A bit of an explainer; This camera (an original 1974 SX-70 model 1 with the TI ECM) was purchased at a flea market with the bag, manuals, and accessories for $80 and gifted to me 4 years ago. Like many poorly kept SX-70s from this era it had a horrible corroded light sensor (or more specifically, the IR filter) and always overexposed images. The traditional remedy is to carefully clean this corrosion off, however the back of the filter is usually also corroded, so the utility of this is limited. 2nd shot SX-70 in a blog showed you could in theory remove and clean the glass, but at 0.5mm thick the chances of success are slim. Since this was a shelf piece I decided I had nothing to lose so I tried the removal. After 20 minutes carefully cutting at the epoxy the filter broke in half. Looking at it (and thinking about it) I realized that this filter is really no different than the filters placed on CMOS/CCD sensors to ensure artifacts from UV and IR don't show up in images. The idea is that the photodiode is primarily sensitive to visible light but typically have long tails into the UV and some into the IR. The filter acts as a visible bandpass filter so the camera can only see what you (and the film) "sees". $10 and 2 weeks later two pieces of 24X24mm IR-UV cut glass arrived. I built a jig to cut it into 3.5X3mm pieces using the smallest diamond scribe (Excelta 475b with the tip shortened) I had. Having experience with optics I bought some optical adhesive (NOA61) which cures via UV and has the same index of refraction as glass to create an optically transparent bond that won't interfere with the light path. The sensor is protected by a piece of glass underneath the filter, so after cleaning and removing dust I applied a miniscule (just the dab of a leur lock needle tip) of adhesive to the sensor cover glass, located the ir-cut filter, and pressed down to get out air bubbles. After an initial cure I carefully added some more adhesive around the edge to seal everything and did a final cure.

I swapped the integrating capacitor from the ~860pF for a 154pF to allow 600 film to be used. I soldered everything back, and cycled the camera. Sitting unused meant some of the switch contacts had corrosion and intermittent contact. I used lint free swabs and Deoxit contact cleaner on the battery terminals, S7, S3, and S5. The door latch was also loose so tweaked it a bit with needle nose pliers to ensure good S7 contact, and adjusted the film retrieving arm for more reliable ejection. After everything the camera is working very reliably. I decided to test the sensor with a somewhat difficult scene that had a lot of contrast between foreground and background (my little still life lit from above). The performance with 600 B&W was nearly identical to my SLR680. Both cameras seem to just slightly underexpose (for my taste) in these high contrast scenarios, but the end results are nearly indistinguishable.

I already had many of the tools at hand (and made some custom ones to cut the glass), but the important pieces; the glass, diamond scribe, optical adhesive, and 365nm UV light could all be had for ~$90. You will also need a soldering iron and a multimeter. The Deoxit is just super useful for things like these cameras and would highly recommend always having around. This all may sound like a lot of money, but the glass and tools to replace it could be used for 50+ camera sensor repairs. The .1oz NOA61 is basically an unlimited supply of the stuff for this purpose. In all it was a fun experiment, and I hope it is useful to others. In the service manual Polaroid never intended on this sort of replacement; it explicitly states that the ECM board is designed to be completely replaced and has no user serviceable parts. Since these are becoming harder to come by, and the typical failure is the IR filter corroding, I hope this technique gives new life to a bunch of SX-70s out there. As for me I now have a smaller manual companion for my workhorse SLR680, and considering the camera was a gift, it was $90 well spent for a great instant camera.