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

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?

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

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.

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.

Sounds good!

Note that if any one of the cells had a partial short, it may take a good few hours on charge for the power board balance circuit to equalise them - and with protected cells, it may not be able to do that at all, as the highest charge one may disconnect itself and confuse things.

I'd also advise soldering any miniature crimp terminals before folding the strain relief tabs over the insulation. The results with anything other than the manufacturers specific tooling can vary tremendously & generic tools often distort the contact, as well as not giving an ideal crimp.

rjenkinsgb said:

Note that if any one of the cells had a partial short, it may take a good few hours on charge for the power board balance circuit to equalise them - and with protected cells, it may not be able to do that at all, as the highest charge one may disconnect itself and confuse things.

Click to expand...

One of the 3 batteries test at a substantially lower voltage than the other two (3.4V vs 4.1V), so they are definitely out of balance. But I've switched to a different (new) set of unprotected batteries now anyway. With the new unprotected batteries and a new battery charging module, I'm able to achieve a full charge. And battery life is pretty good despite supposedly having about 10% less capacity (I got 19.5 hours of on/standby time and 6.25 hours of simulated moderate/heavy use). It's not worth continuing to modify the higher capacity protected batteries.

I do, however, still have a problem with battery charging. While monitoring current draw to know when charging is complete, I noticed some intermittent and large fluctuations in current draw. I think I've narrowed it down to unreliable contact between the battery holders and the batteries (touching the batteries and applying small amounts of pressure causes fluctuations in current draw). Sometimes the current draw will never drop down to a level that signifies that charging is complete unless I wiggle the batteries a few times.

The battery holders I'm using are pretty flimsy/flexible. I think I need to upgrade to something more sturdy with better contacts, which will require some slight changes to PCB layout for different footprints. I think I'm going to order some of these battery holders and hack them onto my current PCB to test them.

rjenkinsgb said:

I'd also advise soldering any miniature crimp terminals before folding the strain relief tabs over the insulation. The results with anything other than the manufacturers specific tooling can vary tremendously & generic tools often distort the contact, as well as not giving an ideal crimp.

Click to expand...

Are you suggesting that I ONLY solder the wire to terminal and not crimp it onto the wire at all (to avoid potential distorting the contact)? Only crimp the strain relief?

I use sidecutters or fine nose pliers to fold the contact area tabs firmly over the wire, then solder them. Then again with small pliers to tighten the strain relief section after soldering.

For the battery holders - try a trace of deoxit or just fine oil on the contacts?

Heya UselessPickles - I am working on a similar project, not nearly as extensive as yours, but I am wanting to use just the handset and a microphone and speaker through the corded cable. Ideally this would work with any car phone with an RJ connection, although I suspect there are differences of the wiring schematics. I am going to go though this thread again and see if you had gotten into the headset wiring, but do you have any tips or advice?

Basically, I would want to be able to plug a car phone headset into a landline and it hear dial tone and microphone works...

SwissBliss One of the earliest things I documented in this thread was reverse-engineering of the wired connection to the handset. I can't say for sure that this is 100% true for ALL car phone handsets, but I suspect they all follow a generally similar design as mine in that it requires some sort of serial data communication to control the handset and enable the speaker and microphone. Although you may get lucky and discover that your particular handset has an always-active speaker and microphone when power is supplied to the handset.

You will definitely not be able to connect a car phone handset to a landline and use it as a landline phone. Not only are the connectors incompatible (RJ45 vs RJ11), but the car phone handset is not a landline phone. The wiring is completely different.

I successfully repaired my spare Hands-Free Controller Unit with a new 5V regulator (L78L05ACUTR)!



Only slightly crooked . The pad and trace leading to the 12V input to the regulator was quite damaged/missing, so I bypassed that trace with a wire.