempted to add a limiter to the receiver to take that out, but it didn't 
> work very well for me.
>
> I wonder if anyone else has had this problem and fixed it?
>   
I've heard many people have had that problem.  I've never got around to 
delving into it.  It's on my to-do list, though.
> Lastly, it turned out that the reason I couldn't get any audio through my 
> audio amp stage was that I had inadvertently installed a 270 K ohm 
> resistor in the cathode lead of the 6HF8, instead of the 270 ohm 
> resistor that it should have had. Therefore the audio amp stage  
> cut-off.
>   
Hee!  Good to know that I'm not the only one to come a'cropper on things 
like that!

73,
TOm
> Ken W7EKB
>
>