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 Post subject: SX-88 dial light drive
PostPosted: Nov Sun 08, 2020 5:09 am 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1695
Location: Champaign IL 61822
Tonight I tried using my Hallicrafters SX-88. The radio tuned what appeared to be very strangely.

That's because the mechanism that moves the dial lamp shutter is broken. The radio and
actual tuning is working fine. It appears that the cogged drive belt is kaput.

The manual has instructions for replacing it. But where do I get a new one?
Has anyone gotten the specs? Without them even after finding a source,
it would sit quite a while taken apart after examining the old one.

The obvious source for cogged drive belts is pic-design.com, which I have used a lot for small
drive parts, including belts. In 55 years this is the first cogged belt that has ever died.

Doug McDonald


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Sun 08, 2020 5:34 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1965
Have you contacted them?


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Sun 08, 2020 5:38 am 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1695
Location: Champaign IL 61822
No, of course not! I only discovered this 5 minutes before posting.

In any case ... they make a huge array of belts, including custom.
They'll have something even if I have to pay big(ger) bucks for new sprockets.

Of course, this IS an SX-88, with what that implies for $$$$.

Doug


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Sun 08, 2020 2:04 pm 
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Joined: Oct Fri 02, 2009 2:41 am
Posts: 141
Location: Chapel Hill NC
Ask on the Hallicrafters list and elsewhere - Years ago, Jim Jorgenson got a batch of belts made special for the SX-88 when he couldn't find anything that fit. Jim is an SK now, but maybe somebody has still got an extra.
https://mailman.qth.net/pipermail/halli ... 26219.html
Also see this info about why it is an oddball pitch -
https://mailman.qth.net/pipermail/halli ... 26221.html

Also W3JN reported this -
"As I've previously related, I found a belt with the right tooth pitch and length but twice as wide at a surplus store. I just split the belt down the middle with a razor and got two good belts out of it. Don't discount an otherwise correct belt that's too wide if there's a standard one in a belt catalog."

Good luck!

_________________
Nick K4NYW
www.navy-radio.com


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Sun 08, 2020 3:09 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 7694
People have already printed cog type belts using a 3D printer and that is probably the most practical way now to get a replacement belt.

I also have a SX-88 and a nephew with several advanced 3D printers and have thinking about seeing if he could make a replacement for me.

Rodger WQ9E


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Sun 08, 2020 7:44 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1695
Location: Champaign IL 61822
rsingl wrote:
People have already printed cog type belts using a 3D printer and that is probably the most practical way now to get a replacement belt.

I also have a SX-88 and a nephew with several advanced 3D printers and have thinking about seeing if he could make a replacement for me.

Rodger WQ9E


That would be interesting! I'd be interested.

I got mine out to find that the rubber cogs had come loose from the cloth backing and totally
crumbled in two places, but the backing was intact so I had to cut it out. Fixing would be possible
but futile as it would just crumble again.

I measured it carefully and get a pitch of the cogs, when stretched out straight,
of 0.2433 inch, 112 cogs, and a calculated length of 27.25 inches (if laid out straight).
I was able to measure it directly and got exactly 27.25 inches.

Nobody seems to sell belts with a 1/4 inch pitch, or even 6 mm.

Give the amount of friction (tiny), and the speed its turned, I suspect that a
plain 1/8 inch O-ring would work. Or, if wrapped once all around each pully ....
dial cord or string (which could be threaded in and tied in a knot. )


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Sun 08, 2020 9:53 pm 
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Posts: 7694
I wondered if a regular O ring type belt would work given the extremely low torque. I remember when I acquired my SX-88 that the belt could be an issue and that 4 tooth per inch belts were long obsolete.

What is the depth of the teeth? I will check with my nephew to see what he thinks about making a couple of these during his slow season (he produces various circuit training kits for several university EE student labs).

Rodger WQ9E


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Sun 08, 2020 10:55 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1695
Location: Champaign IL 61822
The overall thickness is .114 inch. The height of the bumps is 0.053 The base of the bumps is
0.125 inches long. The top of the bumps is 0.060 inches long, The bumps are clearly more
trapezoidal than curvy.

The idea with the dial cord did not work. Rather it worked for several changes and then
started slipping, eventually becoming hopeless.

One could try getting a 3 mm or 0.080 inch pitch belt and cutting off
1/2 or 2/3 of the bumps, but I didn't find any the right length.


