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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (New Jan 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

I have been looking at the disastrous results of adding external backup
battery connection to a tiny2, as the small rare soldered in Li battery had died.
The external batteries outside the metal case had been wired from the anode of low voltage drop D5 to JPB 2 battery 1k earth jumper link.
_ _ _ _ _ _ _ _ _ _ _ _ _
D5 | |
┌─┤<├────────┐+3.6V External

│ _│_ Replacement
├─┤<├─oVCC ▄ Li 2x AA in
│ D4 │ battery Holder
│ 1 o |
│ JPB 2 o─┐- - - - - - - - ┘
│ 3 o │
──┴┬───────────┴─)──┬───oVBB to Ram Memory
│+ │ └───oBACKUP to IC U4
1u === 1k
┴─────────────┴───0V

As you see from the circuit there is very little other than the 1uF protecting the static sensitive memory IC from external electrical shock (static & HF). The Li battery current limiting 1k earth resistor left in the external battery system only making matters worse!

THE DAMAGE
In the end the TNC packed up with a blown up memory 62256 IC & a Z80 S10/0 IC Which handles the data bus to the memory & Z80 CPU. This was found by swapping ICs to another TINY2, luckily all in sockets. The logic IC U4 a 78HC132 took the Zapp OK!

MY SOLUTION
After replacing the ICs, I unsoldered the Li battery & wired the battery connections to 2x AAA Alkaline cells (soldered to batts with special solder or file ends) all tapped up (parcel tape) to provide insulation & some battery leakage protection to the PCB & double side taped the bundle to the PCB where the 82C88 IC hole space is.
┌──────────────────────────────────┐
│12V (~~~~~~~~) DD SW │
│ ( 2x AAA ) 45 [Li] │
│RIG ~~~~~~~~ │
│DIN ┌───┐ ┌───┐┌───┐┌───┐ L│
│sw │ │ │ ││EP ││RAM│ │
│ │Z80│ │Z80││ROM││MEM│ E│
│ │S10│ │CPU││ ││ │ │
│2 │ │ │ │└───┘└───┘ D│
│3 └───┘ └───┘ │
│2 S│
└──────────────────────────────────┘
I expect the 2 AAAs should last as long as the original Li?


I hope this tale stops anyone else from zapping their TNC.


P.S. don't forget to backup your TNC EPROM if you can, as EPROMS only have a 7 year G'tee image lifetime!


Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP



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