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

[Robert & Linda McGraw K4TAX]

In my TV fixing days I recall that the 6HF8's were noted to have some 
problem with hum.  The triode section was used in the sync circuit and the 
pentode section in the video circuits.  Some brands were worse than others. 
I suspect that being the case, the high gain contributes to modulation of 
the cathode via heater leakage.  Never tried it in the case of the TV hum 
but in high gain guitar amps the first stages use DC on the heaters to avoid 
just that.

An interesting point is the hum 60 Hz or 120 Hz.  I bet 60 Hz if heater to 
cathode leakage exists.  Otherwise, it can be power supply related.  And 
too, there might be a ground loop that is being modulated by the 60 Hz line 
inside the box.

73
Bob, K4TAX

.
----- Original Message ----- 
From: "Tom NØJMY - AAR7FV" <tfarl at mchsi.com>
To: <kgordon2006 at verizononline.net>
Cc: <Heathkit at mailman.qth.net>
Sent: Thursday, June 29, 2006 11:07 AM
Subject: [Heathkit] Re: CWF-2 Audio Filter and HW-16s.


> 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
>>
>>
> _______________________________________________
> List Administrator: Duane Fischer, W8DBF        ** For Assistance: 
> dfischer at usol.com **         $$ See the vintage area on the HCI web site - 
> http://www.w9wze.org $$ Heathkit mailing list
> Heathkit at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/heathkit
>