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Tue 10, 2020 6:23 am 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 202
Location: Ashland Oregon
Just a thought, but do any of the manufactures making drive components for robotic RC toys have anything that might work ?


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Tue 10, 2020 7:07 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1695
Location: Champaign IL 61822
michael w wrote:
Just a thought, but do any of the manufactures making drive components for robotic RC toys have anything that might work ?


No. Nobody does ... these thing got restandardized years ago and the needed pitch is
no more.

However ... the needed pitch is 0.250 inch, or 6.25 mm. 3 mm belts exist in the proper length.
If one were to remove every other cog (that length has an even number) it would clearly
fit nicely the driver pulley. It wraps enough around the dial pullies that it would
only "almost" fit, but then, the torwue there is nil. I found at least one source (in England)
that appears to be able to make a 3mm or 1/8 inch wide one for a low price.
If that turns out to be true, any machine shop should be able to mill off every other cog.
A place in Indiana stocks ones 6mm or 9mm wide, and they could of course
be cut narrower.

Attachment:
3mmCogProfile.png
3mmCogProfile.png [ 29.15 KiB | Viewed 695 times ]


With patience and a bit of jigging, one could do it by hand. I'm considering trying.

But .... would it have enough torque?

Also, regular automotive stlye v-belts exist in the right length that are only 10mm
wide at the top. A machine shop could mill the exact correct profile. Being rubber it might take
a diamond mill cutter to do it. (The softer and gooier something is, the sharper a tool need to
be to get a proper cut.) The problem with that is that the strengthening cloth might be too coarse a
weave.


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Wed 11, 2020 1:02 am 
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Joined: Mar Sun 17, 2019 7:32 pm
Posts: 18
A few years ago when I owned my SX-88 (run 3 serial # 357) I tried many of the suggestions given in this forum to replace the drive belt, including kevlar string, wire, and removing every other tooth with a sharp blade from a 3mm pitch belt. Nothing proved to be completely satisfactory. The 3 mm pitch belts that I found at the time had teeth too shallow to get a good grip and eventually slipped. I just now quickly looked at the timing belts that Stock Drive Products has and they might now have something that would work better in their large selection. A great source for belts, pulleys, gears, shafts and other neat stuff. I like the 3D printing idea.

https://www.sdp-si.com/products/details ... detail.php

I found the following quote from an unknown source in my notes:

"I just measured an original SX-88 bandswitch belt. It is possible the length has changed slightly over the years unless the tooth pitch was not an even number. Carefully measuring along the outside surface of the belt I get 27.5" The belt width is 1/8" (.125) and the tooth count is 112. It looks like about 4 teeth per inch."

Les


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Wed 11, 2020 1:28 am 
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Joined: Mar Sun 17, 2019 7:32 pm
Posts: 18
A slip of the finger in my previous post. My SX-88 was run 2 serial #357, I don't believe there ever was a run 3.

Les


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Wed 11, 2020 3:45 am 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1695
Location: Champaign IL 61822
caladonia wrote:
A few years ago when I owned my SX-88 (run 3 serial # 357) I tried many of the suggestions given in this forum to replace the drive belt, including kevlar string, wire, and removing every other tooth with a sharp blade from a 3mm pitch belt. Nothing proved to be completely satisfactory. The 3 mm pitch belts that I found at the time had teeth too shallow to get a good grip and eventually slipped. I

Les


Did you try installing a third idler to make the belt wrap around
more of the circumference of the driving pulley? I

If it were not for the current <forbidden subject> affair it would be trivial.
I've ordered up 6 3mm belts (minimum order). I could order three proper
28 tooth pulleys. The drive pulley could simply be replaced. I could go into
our machine shop and bore them out to the proper (large) diameter. Then slice
them in half-moon shapes and clamp them on.

OR ... since it seems that only the drive pully is the problem, I could just replace it
and try with a proper pulley. The diameter (1.053 versus 1 inch) is immaterial, its the
14 (or 28) cogs that matter. This would make the cogs fit perfectly, albeit at half number,
in the drive pulley. I just thought of that! I'll order up a pulley.


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Wed 11, 2020 8:26 pm 
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Joined: Mar Sun 17, 2019 7:32 pm
Posts: 18
I did try replacing the drive pulley with one with a 3 mm pitch, it was better but still had slippage because of the shallow ribs on the belt. Good luck with your efforts, hope you can find the right combination.

