Here’s a demonstration of what my voice sounds like on the other end of a call from my vintage car phone. This is with the improved hands-free microphone audio quality after I replaced a bunch of capacitors in the hands-free controller unit. Yes, it was even worse before. This might just be the audio quality of a hands-free car phone system from the early 90’s.
Reviving this thread because I’m working on a major improvement to my car phone Bluetooth adapter. I’m redesigning it so that it no longer requires the original car phone transceiver to provide power management. My circuit will now be completely self-sufficient with its own li-ion battery supply, a direct connection to the vehicle power supply and ignition connector, and manage its own power on/off behavior.
And best of all, I’m designing it to fit INSIDE of the original car phone transceiver, replacing the original circuit board, but using the same types of connectors positioned exactly in the right place. The end result will be a completely original-looking car phone with no externally visible signs of modification, no external “plug-n-play” modules.
Here’s a 3D render of the main circuit board:
I printed it out on card stock paper to confirm that everything will line up properly in the original transceiver case:
A separate small circuit board will contain the Bluetooth chip and a few supporting components. This needs to be mounted outside of the metal transceiver case, but still inside of a plastic cover on the transceiver. The metal case would block the Bluetooth signals.
And I recently mocked up a portion of the new circuit design on a breadboard so that I could test all the new power management stuff, including writing new microcontroller code to deal with detecting approximate battery level, when the power button is pressed, when external power is connected, when the ignition is turned on/off, etc., and decide if/when to power on/off.
In that last photo, you can see the first version of my Bluetooth “daughterboard” (bottom center of photo), an off-the-shelf Li-Ion battery module that I customized a bit (right), and the external power supply connection with a switch to simulate ignition on/off (bottom right).
I’m now quite confident that I can pull this off. I just have a few more details to test/refine about the main circuit board before I send the designs out to be printed. I hope to have my first attempt completed in about 4-6 weeks.
If you want all the technical details, you can follow my progress here: Making a Bluetooth adapter for a Car Phone from the 90's | Page 9 | Electronics Forum (Circuits, Projects and Microcontrollers)
Amazing work buddy. Looking to see the future on this one.
I think the rest of us are waiting to get our own boards for purchase so we can do the same thing to our cars. 
If I’m successful and can refine the design to the point that it is practical to do so, I will consider selling conversion kits. I’m having trouble finding some components of specific shapes/sizes that will allow for a simple “drop-in” replacement of the the phone’s original circuit boards and connectors, so it will likely require some minor grinding of a few internal parts to make clearance. You’ll also have to source your own DiamondTel Model 92 telephone.
I’ll also be publishing the circuit and PCB designs, microcontroller source code, etc., free for personal use, so anyone could hypothetically replicate my adapter for just the cost of parts and PCB fabrication, plus time and effort to assemble everything and install software/configuration on the microcontroller and Bluetooth chip.
I’ve made some good progress. I’ve solved the problem of how I will mount the Bluetooth module in the plastic “cover” for the transceiver and connect it to the “motherboard” inside of the metal transceiver case.
Here’s the bottom side of the plastic cover that holds the the original rechargeable NiCad battery pack and battery charging circuitry (under the black metal plate next to the battery). Theres an 8-pin connector that connects to the main transceiver board as you clip the cover into place. It’s a nice self-aligning connector that has been obsolete for many many years, so it’s impossible to buy more of these connectors:
Except I just lied to you a bit. There is no battery or battery charging circuitry in there.
I gutted the useless 30-year-old NiCad battery pack so I could hide my Bluetooth module in there (securely sandwiched between two pieces of foam), de-pinned the original connector, and made a custom wiring harness for the Bluetooth module with the re-used original connector. The connector housing is no longer available, but the wire terminals it uses ARE still available!
Other than gutting the battery pack, no other non-reversible modification was done to the rest of the plastic cover, so it’s a pretty clean conversion.
For the mating socket on the main transceiver circuit board, I have no choice but to de-solder it from the original car phone circuit board so I can re-use it on my custom PCB. In total, I am removing/reusing 3 components from the original car phone electronics:
The 8-pin socket from the main transceiver board, the 8-pin connector from the battery charging circuit in the plastic cover, and the coax connector for the antenna. The antenna connector will not be used in my circuit, but I need to re-use it just so I can physically mount the original portable antenna for a complete original appearance.
When I complete the main circuit board that mounts in the metal transceiver case, I will have a converted and fully functioning vintage car phone that looks 100% original from the outside, even if you remove the plastic cover 
I’ve made a lot of progress on programming new behavior for managing power on/off functionality and monitoring battery voltage. Now that I’m in full control of power on/off behavior, I was able to re-implement original functionality where if you are in a call in your car when you turn your car off, the phone will remain on until the call ends
I’ve also been revising my circuit design to reduce power consumption. Batteries are expected to provide about 5 hours of active “talk time”, or about 18 hours of “standby time” (on, but not in a call). When turned “off”, I got current draw down to 0.05mA, which would take over 2.5 years to drain the 1100mAh batteries in a perfect world (but that 1100mAh probably isn’t “fully usable” in reality, batteries self-discharge over time too, and the battery charging module I’m using probably draws a small amount of current to monitor battery voltage too).
I’m hoping to get the main PCB produced in the next month or so. Fingers crossed that I didn’t make any stupid mistakes in my circuit schematic or PCB layout that prevent it from working 
It’s alive!!!
Here’s my new custom PCB installed in the original car phone transceiver case. Everything fits perfectly, all the external connectors are perfectly aligned, and the circuit itself seems to be working as intended. I still need to do a lot more testing before I can declare it a complete success, but it’s looking good so far. Sound quality seems to be better than my previous external plug-n-play module prototype.
From the outside, it is practically indistinguishable from an original unmodified car phone. I’ll get a side-by-side comparison photo later.
Here’s some side-by-side comparisons of an original car phone and one with my new Bluetooth adapter installed.
The main circuit board in the transceiver
Left: original
Right: modified
Transceiver with the lid installed
Left: original
Right: modified
Connections for external power supply, handset cord, and external microphone (for speakerphone)
Top: modified
Bottom: original
Inside the “transportable cover”
Left: original (battery charging circuit)
Right: modified (Bluetooth module)
My Bluetooth module hidden inside of the emptied out shell of an original battery pack
Left: original
Right: modified
Transportable cover fully assembled
Left: original
Right: modified
Amazing work hats off to you
Here’s some video evidence of my new design working, and disassembly.
Sooooo…whats it gonna take ($, etc) to get you to do this to our model 92 phones? I have a NOS phone that I’d love to convert before putting it in the car. Your progress on this is amazing!
How did you get a NOS phone? I’ve only seen one before on eBay, and I bought it .
It was complete with the “transportable kit”, and the universal vehicle install kit. But I still had to find the vehicle-specific hands-free integration components, which was very difficult.
I still have a few issues I want to try improving, then I’ll consider making a small batch. It won’t be cheap, because it’s still a prototype design that requires a lot of modification/assembly labor. I haven’t really worked on this project since my last post, and I don’t plan to work on it until winter. Next spring is probably the absolute earliest I might have something available, and it would probably be a conversion service rather than a kit.
Don’t wait for a conversion before installing it the car. Assuming that you have the vehicle install kit, the transceiver has a quick-release mounting plate, so you could easily take it out at any time for the conversion.
I found mine on ebay as well and bought it. The VR4 previously had one, but the handpiece was missing, so we have been looking for one. Now we have it.
