Rx7 Pioneer Stereo Restoration and Modification

[BasementEngineering]

Good morning,

this is my first post on this forum. It looks like a great resource and I am looking forward to learning. I am writing to tell you about a project I have been working on for the past couple years and could use some guidance on. I am still a novice with electronics, but have been working towards trying to restore a factory stereo for my old Rx7. I have a project thread on the Rx7club forum that I have been working on for quite a while. https://www.rx7club.com/2nd-generation-specific-1986-1992-17/s5-radio-repair-restoration-having-issues-1148850/

To address the first question, "Why would you bother when there are new radios? Just buy one and install it." I have been working on this car for many years, it is a slow restoration, I am restoring it to drive for my own enjoyment. If you are familiar with Rx7s, you will know that they are a labor of love, albeit one-sided. I am trying to restore all the functions of the car including the creature comforts. I have tried using modern head units, but they don't sound correct. There are additional amplifiers in different locations depending on whether you bought a coupe or convertible, etc. From the progress I have made, which I will describe further down, the factory head unit sounds great. I have just one portion that I am unable to get working.

What I have done is described in the forum thread linked above, but I can recap here to save you having to read the whole thread
- Collected and digitized all the manuals for the different permutations where this sound system was used. Model of the head unit is a Pioneer DEH-K4141ZM. These were only used in the S5 second generation Rx7s, although some components were used in other vehicles of the time.
- Recapped the amplifiers and main head unit (some were leaking or falling apart)
- Replaced drive belts
- Cleaned 30 years of goo out of the unit
- Replaced burnt out bulbs
- Converted After Effect Unit (head-unit amplifier) to coupe configuration. I was only able to salvage radios out of convertibles which have a different amp configuration. I have a problem with the button working correctly, but as long as you don't push that button, it sounds great. Question on that later.
- Restored radio w/ power antenna and tape function. Attempted to get CD player working and that is where progress stopped.
- Attempted to calibrate CD player according to manual using oscilloscope (could use some guidance there)
- Could not set tracking
- Upon inspection, found that the lens carriage suspension was broken. From what I have found and understand, this was a first generation design put out by Sony, where you have 4 wires that not only carry power to the coils that position the lens, but also work as a parallel mechanism to keep the lens in alignment. I put it under my microscope and found that these suspension "wires" are actually made of stamped brass, with just a couple thousandths thickness. These wires had corroded and broken. I tried to solder them back together in a blind-squirrel-can-sometimes-find-a-nut attempt, but it didn't work. These lens units are no longer available for purchase. I have tried purchasing them from multiple websites and have had my money refunded every time. I have also tried finding units that have these lenses in them for scavenging, but the only place I can find them was on ebay for insane amounts of money with no guarantee for an old lens pickup condition.
- I would like to add some sort of Bluetooth connectivity, or mp3 functionality to the radio and have a cheapo bluetooth module that has stereo out and mic in to try to brainstorm with. It works with 5-12v but is pretty dumb without any input other than the mic audio line, and what it gets from my phone.

Here is where I am at with the project.

Restoring the CD functionality would require retrofitting a modern laser pickup unit. I don't know how I would get it hooked up and communicating correctly, or positioned correctly. I have a 3d printer that I could model a new carriage on, but I don't know how the precise the radial positioning would need to be (assuming it wouldn't work)

The other avenue that I would prefer trying would be to try to turn the CD unit into a drive emulator that has Bluetooth/mp3 capabilities, but can use the cd controls to navigate. The reasons I think this could be viable are:
- If I remove the CD mechanism, there is plenty of space to put this circuitry project. There isn't really any free space to put the small BT module that I have inside the head unit, even if I were to splice it into the cassette wiring like most restomods do. It really is the best location I have found.
- The CD unit has access to 3.3v and 12v, with an additional 5v regulator available at the discretion of the main processor in the stereo controls.
- I would like to preserve as much of the original configuration as I can without having to hack up the unit. Some electronics whiz at Pioneer designed things to work this way, so I would like to maintain as much integrity of the original as I can.

What I don't know (the list that keeps growing)
- Can you spoof the unit into thinking a CD is in and running?
- Is it better to insert digital audio into the original DAC and have it put out to the speakers, or just use a modern audio output and have it send the signal to the mix amp?
- How do I use my oscilloscope to decipher commands between the main CPU and the CD unit processor?
- What functionality should I expect? Looking for just basic bluetooth stuff like pause, play, skip, go back, answer/end call.
- Is there suitable solutions with say, the Arduino platform, that would be viable to accomplish some of this?
- What should my next steps be?

This is a lot for a first post, apologies to the moderators. Thank you for having this forum available. I don't expect to finish this project quickly, but I would like to complete it within the next year. Most of my efforts have just been directed towards research, documentation, and restoration. This is the point where I can try to update and improve not only the radio, but my own skills. I was inspired by the videogame retromodding community for a lot of this idea and give a lot of credit to the videos put out by Voultar on youtube for improving a lot of my skills.

Thank you for reading, let me know what you think