I’ve got these cheap but good working wireless headphones branded Welltech 40699/FKH51a.
After several years of use the two AAA NiMH Rechargeable Batteries dies and I replace them with new ones. But hey, they will not recharge, what is wrong? Having a closer look at the headphones I discover that the battery cases have a protective device to prevent non-rechargeable batteries getting charged. This device is a simple contact spring that has to be grounded to the battery minus potential to enable recharging. So the solution was to remove some of the insulation material at the bottom of the batteries, and now the headphones again are recharging the batteries.
I have also done a small modification to the base unit. I have removed the small internal wire antenna and fed the signal to an external sma connector. This enables use of a better antenna which improves coverage when moving around. A simple antenna for the 850 or 900 Mhz GSM bands works good for this unit which operates on the 860 Mhz LPD band.
Here are some pix…