I have an old Yamaha system, it’s not particularly good but it serves me. The problem is that it has a tuner that loses radio the stations I have programmed when the power goes out. This annoys me, it annoys me so much that I have decided to try to fix it.
When it’s connected to the network you can turn it on and off and it always keeps the stations but if there’s a power outage everything starts from zero.
This means that when you turn off the equipment something remains on and I thought I could open the equipment and see where there was voltage and put a battery that would maintain the voltage while it’s off, something like the batteries that computers have.
In the following photo you can see the equipment opened from above:
On the left is the power supply. This supply has two outputs, the one furthest to the left feeds the amplifier, the other the main board of the equipment. On the main board is the microcontroller that manages all the functions of the board, including saving the radio stations. The white and blue cable I put there.
In the following image you can see the two connectors:
In the center is the connector that powers the main board and on the right (half out) the one that powers the amplifier. So I started measuring voltages with the equipment off and the equipment on. The arrows mark two points where there was voltage, point 7 (on the right) was very stable both on and off, it gave 5.8V.
I was incredibly lucky to find the service manual, it explains everything (https://elektrotanya.com/yamaha_crx-e320_mcr-e320_nx-e700.pdf/download.html), the 5.8V voltage is the main voltage that powers the microprocessor, bingo!
The first idea was to put a battery like the ones computers have in parallel with output 7, although it’s much less than 5.8V but I thought it might be enough. You can see it in the following photo:
Well, it wasn’t enough, with this help it kept getting unprogrammed.
I decided to try with 5.8V, to do it I only had a battery holder for 4 AA batteries, a monstrosity, but I wanted to finish it and I mounted 4 batteries with an LM317 regulator. This ugly:
I adjusted the voltage just below the 5.8V voltage to prevent the batteries from discharging. And… it worked!
Assembled it looked like this:
When the equipment is plugged in the consumption from the batteries is 60 μA, when it’s unplugged 33 mA.
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