SX-70 8-Frame Counter Mod

The venerable SX-70 is in my opinion the coolest camera of all time. Modern Impossible / Polaroid film only contains 8 images per pack, as the film is thicker than the original formulation, and with the built in battery, cannot fit 10 shots.

The frame counter on all pre-Impossible cameras is indexed at 10 shots. The counters in these electromechanical cameras are integral to camera function, in addition to displaying the number of shots left. A series of cams control switches and interlocks in both box and folding cameras made by the original Polaroid Corporation.

A couple years ago I began experimenting with the counters on box type cameras, attempting to reshape the control cams on the counter wheels, so they would begin at 8, while still operating the camera correctly. Here is a demo of a box camera I modified the wheel on to begin at 8: https://www.reddit.com/r/Polaroid/comments/xjrm53/boxtype_frame_counter_mod/

This Instructable will be a guide on doing the same thing to the counter found in folding Polaroid cameras.

Here is a short video of a camera after this mod: https://www.youtube.com/shorts/p6NG1fanoWk

First and foremost you will need a very steady hand to complete this mod. The counter on folding cameras is about half the size of that on box cameras. We'll be reshaping cams that are tiny, somewhere around 3-4mm in size.

Physical supplies you'll need are:

A rotary tool, with as small and pointy of a grinding bit as you can find. I got the tool at Harbor Freight, and the bits at Walmart.

JB Weld

Masking tape

Soldering Iron, solder, and a sucker or copper braid

Tweezers

Very small screwdriver

Camera disassembly tools (pliers to peel leather, specialized screw bit)

I don't have pictures of opening the camera, but there are plenty of guides online.

In short, remove the front cover (rollers), remove the bottom leather, unscrew the four screws (store them carefully!) and remove the bottom cover.

I highly recommend looking at pages 4-55 to 4-58 of the Polaroid Repair Manual to see how to remove the film counter unit.

Desolder S6, S8, and S9 switches. Make sure to use proper heat. S8 and S9 share a contact in the center. Carefully remove the flex PCB, including the folded bit to the right of the counter unit where the motor IC lives.

On each side of the counter, remove the screw, and store it carefully. Carefully lift out the counter unit, toward the back of the camera (not down) by the S8/S9 contact area. Mind the indexing pins right next to the screw holes when pulling it out.

There are two shafts running through the counter, a plastic one that the return spring and counter wheel live on, and a metal one that the ratchets live on. The metal shaft is riveted in one side, and push fit into the other, while the plastic one is molded into the left half of the frame, and is melt-riveted to the right frame half.

Use the rotary tool to grind away this plastic rivet, but only so it is flush with the frame. It will need to be JB Welded back together later.

If you are able to without breaking things, gently wiggle the plastic shaft out of its hole, same for the metal one. If the metal one won't budge, try pushing it out with a small screwdriver, or heating the end of it with a soldering iron to soften the plastic.

Once you get the frame apart, set the frame halves, return spring, and reset and incrementing ratchets aside.

On the right frame half is a protruding piece of metal that sticks into the inner part of the counter wheel. When the reset ratchet is engaged, the return spring will rotate the counter until a stop on the inside of the wheel hits this protrusion. We need to make the stop longer so that the counter will reset to just before the 8 position, rather than just before the 10 position.

We'll add JB weld to this inner cavity clockwise of the existing stop. You can tell how far to add the epoxy by the ratchet teeth surrounding the wheel. One tooth = one number. We'll add enough that once ground to size, the counter will stop at just before 8.

Set the wheel in a safe spot to cure overnight.

Once cured, use the rotary tool to grind this stop flat, parallel to the wheel's shaft. Grind it flat perpendicular to the shaft as well, so it is level with the original stop.

Use trial and error, test fitting it on the plastic shaft of the left frame half, and putting the right half back together. You want the 9 to be directly in line with the center contact of S8/S9.

S8 controls dark slide ejection. When it is open, along with S7 (front cover/roller switch) and S6 (camera unfolded) being closed, the dark slide will eject. This means the cam on the left side of the counter needs to be extended counterclockwise, so that S8 will be open when the counter is on 9.

This step isn't strictly necessary, as if S8 is always closed, as it would be if you don't modify this cam, you can simply use the shutter button to eject the dark slide. However, if the camera ever stops mid-cycle, it will need to be wound to completion with the gear train by hand. Dark slide ejection using S8 will force the camera to complete a cycle and reset itself.

