SX-636. Tuner mechanism came apart!!!

[Bassblaster]

So im in the process of re-lamping my SX-636. and somehow the tuner mechanism came apart and i have no idea how to reattach it. The string is fine and i know how its wound, just the black wheel and the bits in the pics came off.
How should i re attach it and still have the pointer at least half accurate?

 

[markthefixer]

rut roah.....

thing to remember - that tuning cap MECHANISM is pretty universal, so while the body containig the fins is different, the gearing is the same, so the procedures below apply.

<snip>
BE CAREFUL !!!!!!!!!!!!

DO NOT PULL THE PULLEY OFF THE SHAFT WITHOUT COUNTER PRESSURE ON THE SHAFT TO PREVENT IT FROM BEING PULLED OUT OF THE Variable capacitor FRAME - WHICH IS EASY TO DO. You will hate it, if it happens - the anti backlash spring and dual gear assembly makes it "interesting" to get back together. You WILL learn to curse like a pro. Best idea is to avoid that particular problem. Also the small gear is SOFTER than the Variable capacitor anti-backlash gears, so if it is wrong, it can destroy itself.

There is ONLY ONE correct position and mesh between the teeth. It's part of the swearing.

http://audiokarma.org/forums/index.php?threads/tuning-cord-installation.674271
Go through this thread CAREFULLY, LOOKING AT EVERY PICTURE BEFORE STARTING!!!!!

From another thread:
http://www.audiokarma.org/forums/showthread.php?p=7525512

Follow and READ ALL the links in this re-post!!!

One gear is very soft pot metal.

Don't do ANYTHING (to it) YET.

This is going to be like surgery: a lot of studying, discussing and planning, so that nothing goes wrong.

Might even help to practice on something less expensive and critical, first. The mechanisms are all the same across many many models and many many years, no fooling!!

In a nutshell - you will have to loosen an almost unreachable screw with a GOOD SHARP #1 Philips screwdriver, remove another similar screw, to get the small gear and the tuner side anti-backlash gear out of mesh, then explore the 1.5 rotation limits imposed upon the small gear.

The unreachable screws are shrouded by the anti-backlash gear, BUT there are positions where the screwdriver shaft will clear the gear or the spring on the gear.

Then you have to align the fins at one extreme of their rotation, and the small gear at it's extreme, get the anti backlash gears (two side by side) under tension and held INDEPENDENTLY of the small gear (you CAN'T DRIVE the anti backlash gear into position with the smaller AND SOFTER drive gear - you will DESTROY the small gear) and then mesh the tuner anti-backlash and small drive gear, then hold them together as they are rotated to clear the empty screw hole for replacement of the screw.

read this thread:
http://www.audiokarma.org/forums/showthread.php?t=495885
CAREFULLY!!

an excerpt:


I use a flat bladed jeweler's screwdriver that fits between the gear teeth to get them to align EXACTLY (NOT one tooth off) and hold them.

see how the teeth are aligned?

 

[zebulon1]

I use a tiny dab of grease (Lithium(?) is good, something lube'y, sticky) to keep the parts together as I insert them back into the gearbox trunnion.
Start with the VC all the way to the stop, one way or the other doesn't matter - your going to pull the gearing in and out several times before you figure out the sequence.

Here is a similar one from my donor pile:
Notice how I have it to one side/stop? Fully closed.

 

[markthefixer]

That little gear is SOFT!!!! Study the procedure and procedures carefully, and anyone CAN get it on the first try, since I made all the mistakes already, and eliminated them from the 10+ pictures of the procedure I posted. THAT post was INTENDED for situations like these.

Half a rotation on the meshing fins is as far as it goes (fully meshed to fully unmeshed), although the big black plastic wheel gets 1 and a half turns.

Mesh the tuning fins all the way together, IF you cannot get them back fully meshed, you have to get the little gear out of contact with the double gears connected by the spring so you can rotate it by hand. That is accomplished by loosening one of the little screws on the u shaped holder. The fins HAVE to be meshed to allow clearance for the screwdriver to get to one of the pins. PERHAPS you can get one screw loose enough to unmesh the gears. Somehow you got it on.

To get TO the screws the plastic wheel has to come off. It is also held on by a screw.

Now that it is loose, rotate the shaft that the plastic wheel was on clockwise until it stops.
Make SURE the tuning capacitor is now fully meshed.
edit- get the dual gear's teeth (with the spring between them) correctly aligned in parallel, hold them with a sharp screwdriver across the valley of both gears.
DON"T USE THE SMALL GEAR TO FORCE THE BIGGER GEAR'S TEETH INTO ALIGNMENT, IT'S TOO SOFT.
Push the two gears back in contact
replace the screw you removed - IF YOU have to rotate to clear the screw - either to clear the flat part of the gear for the screw closer to the PC board
or
the screw further from the pc board will need to be lined up with the spring on the double gear.

DO NOT let the gears slip while getting the screw clear for the screwdriver.

put the screw back in. check for 1.5 turns on the shaft that holds the plastic wheel, from full mesh to full unmesh.

put the plastic wheel back on, align the tuning dial slider on the face, in a fully meshed state that is the 88 MHz part of the dial.
tighten down the plastic wheel
rotate the front panel tuning knob to 108 MHz, checking that the cap unmeshes almost fully.


BTW, the double gear with the spring? That's an anti-backlash gear, there is always a few thousandths of play between the gears to keep them from binding, SO with the double gear, the alignment between the paired gear's teeth makes the teeth look a few thousandth wider, because they are forced slightly apart, (in opposite directions from each other: CW & CCW) by the spring. Then as the small gear and big paired gears are forced together into mesh, the paired gear's teeth are forced together against the pressure of the spring, exactly filling the gap and eliminating all the slop. The size of the gap grows or shrinks as the gears rotate (and wear with age), but the pressure of the spring keeps both gears pressing in opposite directions, filling any gaps with zero slop. By using more pressure on the spring than is needed to turn the tuning cap, the perfect fit of the teeth in the gap's changing size is not affected by any turning.

from wikipedia:
Gear trains where positioning is key but power transmission is light

The best example here is an analog radio tuning dial where one may make precise tuning movements both forwards and backwards. Specialized gear designs allow this. One of the more common designs splits the gear into two gears, each half the thickness of the original. One half of the gear is fixed to its shaft while the other half of the gear is allowed to turn on the shaft, but pre-loaded in rotation by small coil springs that rotate the free gear relative to the fixed gear. In this way, the spring tension rotates the free gear until all of the backlash in the system has been taken out; the teeth of the fixed gear press against one side of the teeth of the pinion while the teeth of the free gear press against the other side of the teeth on the pinion. Loads smaller than the force of the springs do not compress the springs and with no gaps between the teeth to be taken up, backlash is eliminated.

 

[zebulon1]

Well done Mark.
I saved it to my drive.

 

[Bassblaster]

Forgot about this thread, but i was able to get the gear bits back together but the dial pointer is almost 2mHz behind, but i dont really care. i rarely lisiten to FM anymore. maybe i might try to fix the pointer being off, but for now its fine

 

[markthefixer]

At least all of the parts are in a predictable place.

 

[Pio1980]

This isn't an unusual occurrence with this very commonly used tuner mechanism, the pulley shaft tends to want to come out of the gear frame. Dunno why.