Making a Bluetooth adapter for a Car Phone from the 90's

[UselessPickles]

I found the source of the problem. The "speakerphone" IC (and a few other ICs) are not receiving any power supply at all. I traced the power supply track back to a small IC that I now assume is a 5V regulator, because the other pins on it are ground and constant 12V power (even when the car is turned off).

(everything is glossy due to conformal coating I sprayed on the board after cleaning and repairing)

The constant 12V input pin on this IC is one that was severely corroded by water damage previously, almost completely dissolved away. An electronics repair shop reconnected the small remaining stub of a pin with a solder bridge (giant blob). I suspect there was still some bad metal on that pin and the corrosion continued to spread and severed the repaired connection, or it just wasn't a good solder joint and it failed.

So good news: I'm pretty confident my new Bluetooth adapter design did not cause any problems.

Bad news: I'm not sure if I'll be able to save this.

I can't identify this IC. It has some faint markings on it. I think it may be "8606" or "8C06" or "8006". I could try replacing it with an arbitrary 5V regulator with the same footprint (3-Pin SOT-89?) and pinout that can handle ~12-14V input. Or maybe I could try removing the solder blob, scrape away a bit of the plastic case of the IC to expose more of the pin, clean it up, and repair the connection again with a piece of wire?

 

[rjenkinsgb]

One of these may be suitable? It looks like the tab on the right is 0V??

https://www.mouser.co.uk/ProductDetail/Texas-Instruments/LM2940IMPX-5.0-NOPB?qs=X1J7HmVL2ZELzMHXu8bpyQ%3D%3D

 

[UselessPickles]

Looks like Vin and Vout are swapped on that component. In my photo, bottom is Vin, top is Vout, middle and right side is Gnd.

I found this L78L05ACUTR, which looks promising. The only spec I'm uncertain about is the max output current of 100 mA.

Before I order anything, I'll take a measurement of the output voltage on my remaining functioning module to confirm it is 5V, and also take a better look at all the components that receive power supply from this voltage regulator to see if 100 mA max seems reasonable. But the output voltage confirmation will have to wait a while because my remaining functioning module is in my car, which is still covered up for winter storage.

 

[UselessPickles]

First, the good news: I have taken my car out of winter storage mode and tested my new design in the car with full hands-free integration, and everything seems to be working great! Its really nice to have the car phone installed exactly as originally intended without any additional external conversion devices/wires. It just works, looks all original, and can even be removed from the car and carried in portable mode as originally intended. No excuses and no compromises.

The less good news: I have now put together a second converted car phone with the new custom PCBs, but have encountered a few issues of differences in performance/quality between the two. I need to identify the cause of these differences and figure out how to reliably avoid them before I can consider building more of these for sale:

• One of the phones has more noise in the Bluetooth phone audio, similar to the noise I had with my breadboard prototype due to poor grounding (even the better phone has some of this noise that I was hoping would be eliminated by the custom PCBs with plenty of ground fill and via stitching).
  • By swapping parts between the two phones I have narrowed it down to primarily the Bluetooth module/daughterboard, but also the the custom cable that connects the Bluetooth daughterboard to the motherboard is contributing to some difference in audio noise.
  • I suspect the issue with the Bluetooth daughterboard may be inconsistent/poor solder joint quality on the ground pins of the BM62 bluetooth module. I did have issues hand-soldering these pins because the heat of the solder iron tip was quickly wicked away by the grounded shield on the BM62, resulting in ugly cold solder joints. I'm preparing to order a couple of these PCBs from JLCPCB with assembly of the side of the board containing the BM62 so I can test with good professional reflow soldered connections.
  • There was a bit of a learning curve when crimping terminals for my custom cable. I had a couple non-ideal crimps on the first cable I assembled that I thought I had fixed, but that cable causes a bit more audio noise than the second cable I assembled. I'll be ordering more terminals and rebuilding the first cable.
• The second phone I assembled initially had a rapid clicking sound in all audio (including audio produced by the MCU; not just Bluetooth audio). I discovered this was caused by the power supply output of the Dayton Audio battery module lightly contacting a grounded part of the transceiver case (stupid oversight in how the module was positioned without sufficient insulation).
  • Solving the shorting problem immediately cleaned up the audio clicking. But then there was still sometimes some audio clicking.
  • I then discovered that the piece of foam I used to secure the battery module in place is actually somewhat conductive! So it was also partially shorting out random parts of the battery module and causing power supply fluctuations. The piece of foam I used in my first phone was different and not conductive at all.
  • I have solved all the shorting issues, but now that battery module is not fully charging the batteries. So I think I damaged the battery module.
  • Another battery module is already ordered, and I'll have to modify it and more carefully insulate/install it.

I also have been unhappy with the li-ion batteries I'm using because they are a bit too long to fit well in the AA battery holders (55mm length, compared to standard AA size of 50mm length). They need to be forced in, and they distort the shape of the holders. I have now learned that this is because I bought "protected" batteries, which have an over-charge/discharge circuit added to the negative end of the battery (increasing its length). The best unprotected batteries I can find that are actually the same length as standard AA batteries have about 10% less capacity than the protected batteries I'm using. I also learned that it's easy to remove the protection circuit and re-shrink-wrap the batteries (I already did this). I'll be testing/comparing the 1000 mAh unprotected batteries with the modified-to-be-unprotected 1100 mAh batteries to decide whether it's worth the hassle to modify the protected batteries for that extra 100 mAh capacity.

If I can solve all of these issues reliably, and assuming I don't encounter any new issues over the next couple months, then I think I'll have enough confidence to make a few more of these to sell. The proceeds would help recoup some of the money I've put into this project, and fund the cost of further development. I really want to develop a replacement for the Dayton Audio battery module that I can incorporate directly into my custom PCB design, because modifying that battery module to connect to my PCB is quite tedious.