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Sunday, June 25, 2017

Mod to Correct DESIGN ERROR in Heath HW-8

130k resistor inside heat shrink, across R-24
SPRAT 161 (Winter 2014/15) had a very intriguing article about the Heath HW-8 by Dave M0CEM.  Dave wrote about a possible design error in the active filter of the receiver.  The center frequency and gain shift significantly when you switch from "Wide" to "Narrow."   I was very interested because for many years I have noticed this problem in my own HW-8.  I just left it in "Wide" and never used the more narrow filter.  
 
Dave did some excellent circuit detective work and determined that the problem was R24.  He prescribed a change:  instead of 82k it should be 49.8k.  The placement of a 130k resistor across the 81k R24 would have the same effect.

Yesterday morning I installed Dave's mod.  I did as he suggested and put a 130k resistor across R24.  Brilliant!   What a difference!  In the past, I noticed a significant degradation in signal strength when I put the filter in the "Narrow"  position.  This doesn't happen now -- the response obviously does narrow, but the center freq stays the same.

It is really amazing to me that the Heath engineers made this error, and that it went uncorrected for so many years.  There have been many, many modification articles for the HW-8 over the years, but I haven't seen any that really address this problem. 

If anyone has more info on how this design error happened and why it went uncorrected for so long, please let me know.

Thanks to Dave and SPRAT for getting us back on the straight and narrow. 

4 comments:

  1. Hi Guys,

    Back in my early days of ham radio, I purchased an HW-8 from the New Orleans/Metairie, LA Heathkit Store. Remember those? :D. My Dad and I (W5VXQ SK) took turns soldering the thing, and after a few days we had a working rig. It worked surprising well with a pair of phased dipoles @ 30 feet, but with one caveat -- the receiver was broad as a barn door, and with a direct-conversion receiver, that can be a bit challenging, to say the least, especially if you Dad was a radio hog and forced you to operate late at night and into the early morning. :D

    I did notice the HW-8 Handbook being advertised back in the day, but I never really thought about purchasing it, mainly because a lot of folks in the QRP rags were saying things like, "Well, it really depends on who did the mods. One guy can do them and totally enhance the quality of the rig, yet another can do them and totally ruin the thing." With all that in mind, I decided to leave my HW-8 in "virgin" condition, just in case I ever wanted to sell it or do some horse trading with the rig.

    Well, that all changed when I read the post here on this simple modification. I figured, "What the heck can go wrong with this, and if it does screw things up, I'll just remove the mod and no harm done." Man, am I really glad I decided to make the change! My HW-8 has never sounded better, and now I'm going to put up some kind of external antenna for 40 and see if I can relive the days of my Dad yelling, "I just worked Canada twice!" :D.
    Thanks so much for this mod!

    BTW, I'll be hanging around the lower end of 40, say 7.030. My code speed will probably suck do to lack of activity, but I will answer any and all calls that can hear me!

    73,

    Randy AB5NI

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  2. I did this modification also but had to change all the resistors around the active filter by 1% metal film ones as well before I was quite satisfied.
    Gerard

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  3. Bill - I finally got around to opening up the HW-8 to investigate the AF filter center frequency problem you mentioned with the fix in Sprat #161.

    To tell you the truth I never really noticed the change in center frequency when moving from the wide to narrow positions but I fed it some 7.0MHz RF and measured the frequency where the audio signal peaked and sure ‘nuff it was about 100Hz low from the wide position at around 650Hz.

    So I tried the fix outlined in the Sprat issue by tack soldering a 130k resistor across R24 and the author recommended and viola the center frequency now rests at ~750Hz.  The additional MFJ CWF-2 filter that I installed tracked almost exactly over the three narrower bandwidths as well right at 750Hz.

    BTW, with the S-meter mod it makes it very easy to peak the signal then simply read the frequency with the highest response on the scope.

    Thanks for the tip and like he said, wonder why nobody ever saw that before?

    Have a great week!

    73 - Dino KL0S

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  4. In the Yahoo HW-8 group Jim WA1BOQ pointed me to the May 1996 issue of 73 magazine. There George K8MKB wrote of mods to the HW8 including the change to R24 that M0CEM came up with independently.

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