MV-6F...new battery discharging quickly!!??..help please!!.

I need your help guys...thanks!!
Well i just got around to replacing all the back-up battery's in my mvs machines..3 cab's all replaced with Ni-MH 3.6v 80mAh...all great tidy jobs bar one!!.
One of the cabs is losing back-up data FAST!!!..so i checked it out again...removed the mobo, & checked wiring & fitting / soldered joints etc..all perfect..re-build & still the same...now i thought it was a duff battery...so replaced it for another....still same, so i got my multimeter & started to check voltages...battery was disscharged!!...switch on the mvs machine & it starts to charge the battery back up to say 3.7v..& holds...when you switch off pwr & check with the meter..the volts drop really quickly!!...(somethings zapping it down)...so the charge circuit is ok...& the battery is too.
The other machines have been switched off for days & battery check holds a constant 3.739 volts...no change at all.
Is there a storage capacitor OR one-way resistor that could be at fault on this circuit??...what could be running the battery down so quickly??..the wiring to the battery is perfect & no shorts at all...this is baffling me!!.
Thanks so much for any insight into this problem..cheers!!!

Strange, I would connect an ameter in series with the battery to monitor the current draw.
In the charge circuit there is a diode and 470 ohm resistor in series, if the diode was short it may cause this, as the battery tries to supply the 5 volt rail whilst the psu is not powered up.

Hmm thanks for your input ...i'm competant in using multimeters & soldering...but i'm more mechanicaly minded than electronically!! ....testing wiring i'm fine...electronics is not my forte...sorry.
Close to this circuit..there is a 16v 47uf capacitor...it looked a little green/corroded at the bottom...tried to clean it...it does not look like a leak...
Also to the left of that is a row of diodes..one had slight corrosion..i have cleaned & checked the joint...
I have dissconected all the wiring to the mobo....& it still plumetts down to 1.8v..in less than 1 min!!!..i think the fault is on the + side..but how can it bring the charge of the battery down like that?..

NEO GOLD said:

Hmm thanks for your input ...i'm competant in using multimeters & soldering...but i'm more mechanicaly minded than electronically!! ....testing wiring i'm fine...electronics is not my forte...sorry.
Close to this circuit..there is a 16v 47uf capacitor...it looked a little green/corroded at the bottom...tried to clean it...it does not look like a leak...
Also to the left of that is a row of diodes..one had slight corrosion..i have cleaned & checked the joint...
I have dissconected all the wiring to the mobo....& it still plumetts down to 1.8v..in less than 1 min!!!..i think the fault is on the + side..but how can it bring the charge of the battery down like that?..

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Clean the board, check the diodes in that area and remove any dodgy looking elecktros. See what happens

I can't think of anything

But does it make a difference the way the battery is put in..the direction it faces as maybe it's backwards..

chris1 said:

But does it make a difference the way the battery is put in..the direction it faces as maybe it's backwards..

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Heh...cheers chris...the polarity is ok,...i have the pcb out again & dissmantled to the base...triple checked my work & condition of diodes & capacitors...
So the top six-slot mobo is removed & put away in a safe place.
Right...when i connect the battery & check volts with the meter...it falls quite quickly...like a dead short of live to earth?.....right down to say 1.8v if left a little longer....right now when i dissconnect the + wire to the batt the volts actually creap back up to around 3.2v...if i powered up the unit it would start to charge the batt up to a max of 3.7v...this is weird.
I cannot see it being a prob with the chip-sets...i cannot fathom how a capacitor or diode could even cause the volts to drop...i need a deeper insight.

NEO GOLD said:

i cannot fathom how a capacitor or diode could even cause the volts to drop...i need a deeper insight.

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If the diode was faulty the battery will attempt to supply the 5 volts vcc on all the ICs via the 470 ohm resistor when the board is not powered up.

There are two diodes connected directly to the + supply side on the pcb...shall i replace these?..

NEO GOLD said:

There are two diodes connected directly to the + supply side on the pcb...shall i replace these?..

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I have a couple of six slots (somewhere), will confirm the component locations

No, one is in the recharging circuit across 5v and battery+ (through 470 resistor, toddy refers to this) while the other (a zener?) is in the supplying circuit across battery+ and the backup ram.

MKL said:

No, one is in the recharging circuit across 5v and battery+ (through 470 resistor, toddy refers to this) while the other (a zener?) is in the supplying circuit across battery+ and the backup ram.

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OK just located the hardware. There are no real circuit references on these boards, looking at the board component side with the battery on the top and edge connect on the RHS, just to the bottom LHS of the battery there is 470 ohm resistor and just below it is a 1S1588 small signal diode for the trickle charge circuit (maybe use a common 1N4148) No zener as far as I can see.

I really appreciate the help here guys...as i'm checking the pcb...the charge circuit is working & powering up the batt to 3.7v...if i hit the switch off the volts count down..they should hold..