These Philips wireless headphones - model SBC HC8545 specifically - were really nice - when they worked. I bought them maybe 2006 or 2007, and I remember using them quite a lot when our kid was small. Unfortunately they started acting up more and more - specifically headphones did not pick up then wireless signal until maybe after many on-off cycles - until later they just would no longer pick up the signal at all. Bummer, because they offered a lot of freedom and, unlike many headphones, were actually comfortable to wear even with glasses.
They have been sitting on my shelf for a long time now, and since I had some unexpected free time last week I decided to see what's inside.
Headphones had rechargeable batteries (two, AAA size, 1.2v 550 mAh NiMH) and they were charged when on the base station.
Base has two screw on bottom, and after that top part comes loose - interesting design choice.
RF can has PCB in it, with seemingly just passives and few (likely) transistors in it. U5 at the right is EM78P156 - apparently simple microprocessor. I guess it controls charging, FM transmitter, status LEDs and maybe thing or other thigs. Wires you see at bottom go to charging contacts on top part.
I was looking for chip datecodes until I saw huge date stamp marking on it, placing manufacturing date to november 2006.
Unfortunately for me almost all built-to-price RF circuits are effectively black magic; I can work out how, say, spectrum analyzer RF front end works, as you can clearly see all chips and radio path involved, but these things where oscillators are built with just few passives and maybe a transistor are something I've never managed to grasp.
I thought that is might be possible that the problem might be transmitter or receiver frequency slowly crawling off the spec so receiver could no longer lock on to it.
Headphones opened up. It appears that other cup has mainly power and charging related parts (including batteries), and other has RF parts, and there is nothing tunable on RF side. Copper wire on lower right functions as antenna here. Under the RF cover is RS832U chip; I could not find any information about it. I don't think those blue ones (brown board) are related to RF stuff; brown board is connected to RF board with two pairs of wires, other being power and other audio.
This thing has been sitting on my shelf, unused, for close to ten years. But since I now had it open (previously I had opened only headphone part, finding no adjustments and given up) I thought I might as well try to adjust it, assuming it were the RF being out of tune.
My plan was: I use function generator to enter 1 kHz sine wave to one audio channel, and spectrum analyzer (with very narrow span and resolution bandwidth setting) to find RF carrier at specified frequency (helpfully frequencies - selectable, in case there are multiple devices operating at same area - are listed at bottom of base station) and try adjusting those yellow resistors on transmitter.
But guess what. When I try this, frequency seems to be spot on (granted, considering that I do not have exact 10MHz reference for analyzer, this isn't exact science) and headphones lock on to signal immediately.
This was with fresh non-rechargeable AAAs though. I switched back to rechargeable NiMHs (new ones, original ones were totally dead) and no more audio. Hmm. Curious.
At this point I switched to adjustable desk power supply. Headphones would lock to signal at around 2.5v voltage level, and lose it below that. That's on a bit high side (considering NiMHs are 1.2v nominal), so I now focused to those blue adjustable pots on power board.
After a bit of experimentation I figured out that one very likely adjusts undervoltage sensitivity; I guess that under a specific voltage it cuts power completely from RF board. Other adjusts LED behavior, at which voltage it starts to flash.
So, new plan: Set my desktop power supply to around 2.3v and start adjusting those two pots so that audio and LED drop out just below that voltage (1.15v being around 10% left on NiMH according to voltage curves, leaving a bit overhead). This turned out to be less than exact science, though, so eventually I ended up just tuning them to "works, plus some" level.
So, now I again have working headphones. If they last for year or so I consider this a big win.
A bit warning if you plan to do this yourself though: Although adjustment takes only a phillips-head screwdriver, those wires inside headphones are very fragile and are torn off the board very easily. Be very very careful with them if you choose to open your headphones up, as soldering them back can be challenging without proper tools.
