I'm hoping someone might be able to steer me to fixing a lead acid battery charger that isn't working, please. (While I have a very basic grounding in electronics, most of it dates from 2 or more decades ago, and it's very rusty!)
I have a portable amplifier (Bretford Entertaina Plus) that can operate on an integral 12V lead acid battery or on the mains via an external power supply. The basic problem is that the battery simply isn't charging. The amplifier works on battery when the battery is charged elsewhere, and it works on the mains. It would be really good to get the charging circuitry working again. (I bought the amplifier in this state and I do have some reservations as to its pedigree!) I have not managed to find a circuit diagram for it anywhere.
I have replaced the battery, and so I don't believe that is at fault. Right now, the battery is showing just under 5V because it has been running the amplifier for a while. With the battery disconnected, the charging circuitry is delivering just under 15V on the battery leads. However, when I measure the charging current with the battery connected, it's a few mA which clearly isn't going to charge the battery!
As far as I am aware, the circuitry includes overcharge protection, which would make sense given that the amplifier can be run from the mains.
Without the circuit diagram, I'm shooting in the dark to fix it, I know, but I feel it's worth a try. Looking around the relevant part of the (fairly small) circuit board, I can identify the area responsible for the power, because of the thicker tracks around there, and because of where the battery leads are connected. In that area, there are 3 devices that suggest themselves to me - two voltage regulators (KA7809A and KA7812) and one MOSFET (SFP9540). There is no visible damage or scorching to the board or any of the components.
Would it be a reasonable first assumption that the MOSFET will be responsible for the overcharge protection, and that it has probably died?
Any suggestions would be most gratefully accepted.
T
I have a portable amplifier (Bretford Entertaina Plus) that can operate on an integral 12V lead acid battery or on the mains via an external power supply. The basic problem is that the battery simply isn't charging. The amplifier works on battery when the battery is charged elsewhere, and it works on the mains. It would be really good to get the charging circuitry working again. (I bought the amplifier in this state and I do have some reservations as to its pedigree!) I have not managed to find a circuit diagram for it anywhere.
I have replaced the battery, and so I don't believe that is at fault. Right now, the battery is showing just under 5V because it has been running the amplifier for a while. With the battery disconnected, the charging circuitry is delivering just under 15V on the battery leads. However, when I measure the charging current with the battery connected, it's a few mA which clearly isn't going to charge the battery!
As far as I am aware, the circuitry includes overcharge protection, which would make sense given that the amplifier can be run from the mains.
Without the circuit diagram, I'm shooting in the dark to fix it, I know, but I feel it's worth a try. Looking around the relevant part of the (fairly small) circuit board, I can identify the area responsible for the power, because of the thicker tracks around there, and because of where the battery leads are connected. In that area, there are 3 devices that suggest themselves to me - two voltage regulators (KA7809A and KA7812) and one MOSFET (SFP9540). There is no visible damage or scorching to the board or any of the components.
Would it be a reasonable first assumption that the MOSFET will be responsible for the overcharge protection, and that it has probably died?
Any suggestions would be most gratefully accepted.
T