A fast bob & weave to what the odds predict will probably be my last successful repair before I return to my usual form of breaking stuff. This little diversion involves a Liitokala 9V battery, and a poor wiring decision, made way back in the mists of time, that came back to haunt me. The batteries in question were being used in my old detector, and as can be seen below, multiple batteries are needed to power this beastie.
Two are used, in series, to power most of the electronics, while the third, labelled 'S' in the above pic, powers the speaker. This somewhat complicated wiring setup needed to be routed appropriately, which is what the piece of vero-board, screwed to the battery compartment, aids in doing. This setup worked perfectly for decades. But if you look closely, you may notice that the three battery power leads are routed behind, and clamped in place, with the sliver of vero-board, courtesy of a few screws. Also visible are the multiple soldered vero-board connections, that keep everything civilised. What is not visible or obvious is that many of these soldered wires protrude through the holes, to the other side of the board. Although all were trimmed, one, coupled with the clamping effect of the board, and in the fullness of time, still managed to skewer one of the battery leads, forming a dead short.
But back to the battery. Below is the battery PCB, with a replacement transistor fitted (bottom right) - that didn't work! The original transistor had its identifier code burnt off of it (see pic), so I was working in the dark, and with limited options - all of my 'replacement parts' coming from scrap PCBs.
Fortunately, I got lucky and found a transistor (pic below) that worked!
More or less anyway. The On-resistance of the transistor seems to be a bit higher than the original, which results in more of a voltage drop under load (0.2 - 0.3V @ 200mA tested) but still better than a sharp stick to the eye. I also checked the Li-ion battery capacity, with & without the Boost circuitry;
Repaired Battery voltage 4.16V, no Boost - Capacity 1080mAh
Repaired Battery voltage 8.6V, Boosted - Capacity 350mAh
Interestingly, a capacity comparison between the 'Good' and the 'Repaired' battery, using mAh and Wh's, produced the following;
Good Battery Capacity 360mAh and 3.24Wh
Repaired Battery Capacity - 350mAh and 2.98Wh
Which are close enough to make it a job well done! Incidentally, the measured, (1080mAh) versus the stated (1100mAh, by Liitoala) are satisfyingly close, especially since they've had a fair bit of use. Of course, stating battery capacity in mAh's for Li-ion (boosted) 9V batteries is completely misleading, and is done purely to mislead.
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