Les


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Wed 11, 2020 8:44 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1695
Location: Champaign IL 61822
When you had the 3mm drive pulley, was it slipping on the driv e one or on
the ones at the dials? Did you have the belt wrap around more of the
drive pulley?


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Thu 12, 2020 2:30 am 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1695
Location: Champaign IL 61822
It appears that the problem is actually that the two dial sprockets are
a different pitch from the belt and the driver sprocket!

They do appear a bit worn.

The only cure for this requires a machine shop to create a proper mount for
new sprockets. Unless I can find a commercial shop around here that can do it
I'll have to wait until <forbidden topic> is over and I can get to our shop.
The two machinists I know that ran suitable ones retired just before I did.
This would not be reversible ... but unmatching sprockets is terminal.

In the meantime ... an electrical solution. There is room on the driver shaft
for another switch section. I could remove the slotted rotary pieces and replace them
with solid ones with an LED (or for purists, lamp) for each band, switched
by the new switch. This could be fabricated with hand tools. This of course would be reversible.

For the gaudy, one could use different colors for each band.

Doug McDonald


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Thu 12, 2020 9:46 am 
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Joined: Mar Tue 10, 2020 5:11 am
Posts: 445
Location: Mission Viejo, CA
I'm just tossing a wild idea out there for perusing:

Way back when (but well after the SX-88 came out), WRL produced the Galaxy III and Galaxy V transceivers. I dunno about the III model... but the Galaxy V had a bandswitch arrangement I thought interesting: the final amplifier compartment bandswitch shaft stuck downward through the chassis vertically to provide shortest connection between the switch wafer on top of the chassis, and the 10/15m pi-net coils, while the main bandswitch, a few inches away, ran horizontally through the rig just like you'd expect. They ganged them together using a pair of plain brass hubs, joined with a strap of brass. This strap was bolted onto one of the hubs, essentially creating a "belt". Since it did not have to turn too many degrees (I suppose perhaps just 210 degrees, to include three 10m segments of 500 kc for 28.0 - 29.5 Mc), the screws did not get in the way of the rotating "belt". Thus, to turn the final bandswitch, the "belt" twisted 90 degrees from horizontal for the main bandswitch, to vertically for the final bandswitch (or vice versa, depending upon your viewpoint), in between the two hubs (which were mounted on each switch shaft), and provided solid connection between the two bandswitches. A simple idea but I've never seen it used in a homebrew rig. I'll admit that this technique apparently did increase the torque required to turn the whole bandswitch, however; you actually had to grunt sometimes while changing bands :D

If plain hubs were made for the SX-88 mechanism, perhaps a similar arrangement using a solid metal "belt" might provide a workable alternative?

SteveH


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Thu 12, 2020 4:09 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 7694
There are a lot of modification possibilities but I am hopeful that we can come up with a belt that replaces the original.

A non-destructive mod that might work is a variation on how Ten Tec set up the slide rule dial indicators on some of their vintage gear, an elastic cord (a spring could also be used) biases the dial indicator to one end and it is pulled over as the VFO is tuned. The Hallicrafters dial shutters could be similarly biased to a resting point through a cord and spring arrangement and as the band switch is turned a dial cord rapped around each of the shutter pulleys and the band switch pulley pulls the shutters through their range. Hallicrafters used a similar system with the sliding dial lights on some of their earlier receivers using a spring and cord combination.

But I hate going Rube Goldberg on this scarce receiver.

Rodger WQ9E


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Thu 12, 2020 6:20 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1965
Your mention of Rube Goldberg gave me an idea. Could you cobble up a setup involving a cat and catnip pulling a string when you turn the knob? You could put in a little ramp where a ball rolls down and wakens the cat, who then goes for the catnip and pulls a string that moves the dial lamps. A little far fetched but that's Rube for you.


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 Post subject: Re: SX-88 dial light drive
PostPosted: Nov Fri 13, 2020 12:33 am 
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Joined: Mar Sun 17, 2019 7:32 pm
Posts: 18
dtvmcdonald wrote:

"When you had the 3mm drive pulley, was it slipping on the drive one or on
the ones at the dials? Did you have the belt wrap around more of the
drive pulley?"

I believe that the slippage was on the drive pully, I had it wrapped around as much as the tensioning posts would allow, but more would have worked better. I would try the belts you have ordered with the stock drive pulley, removing the ribs by hand is a bit tedious, but can be done with a sharp hobby knife blade.

Les


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