Getting Medieval with Bluetooth for a Car Diagnostics Interface

23 Sep 2022

I was recently given a circuit board for a car diagnostics tool (also perhaps known as a vehicle communication interface or VCI) to repair. It is used to attach to vehicle diagnostic connectors like OBD-II so that a PC can be used to interrogate the vehicle’s electronics.

The enclosure had long been lost by the mechanic, and the USB connector was broken. The mechanic had been using the bare PCB as-is, for over a year; it was waiting for a disaster to occur such as a short-circuit to a car chassis!

From the shape of the cable side plastic end-piece, presumably, the device looked something like this when new:

(Image source: Google Images)

The USB connector was an easy fix, but as I was about to start work on it, I noticed this:

Someone (presumably the mechanic) had felt the need to attach a big fat yellow wire onto the antenna : ) I couldn’t blame the mechanic, because the device has a flaw; there is a ground plane on the reverse side, covering the entire board! The module antenna has no chance to develop decent radiation capability. This isn’t the first time I’ve seen designs like this; unfortunately, I think there are a lot of firms where engineers do not get the opportunity to train up. This flaw should have been picked up in a design review.

To properly fix the problem, I would need the datasheet for the Bluetooth chip (Beken BK3513) however I couldn’t find any useful information on the Internet. It’s presumably quite an old chip, it supports Bluetooth 2.1. I could not tell if the chip expected a 50-ohm impedance for the antenna or not. If I knew, then I could remove the antenna matching components on the module, and attach a short thin coax to an external antenna.

It could also be possible to pull out a Vector Network Analyzer (VNA) and determine the impedance of the antenna and matching circuit so that it could be replaced with appropriate matching for an external antenna, however, I wasn’t going to spend that much time redesigning the antenna circuitry for a one-off repair. There is information on how to do such a thing in the FPC1500 VNA RoadTest report. and there is an impedance matching video here (the video concerns impedance matching for a filter, but the principle is the same for an antenna; more care is needed at 2.4 GHz than for the filter in the video).

One more possibility that I briefly considered was to strip off the ground plane on the other side of the board, however, I think it’s a 4-layer board, and there was no guarantee there were no signals or planes close to the Bluetooth module on the other layers.

I decided to do some ugly slicing on the module instead:

I used a small utility knife to hack off the antenna portion of the module. It needed several blades because the sharp edge blunts quickly on fiberglass PCB. Then, using a small flat screwdriver pushed under the antenna portion, I was able to snap off the board without damaging the soldered joints (but I fluxed and re-melted all the joints just in case. 8341-10ML Flux paste is great for this, I use it with a thin pointy nozzle).

There was about 1mm of antenna that I couldn’t hack away, it was too close to the rest of the module, so I later removed the copper from that portion with the knife as well, and then used a blob of solder to hold the antenna perpendicular, about 1mm off the antenna matching circuit. It’s not great, but I couldn’t think of what else to do. It is quite intricate working to tiny 2.4 GHz antenna tolerances in such situations.

Since the antenna was now physically weak, I used good old Polydoh (I use it all the time, it’s like 3D printer plastic but in small beads) melted with a hot air tool, and then pressed into position by hand, to make a couple of antenna supports.
Polydoh is not adhesive, so the supports were fixed using epoxy glue on the main PCB. This way, if I ever need to remove it, I can hack at the glue without damaging the antenna or the rest of the module.

Incidentally, after doing this fix, I googled and found comments that discuss a similar problem, I don’t know what device, but it’s possible it is referring to the exact same one.

The work is almost done. I’m not returning it to the mechanic until I’ve stuck it into an enclosure, and probably place a chassis-mount USB connector on it too. I’ve ordered a nice-looking 30x140x190 enclosure (currently only $3.50 in the US, but three times the price in the UK : ( and what looks like a fairly rugged USB Type B chassis-screwable connector .

There are photos of the entire board below, in case you're curious about what it looks like. Presumably, it’s just a microcontroller connected to discrete line drivers and receivers, and the connector on the end can be swapped out I think, to support OBD-II and other interfaces perhaps.