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Author Topic: Low HW-8 sensitivity on 40 M  (Read 3306 times)
WB2JNA
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Posts: 88




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« on: November 30, 2013, 10:35:03 AM »

I notice that my Heathkit HW-8 has obviously lower sensitivity receiving on 40 M (compared to the other bands). I've tried the usual alignment process. Anyone have any ideas? I know that lots of hams enjoy working on these nice QRP rigs so I'm hoping for some help. Thanks.
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W9GB
Member

Posts: 2597




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« Reply #1 on: November 30, 2013, 01:41:51 PM »

Heathkit HW-8 Service Bulletins (1976-1989, Complete)
https://puck.nether.net/pipermail/heath/1998-April/029183.html

Poor sensitivity on 1 band is usually caused by Q1 (# 417-169)

==
KK4KF's Heathkit HW-7/8/9 Information Page
http://www.qsl.net/kk4kf/heathkit.html

Heathkit HW-8
by William Eric McFadden
http://home.frognet.net/~mcfadden/wd8rif/hw8.htm

Heathkit HW-8
Assembly and Operation Manual
http://www.utm.edu/staff/leeb/hw8.pdf

WB3T - Heathkit HW QRP Restorer
http://radiowb3t.homestead.com/heathkit.html
« Last Edit: November 30, 2013, 01:55:51 PM by W9GB » Logged
WB6BYU
Member

Posts: 13017




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« Reply #2 on: December 01, 2013, 07:02:00 AM »

If it is just one band then I'd check the band switch and input network for that band.  The
Switches are normally pretty reliable, but if someone overheated one of the pins during
soldering you could get a poor contact buried inside the body.  Test that by jumpering the
pins externally where they should be making contact internally.

Was the radio ever working properly?  There were some cases where the wrong core was
provided for one band, or the builder grabbed the wrong core, so the inductance was off
and the circuit wouldn't resonate on the right frequency.

Other than the antenna input, the circuit uses mostly DC bandswitching, so you can
use DC voltage checks to make sure that the right circuits are being selected.  (A bad
switching diode can cause your problem, too.)  The input you can test by applying
the signal to both sides of the band switch terminals to see if it makes a difference.
The oscillator is also DC switched - after you confirm the right DC voltage on the
tuned circuit then check that the crystal is oscillating and still on the right frequency.

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WB2JNA
Member

Posts: 88




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« Reply #3 on: December 01, 2013, 05:52:58 PM »

Thanks.
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WB2JNA
Member

Posts: 88




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« Reply #4 on: December 03, 2013, 01:46:18 PM »

By the way, does anyone know what kind of diodes do you use for D2 and D6? Thanks again. The radio doesn't receive at all on 40 m now. I'll keep at it.
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W9GB
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Posts: 2597




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« Reply #5 on: December 03, 2013, 04:36:18 PM »

1N914 / 1N4148 are suitable substitutes for these bad diodes used in HW-8 band switching.

Radio Shack use to stock these diodes at their retail stores.
http://www.radioshack.com/product/index.jsp?productId=2062576
Use the FIND IT NEARBY button on web page, to check stock at local store.

EDUCATIONAL YouTube video with Heath-kit HW-9 (How diode switching works).
http://youtu.be/rPM1vvGorTo
« Last Edit: December 03, 2013, 04:44:24 PM by W9GB » Logged
WB2JNA
Member

Posts: 88




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« Reply #6 on: December 03, 2013, 05:06:45 PM »

Thanks. I'll start with the diodes.
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WB6BYU
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Posts: 13017




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« Reply #7 on: December 03, 2013, 06:13:54 PM »

If the diodes are bad you'll see it in the DC voltage measurements on the
tuned circuits involved.
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WB2JNA
Member

Posts: 88




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« Reply #8 on: December 21, 2013, 08:27:16 AM »

I probably will want to replace Q1. Does anyone know if a transistor besides the MP-102 FET will work? Thanks.
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