[ARC5] [Boatanchors] BC-348 Backlash Suggestions

Richard Knoppow 1oldlens1 at ix.netcom.com
Wed Apr 24 15:24:05 EDT 2013


      I can't tell from the photo how the worm shaft is 
fastened at the knob end.  It looks as though there is a 
spring washer between the gear and the chassis, is this the 
case?  If so there must be something for it to work against. 
I would expect something like a C clip at the knob end. 
Perhaps if its missing it would account for the backlash.
     BTW, is this backlash or play, they are different. 
Backlash is when you turn a control to a certain position 
and it moves back the other way slightly. Play is where 
there is a gap between movement in one direction and moving 
the other way. Where there is play but no backlash the 
control may be difficult to adjust precisely but does not 
move after adjusting.  Most of the so-called anti-backlash 
arrangements of gears is really to eliminate play.
     As far as tensioning the anti-backlash (might as well 
use that term) gears, perhaps the bearing is just stiff. In 
general springs don't get weaker simply from being 
compressed or extended unless they are stretched so far as 
to exceed the elastic limit of the material.  What does 
weaken springs is use; they suffer from metal fatigue and 
loose their springiness.  I would investigate the bearing 
where the anti-backlash gear turns on the driven gear to see 
if its free. That may be difficult where one can't easily 
remove or disengage the gear.  Its possible some penetrating 
oil may help without having to dissemble the gear.



--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com

----- Original Message ----- 
From: "David Stinson" <arc5 at ix.netcom.com>
To: "ARC-5 List" <arc5 at mailman.qth.net>; 
<milsurplus at mailman.qth.net>; <boatanchors at theporch.com>; 
<boatanchors at mailman.qth.net>
Sent: Wednesday, April 24, 2013 7:23 AM
Subject: [Boatanchors] BC-348 Backlash Suggestions


> Been reviving a BC-348 for a friend.   It had a 
> "doofuss-try" I hadn't seen before- someone rotated the 
> bandswitches 180 degrees.  Didn't know you could even do 
> that.
>
> After this and lots of other work, it's playing well
> and is one of the best sounding 348s I've ever done. Even 
> the crystal filter works well.
> One common problem remains- tuning backlash.
>
> The three major contributors to backlash in a 348 are the 
> tuning capacitor gear, the worm wheel and
> the worm shaft.  Here is a photo:
>
> http://home.netcom.com/~arc5/348gears.jpg
>
> First, make sure all the gears are clean and lubed.
> Put some drops of GOOD penetrating oil on the edges of 
> each gear and let them sit.
> Do this until the two halves of the gears move freely. 
> You can test this with a small
> screwdriver inserted between two edge teeth.
> Mesh the teeth and they should spring back.
>
> The gear on the end of the cap is not too tough
> and that's good, since it's usually the big baddie.
> Just takes patience and practice.  Close the cap all the 
> way and make sure this coincides with the end
> stop on the tuning knob.  Loosen the two screws on
> the gear end, but don't remove them.  You want to be able 
> to disengage the gear from the pinion, but just barely.
> This is best done at a 45-degree angle toward the front.
> I use a pair of hemostats to engage both sides of the gear 
> and well-compress the spring.  Then, making sure the 
> tuning knob is still on the stop and the capacitor is 
> still fully meshed,  re-engage the gear and tighten the 
> cap back down.
>
> The parts where I could use suggestions.
> The worm gear wheel also needs to be retensioned, but the 
> assembly is pinned together and the ends of the pins 
> "boogered-up" so you can't drive them out
> without damaging the whole assembly.
>
> I can't disengage the worm gear wheel to retension it like 
> the cap gear, nor can I remove the worm gear shaft, as it 
> is also pinned.
> In the past, I've used hemostats to removed one of the 
> worm wheel springs, stretch it out, try to put it back, 
> cursed at the 27th try, then watched the spring 
> "sproing-ing-ing" off into oblivion,
> found another spring in the junkbox, stretched it too much 
> so that, once I did get it on,
> it stuck -out the sides of the gear etc. etc.
>
> And then there's the worm gear shaft- this problem is 
> nasty and is also common on later production 348s.   The 
> worm shaft slops "in and out" towards the back and front.
> Common symptom is "wibble-wobble" while tuning
> that is most pronouced on the higher bands.
> The shaft has a wimpy leaf spring under the worm
> and a felt pad under the tuning knob that help some,
> but it's no cure.
> I cannot remove the worm to add a washer or bearings
> under it.  Only someone with an advanced machine shop 
> should attempt it since you will absolutely wreck the 
> thing if you try to hammer and whack the pins out.
> So I'm looking for suggestions on how to address
> the slop in these two gears.
>
> How about a couple of snap-rings under the worm shaft?
>
> TNX ES 73 DE Dave AB5S
>



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