[HBR] So i'm gonna build an HBR-XX
G4IZS
ronald.sexton at btinternet.com
Thu Feb 19 14:53:06 EST 2009
Hi Bob,
You are too kind ....
I guess I missed out on the advice that if each of the pulleys is mounted on a small adjustable (probably soft aluminium) bracket, some judicious bending can ensure the string tracks properly.
The real trick is in having a simple mechanism whereby the string tension can be adjusted for freedom of motion throughout the system without backlash and to ensure the "feel" is right for the operator. Tying knots and hoping to get things right does not work well. I reckon the string has to be the right "length" within 0.01 inch.
I guess one other warning if someone does try to make a modern string drive - the variable capacitor itself, as always, should be free-turning and not gummed up with backlash-inducing grease.
Simplicity is the art of design - .
I see a number of responses concerning the actual dial cord. On this side of the Pond dial cord was always to be found at "Radiospares" - the company now known as RS Components, but sadly no hint of dial cord amongst their current offerings. I think someone must still make it because there are still many inexpensive commercial transistor radios with string drives.
For anyone interested here is a link to a company (with an interesting name!) who are selling the "proper" stuff.
http://www.6v6.co.uk/cp/scripts/prodList.asp?idCategory=65
They have some other amusing bits and pieces too.
When I did a Google search for "dial cord" I found quite a number of potential suppliers. .......
As for "proper" Eddystone 898 dial mechanisms, someone was asking about prices. For a good, original untouched dial - maybe as much as £150, say $200. A used but serviceable one, maybe $120. It will often be less expensive to dismantle an old (Eddystone?) radio for parts.
A significant problem with the 898 dial is the tuning capacitor will often have to be mounted well above a chassis in order to line up the shaft mechanisms. The flexible connector that really should then be used to align the shafts is a very significant source of backlash. A pulley directly on the shaft of the capacitor will not, of itself, create backlash.
Having said that, an 898 dial is a really super thing to own and use (and represents a good investment in difficult times).
73,
Ron, G4IZS
----- Original Message -----
From: Bob Betts
To: HBR Receiver List
Sent: Thursday, February 19, 2009 10:35 AM
Subject: Re: [HBR] So i'm gonna build an HBR-XX
Ron:
That was just about the best explanation of methods and technique I have read in a long time. I have been a product design engineer since 1963, and based on the many schemes I have been exposed to, what you describe is the simple, yet elegant concept in those kinds of drive/indicator mechanisms. Some of this stuff is (and has been) the de facto industry method (think high-end audio products of the 1950s and 60s).
Yet other old stuff was overly complicated, and to me, never had the feel of a quality, well-designed tuning system. Fortunately, most of the good (expensive) equipment was well thought out and a pleasure to use...as you have described.
HBR's not withstanding, I have seen some wonderful prototypes (and homebrew) creations compromised by the lack of design technique in the tuning mechanisms. That's a shame, because otherwise many of them were technically advanced electronically, but not mechanically.
We have developed many different drive systems over the years, for consumer goods, military contracts, and commercial/professional applications. Due to specification requirements, we utilized gear drives, chain drives, and cam/lever systems, but to me, nothing has the "feel" of a well-executed string and pulley arrangement. For what it's worth, we have successfully used a tinsel wire (woven metalic fibers) covered with a fiberglass braid. I have no idea who may make it, or where that material can be obtained these days, but if it can be found, it would certainly be treat to homebrewers. It was variously called 'wire core dial cord' or 'metal core dial cord' ...something like that. For the hobbyist, and repair shops, (back then), it was marketed by Waldom and General Cement (GC). I think I used to buy it from Allied, in Chicago as well.
Anyhow, good post.
Bob, N1KPR
http://www.bobsamerica.com
Music, Audio, Radio
"It's all good."
--- On Thu, 2/19/09, G4IZS <ronald.sexton at btinternet.com> wrote:
From: G4IZS <ronald.sexton at btinternet.com>
Subject: Re: [HBR] So i'm gonna build an HBR-XX
To: match at ece.utah.edu, "HBR Receiver List" <hbr at mailman.qth.net>
Date: Thursday, February 19, 2009, 2:56 AM
Hi Marvin,
I used to do design work for Eddystone - you do NOT need to have
Anti-backlash gears, or even spend very much! The problem with string drives
is getting the tension right in the cord (and maybe finding proper dial
cord).
The answer? Take a ball race (important) front panel mounting shaft
(best are those by Jackson). I expect you will use a 1/4 inch one. You want
a long one so you can fit a flywheel on the rear of the shaft and a heavy
control knob on the front.
