[Heathkit] Audio filter on HR-10B

[Kenneth G. Gordon]

On 4 Jul 2006 at 9:03, TJRachwal wrote:

> Hello all,
>  After posting about mods for improving CW selectivity and other mods
> and receiving many responses I decided to follow the advice of adding
> an audio filter rather than making mods to the HR-10B.  It was a good
> move and a very good suggestion.  I opted for the SCAF filter from
> Idiom Press rather than the MFJ much cheaper filter.

<snip>

> Wow!  The performance difference is
> amazing!  It wasn't "quite" the same as the 500 hz filter but leaps
> and bounds over the "wide as a barn door" front end in the HR-10B.  I
> could copy CW stations clearly that were down in the mud without the
> filter and not workable.

My experiences are identical with yours.

>  Since I didn't have an MFJ to compare it against I can't say the SCAF
> by Idiom press is any better.

And, since I have never used an SCAF filter, I cannot say which is 
better either. :-)

However, I have several of the older model MFJ CWF-2 audio filters, 
which are identical with the newer VEC-820K filters by MFJ's partner, 
Vectronics. 

These audio filters provide 180 Hz, 110, hz, and 80 Hz selectivity 
choices, at a center frequency of about 750 Hz, and in the 80 Hz 
position (at least), provide about 3 db of gain. There is actually a fourth 
choice of selectivity, 90 Hz, which is not normally connected in the 
commercially sold filters.

I have used these externally for a couple of years with a Heathkit HW-
16, and a series of military boatanchors. When copying weak CW 
signals, the filter REALLY makes a major difference!

When I recently did some other work on my HW-16/VF-1 combo, I 
mounted a CWF-2 internally, between the product detector output and 
the input of the 1st audio stage, and used a front panel mounted 
(between the RF gain control and the Power Output control) miniature 6 
position rotary switch to choose OUT, 180, 110, 90, and 80 Hz audio 
selectivity. I power it with a voltage-doubler from the filament voltage 
line.

I find I am constantly using the 90 Hz position.

I do a lot of CW net operating and like to work the QRP guys, and, 
always, when I cannot copy a station because it is too weak and 
interfered with by QRN or QRM, switching the filter in makes copy solid 
Q-5 from then on.

I can very highly recommend them!

And at $19.95 for the MFJ/Vectronics filter vs $140.00 for the SCAF, 
even if it has other shortcomings vs the SCAF filter (I have never used 
one),  the MFJ/Vectronics filter is an excellent investment.

BTW, Tom N0JMY, a member of this list, who has restored and 
serviced many HW-16s over the past few years, and at the moment is 
selling a very very nice overlay for those HW-16s whose front panels 
are in bad condition, installs these CWF-2s internally as one of his 
services.

You might ask him what his experience with those has been.

Ken Gordon W7EKB