The HW-8 rehab project of Ed, WA3WSJ, has led to a nice discussion on QRP-L about the possibility of using the HW-8 as a DSB rig. Mike, KL7R, did such a mod on his HW-8 many years ago and used it successfully in the field in Alaska. Read his story here: http://kl7r.ham-radio.ch/hw8/hw8story.html
Nick, WA5BDU, and I have been discussing the possible need for an additional mod to put the HW-8's final in linear mode. The picture above is of Mike's rig, with the finals unbiased. Check out the scope. Looks pretty good to me!
Here's the e-mail exchange:
From:
"Nick-WA5BDU"
To:
"Bill Meara"
Cc:
qrp-l@mailman.qth.net, sigcom@juno.cm
The balanced modulator is inserted between the driver and final amplifier, which has no fixed bias and has its emitter grounded, so it's definitely class-C. I do note that the balanced modulator is passive and probably has 6dB or more of loss, so you're getting a pretty good drive reduction. And Mike said he adjusted the audio while watching the RF output until there was minimal flat topping on a scope. So I wonder if it's possible to keep things fairly linear if the power is kept quite low?
At any rate, I love the idea. Here's Mike in the 70s. He has an HW-8 he's familiar with, and he's reading SSDRA. Going over the simple DSB generation examples, he thinks -- why not stick this in the RF train of my HW-8?
72-
Nick, WA5BDU
On 9/19/2011 4:36 AM, Bill Meara wrote: Nick: I enjoyed reading about your HW-8 adventure, especially the part about Mike's work with this rig. I remember talking to him about it, and I've since come across some articles describing similar DSB mods. One thing that all of these plans lack, however, is a modification to the PA. It is Class C in the HW-8, and would need to be made linear for DSB service, correct? 73 Bill N2CQR
--- On Sun, 9/18/11, Nick-WA5BDU
From: Nick-WA5BDU
Subject: Re: [QRP-L] WA3WSJ Heathkit HW-8 Rebuild Status
To: qrp-l@mailman.qth.net
Date: Sunday, September 18, 2011, 9:44 PM
Sounds like a great project (or projects!) Ed.
I'm working on an HW-8 also. Bought it at a hamfest this Spring at a
good price, but suspected it might have some issues. When I started
playing with it I noted that transmit power was down, the loading
control was stuck, and there was no sound from the receiver.
The receiver issue was the most challenging and I eventually tracked it
to a leaky transistor in the T/R circuit which was activating the mute
function even when key up. I suspect that the transistor's problem may
have been due to the lack of a de-spiking diode on the T/R relay's coil
so I added one.
So far I'm not keen on modifying the rig and am even keeping the RCA
connector (spent an outrageous $5 for a RCA to BNC adapter from Radio
Shack). Well, maybe not outrageous, but I'm pretty cheap. Other than the
diode, I did add a 1A fuse and a reverse polarized diode downstream of
it for reverse-polarity protection.
After going thoThe HW-8 rehab project of Ed, WA3WSJ, has led to a nice discussion on QRP-L about the possibility of using the HW-8 as a DSB rig. Mike, KL7R, did a mod on his HW-8 many years ago and used it successfully in the field in Alaska. Read his story here:
http://kl7r.ham-radio.ch/hw8/hw8story.html
Nick, WA5BDU, and I have been discussing the possible need for an additional mod to put the HW-8's final in linear mode. The picture above is of Mike's rig, with the finals unbiased. Check out the scope. Looks pretty good to me!
Here's the e-mail exchange: ugh most of the alignment procedure (I don't have a tool
that will reach the bottom coils of the dual-coil adjustable inductors),
the VFO looks good and the power output seems about right.
It was nice that QRP Afield and the Washington Salmon Run were going
right after I did my repairs, so I was able to make ten or so QSOs to
try it out. It also reaffirmed the idea that QSOs can come pretty easily
at 1 or 2 watts out -- you don't need the full pentawatt. I had my first
QSO with the rig the night before, when it was fitting that I worked a
guy running a HW-101 who said he also had a HW-8. Heath to Heath at
random -- what are the odds?
I find that if I buy a used rig and have to work on it to get it going,
I develop a greater appreciation for it. If the repair is successful,
that is.
The HW-8 rehab project of Ed, WA3WSJ, has led to a nice discussion on QRP-L about the possibility of using the HW-8 as a DSB rig. Mike, KL7R, did a mod on his HW-8 many years ago and used it successfully in the field in Alaska. Read his story here:
http://kl7r.ham-radio.ch/hw8/hw8story.html
Nick, WA5BDU, and I have been discussing the possible need for an additional mod to put the HW-8's final in linear mode. The picture above is of Mike's rig, with the finals unbiased. Check out the scope. Looks pretty good to me!
Here's the e-mail exchange:
The HW-8 has an interesting mixture of modern and semi-vintage (70s)
features. Some things I found interesting were -
It uses a LM3900 quad Norton op-amp for the active audio filter, one
stage of audio amplifier, and the sidetone oscillator.
The final amplifier is a 2N4427 with a small heat sink. According to the
data sheet it is rated for 1W output at 175MHz and has GWB of 500MHz.(I
assume that's original -- I was sort of surprised it wasn't "house marked".)
It has a direct conversion receiver but has a heterodyne frequency
generation system. The product detector is an MC1496 balanced modulator IC.
The VFO tunes the same linear scale 250kHz on each band, 8.645MHz to
8.895MHz with a reduction drive. Pretty state of the art for the 70s.
Heath rates power the old way with power /input/ of 3.5, 3.0, 3.0 and
2.5 watts input from 80 through 15.
Bandswitching, covers 80, 40, 20, 15 meters.
The audio output stage is a small transistor in a common emitter
configuration with a 1kΩ resistor in the collector lead.So it can only
put out a small amount of AF power and into a high impedance (1k)
load.Has a mono ¼ inch phone jack and no speaker.
Keying is by pulling 12V to ground like in a modern transceiver.There’s
a keying offset of 750 Hz and you tune to the HIGH side to get on frequency.
Antenna switching is done by a relay with adjustable drop-out time for
semi-QSK.
Relative power output meter, not used on receive.
While doing web searches on the HW-8, I came across the web site of
Mike, KL7R (SK), late co-host of Solder Smoke.
http://kl7r.ham-radio.ch/hw8/hw8story.html
The idea that he put the HW-8 on DSB with such a simple mod really
impressed me. I think I'm going to fool with some DSB stuff in the
future, although I'm not sure I'll try it with the HW-8.
72,
Nick, WA5BDU
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics"http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20
Posts
Can't argue w/ the waveforms. Knowing how a Class 'C' amp. works (and the HW-8 PA is definitely Class 'C') I don't understand how it could be so clean with SSB input.
ReplyDeleteBe that as it may, that was a very cleaver mod. Mike made.
With a mic. in one hand and a puzzled look on my face.......
73.......Steve Smith WB6TNL
"Snort Rosin"
Hello - I performed the mod as described, and could never get it to work properly as the SSB output was clearly distorted. I sold the radio on eBay 'as is' - everything else worked including the RIT, keyer, S-meter - etd.
ReplyDelete73's - Joe - K3JLS