The next "trick" is to only use two BRASS pulleys with steel
bearings to
guide the string across the top of the visible dial itself. You need ONLY
these two pulleys for the whole mechanism or maybe just one other if you
need the sting to miss other components. Fewer is better. You can take the
string across the bottom of the dial but it is than more difficult to get a
smooth feel because the pointer tends to drag. You can reduce this with some
PTFE sleeving at the very tip of the pointer.
The next trick is to find a LARGE diameter pulley to fit on the shaft of
the capacitor. You could say the larger the better. just under half the
circumference is going to be the visible dial length.
Now, the capacitor is mounted on your chassis. It goes in with the shaft
VERTICAL and in such a position that the string track around the rim of the
large pulley is asymptotic to the line of string across the top of the dial.
The pulley should be at the height of the control shaft. This means the
position of the capacitor is fully defined in 2 dimensions, you can choose
where you want it "left to right" so long as it is within the width
of the
proposed dial "window".
The next drill a hole in the flat face of the large pulley so you can
fit a long solder tag or similar. The string tension spring goes through the
hole at the tip. You want a short stiff spring.
String the dial, two full turns around the front panel shaft, one turn
on the large pulley.
Fit the flywheel and a heavy, well balanced, pleasant feeling, knob.
The final trick? Finely adjust the string tension using the "solder
tag"
mounted on the large pulley.
You will easily be able to make a dial whereby you have the feeling if a
fly leaned on the control knob it the dial would move imperceptibly slowly
and if you give a single twist of the wrist the pointer will travel gently
all the way across the dial (this, not the part with the fly, was the test
for Eddystone dials).
Backlash? None if you get the tension right AND use PROPER dial string.
Disadvantages? Because the large diameter pulley lands up horizontally ,
it can take up a lot of otherwise useable chassis area.
Advantages? Deliciously homebrew and very inexpensive - you can adjust
the "gear ratio" easily by turning down the groove in the panel
shaft, just
do not go too far. Which brings me to a last, final, trick. Make sure the
string groove in the shaft is absolutely in line with the plane of all the
pulleys taking account that the string runs in the "groove" in the
rim of
the pulley.
Sounds difficult, but try it! It takes me about 2 minutes to get the
string tension right using the "adjuster" on the pulley.
73, Ron G4IZS
----- Original Message -----
From: "Marvin Match" <match at ece.utah.edu>
To: <hbr at mailman.qth.net>
Sent: Wednesday, February 18, 2009 10:49 PM
Subject: Re: [HBR] So i'm gonna build an HBR-XX
> On 18 Feb 2009 at 12:00, xjakobx at gmail.com wrote:
>
>> Hello List!
>>
>> I'm starting to gather together parts for my HBR-styled
construction
>> project. It's a long way from being started, but hey, i gotta get
the
>> parts
>> together sometime, right?
>
> ---snip---
>
> A dial mechanism will be difficult. I've been looking for some time,
> and I have a National or two, but I really want an Eddystone. At this
> point I don't expect to find one.
>
> So, I'll be making my own slide-rule mechanism. I'll share it all
> with the list, but I don't expect to make much progress for another
> month or so. I really wanted a gear-reduction dial made with anti-
> backlash gears, but the gears themselves are expensive, so I found a
> variable cap made by TRW with an integral 6:1 ball reduction built
> into the shaft and I'll put a 6:1 Jackson ball reduction drive in
> front of it. That'll get me 36:1 reduction. I'd prefer about twice
> that, or three times that with a two-speed reduction, but I won't
> live long enough to find the parts to build it that way.
>
> BTW, the National and Jackson reduction units are not all the same
> either. Line up 10 identical Nationals and each one will feel
> different. The Jacksons are more consistant and generally feel better
> than the Nationals, but the same holds true. The best that i've found
> are the ball reduction drives that you see on some military equipment
> that have threaded parts that you can take apart. Not sure who made
> them. Next is the Jackson like found in Heathkits, last is (most) of
> the Nationals, although like I said, once in a while you find a
> National that feels really good.
>
______________________________________________________________
HBR mailing list
Home: http://mailman.qth.net/mailman/listinfo/hbr
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:HBR at mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
------------------------------------------------------------------------------
______________________________________________________________
HBR mailing list
Home: http://mailman.qth.net/mailman/listinfo/hbr
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:HBR at mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.qth.net/pipermail/hbr/attachments/20090219/b6d68fee/attachment.html
More information about the HBR
mailing list