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Old 24th Apr 2017, 6:09 pm   #1
Vintage Engr
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Join Date: Jan 2008
Location: Shrewsbury, Shropshire, UK.
Posts: 824
Default BT 290A Battery leakage.

I have a BT 290A linesman's phone which I have had for many years, which has worked faultlessly.
I did not realise that it contained a wired-in Lithium battery... The original handbook makes no mention of an internal battery at all.

I was rummaging through my 'small items of test equipment' cupboard when I discovered a puddle of very corrosive brown liquid on a lower shelf.

I quickly moved everything out, & cleaned up the shelf, but could not find the source. It was only next day that I needed to use the 290A phone, which was on a higher shelf, but I removed it from the cupboard that evening, and placed it on top of the exchange simulator ready for the next morning. There were no signs of leakage from the phone at that time. By morning, the source of the leakage became obvious, as there was now another large puddle on top of the the simulator.
The 290A's case is gasket sealed against ingress of water, but the case was now under pressure from the inside, so the leakage of electrolyte came out through one of the switches!
I have now removed the offending battery, and cleaned the pcb, but a fair amount of print track has vanished on the battery side of the board. The board is double-sided and some of the through-plated holes are no more.

Does anyone have a circuit diagram of the 290A please?
I'd rather like to get it working again.

Thanks,

David.
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