Pages: [1]   Go Down

Author Topic: HW7 new front end  (Read 2795 times)

9H1FQ

  • Posts: 216
    • HomeURL
HW7 new front end
« on: February 08, 2018, 12:56:43 AM »

Anyone experimented with replacing the front end of the Heathkit HW7 with an FST 3253  switch detector, Similar to a Softrock ?
Logged
An unknown gem of the Mediterranean sea is the Island of Malta. blessed with warm sun , all year round.
In just one day, you can see places which will make you travel in history from the Prehistoric temples throu the middle ages, the Knights of Malta, all the way to world war two. All the major civ

K8BYP

  • Member
  • Posts: 256
RE: HW7 new front end
« Reply #1 on: February 14, 2018, 05:46:22 AM »

1. I have one that Ive restored, it was my Novice rig.
2. HW-7 is not a boat anchor. Boat anchors are large, heavy radios. Just because its old doesnt make it a boat anchor.
3. That MOSFET is a combination of RF amp and mixer. There are no switching functions.
4. That part number is for a digital switch device (digital bus buffer), has no relevance to this question.
4. The "front end" is a dual gate MOSFET, the replacement is supposed to be BF991, I recall reading someone had used one, I have not since the original in my rig still works. This device has ESD diodes built in.
5. If one finds the "original" MOSFET advertising the SHORTING SPRINGS HAVE BEEN REMOVED, DO NOT BUY THEM. Some Hucksters had advertised that... those original MOSFETS were extremely sensitive to ESD and just the act of removing the spring can destroy it, unless its connected to the circuit. Was there, did that, blew the first one up.


If youre going to be in there working on it, a couple cautions

a. DONT reroute the long RG 174 cable from the TR relay to RF preselector, its routed behind the circuit board for a reason.
b. Remove the TR relay and look for corrosion under neath.
c. Ignore the Hams websites discussing design changes, especially the break in transistor circuit, because the information is wrong. The fault in the break in circuit is an old leaky capacitor (or just a bad transistor) and the advice to change resistor values WILL apply DC voltages in excess of the ratings of that transistor. Beware when Hobbyists think they know more than the Engineers that design things originally...
« Last Edit: February 14, 2018, 05:52:29 AM by K8BYP »
Logged

KX4OM

  • Posts: 533
    • HomeURL
RE: HW7 new front end
« Reply #2 on: February 14, 2018, 08:18:08 AM »

There are numerous articles on H-mode mixers using fast bus switches such as the one you mentioned. They are exceptional when employed correctly. For any rig, a switching mixer, including a diode ring design, has significant advantages over active mixers. Whether or not an RF amplifier along with a front end filter depends on the operating frequency (above 40 meters, for example) and also the rest of the design of the receiver. A direct conversion has most of the gain in the audio circuits.

The question that you asked is complex, and requires significant design study.

Ted, KX4OM
Logged

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
RE: HW7 new front end
« Reply #3 on: February 14, 2018, 09:07:16 PM »

You'd need to make sure that the rest of the radio was capable of enough performance
(dynamic range, gain, etc.) to make the FST 3253 mixer worthwhile.  But there was a
simpler version from Australia - called the "Ned Kelley" mixer, after an outlaw who wore bullet-proof armor.  Used a 4051(?) CMOS switch chip IIRC into a pair of op amps.  I prototyped one
and it seemed to work well, with no transformers to wind.  It was in G3VA's Technical Topics
in RadCom, perhaps in the late 1980's.

You'd probably still need a good preamp ahead of it to set the noise floor, but my guess
(without the circuit in front of me) is that it would be an easier conversion and still give
most of the improvement.

Of course, converting to super-het, or a half- or double-frequency mixer, would improve
some of the other direct-conversion idiosyncrasies...

KX4OM

  • Posts: 533
    • HomeURL
RE: HW7 new front end
« Reply #4 on: February 15, 2018, 07:04:49 AM »

Here is an article by Mike, WA8MCQ who has been a technical columnist for QRP Quarterly and other magazines for many years:  "Better Ears for the HW-7"

http://www.qsl.net/kk4kf/hw7-ears.html

In that article, the dual gate MOSFET front end mixer is replaced by a MiniCircuits double balanced, ring-diode based mixer. SBL-1, ADE-1 and other similar devices are available in eBay. A homebrew version can be made using a two ferrite toroids and four IN5711 Schotttky diodes or two surface mount BAT54SL diode pairs. 1N914 or 1N4148 switching diodes will work. Refer to Farhan, VU2ESE's designs of the famous Bitx20, Bitx40 and uBitx transceivers. Several designers and engineers including Wes Hayward, W7ZOI no longer recommend dual gate MOSFETs for mixer use.

This website link is Farhan's description of the uBitx, with a section on diode ring mixers:

http://www.hfsignals.com/index.php/ubitx-circuit-description/

Ted, KX4OM
« Last Edit: February 15, 2018, 07:08:09 AM by KX4OM »
Logged
Pages: [1]   Go Up