Reshaping this cam is more difficult than extending the stop, because it actually has to be shaped, rather than simply being in the right spot. Further, this side of the wheel is where the reset spring lives, so you need to be very careful not to get JB Weld all over, or you'll have a bear of a time grinding it out, and getting the spring to sit nicely inside.

As before, use the ratchet teeth to count back two frames. This is approximately where the end of the cam that actuates S8 will be. Use masking tape and a tiny screwdriver to make a boundary where the JB Weld will end up. Use the screwdriver to securely adhere the tape to the inside of the wheel. You're trying to essentially make a pocket that will be filled up.

Once the tape is correctly placed and sealed against the inside of the wheel, fill this pocket with JB weld. Fill it above the existing cam, but be sure to not let it spill over into the ratchet teeth. Let it sit overnight.

Another option is also to do this in stages, filling in levels from bottom to top, until there is enough material that you can grind a new cam profile.

Once there is enough new material, begin grinding it away. I started on the top, grinding it flat until the JB Weld was level with the old cam. Next I did the inner profile, rounding it out until it looked similar to the preexisting cam. When working on the outer profile, model the new left edge on the existing left edge of the old cam - you should be able to see it if you flattened the top already.

As in the last step, use trial and error to test if you need to grind more. Put the wheel on its shaft, assemble the frame, and rotate it against the stop on the right frame half. S8 is the lower contacts, you want them to just touch, but not flex significantly when the cam closes them.

slide the ratchets onto the metal shaft as shown in the pictures. Slide the reset spring and the wheel onto the plastic shaft. Wind the counter forward (so it counts down) 1-2 revolutions and have it indexed so 9 or 10 are aligned with the center contact of S8/S9. Hold the spring end near the hole in the left frame half. The incrementing ratchet has a protrusion on the back that the reset spring hooks on, and will poke into this hole.

I held the wheel with my left middle finger, the spring with my index finger, and the right frame half (and ratchets) with my right hand.

Carefully slide the metal shaft into its hole slightly, and hook the spring with the incrementing ratchet before sliding/rotating the right frame half into place. Before you can push the halves together, the stop on the right frame half must be inserted inside the wheel.

Push the halves together. The metal shaft will be all that is holding the frame together, but the plastic shaft should be in its hole, and not loose. Position the metal shaft so it is inserted the correct distance into its hole. You can tell this by whether the frame is warped, and the halves are parallel.

At this point I recommend test fitting the counter in the camera if possible. See the next step for that.

Before final installation, JB Weld the plastic shaft into place and let it cure overnight. You don't want a ton of epoxy there, as that is where the motor chip lives, and you don't want interference.

In the camera, the ratchet that actually increments the wheel is linked to the mirror reset mechanism. At the bottom of the area where the counter is, there is a metal finger that links to the counter's reset ratchet. The counter needs to fit between these two actuators for it to seat and work properly.

I recommend consulting the official repair manual again.

The wide protrusion on the reset ratchet will slide inside the finger at the bottom of the camera. I recommend having a film pack inserted, as this pushes the finger down, and makes positioning the counter easier.

The body's incrementing ratchet is spring loaded to pull downward, and needs to be pulled up while the counter is inserted. The repair guide specifies using a dental pick to hook the ratchet, which can be slid out once the counter is nearly in place.

Both these actuators need to be fit against the counter more or less at the same time. I held the incrementor up with a small screwdriver, while I placed the counter in the camera, making sure to get the reset ratchet properly located in the finger, before letting the body's incrementing ratchet go from the screwdriver.

Make sure to have the counter placed on the indexing pins in the body, to align the screw holes.

In this whole process you'll need to push the flex PCB to the right, but be careful not to overstress it.

Screw the counter in to the body, and carefully fold the motor IC back into its holder.

Use the tweezers to carefully push the flex PCB back over the S6, S8, and S9 contacts, and resolder them.

If you haven't already, sand off or paint over numbers 9 and 10 on the counter.

Install the front cover.

Check proper operation with a film pack. The counter should reset to just before the 8, formerly where 9 was. When you close the front cover, the camera should run the dark slide ejection cycle, and the counter should stop at 8.

Cycle the camera 8 times or ratchet the counter down by hand, making sure nothing binds, and the camera operates properly.

Remove the front cover, and screw the bottom cover back on. Reinstall the front cover, and place a new bottom leather on the camera.

And that's it! Congrats!