[sdiy] Cheetah MS800 replacing NiCd Varta Mempac

Richie Burnett rburnett at richieburnett.co.uk
Fri Jun 4 17:53:52 CEST 2021


Hi Tom,

I've read about this suggestion of fitting a CR2032 battery instead, and I 
know that some of the synths that came a bit later like Casio and Korg used 
these batteries instead for memory backup duty.  The thing that concerns me 
about this is that it is quite a departure from how the original Cheetah was 
designed to work.  The battery voltage is lower, and the charging needs to 
be disabled.  It is also not clear how the user would go about changing this 
battery in the future without risking loosing data.  I guess they would need 
to bridge the CR2032 battery holder with another external voltage source 
whilst taking out the old battery and inserting the new one?  This sounds 
like something a technician would have to perform in order to guarantee no 
loss of data!

-Richie,


-----Original Message----- 
From: Tom Wiltshire
Sent: Thursday, June 3, 2021 4:25 PM
To: rburnett at richieburnett.co.uk Burnett
Cc: synth-diy mailing list
Subject: Re: [sdiy] Cheetah MS800 replacing NiCd Varta Mempac

On the Polysixes I’ve worked on, I always replaced the NiCd with a coincell 
battery holder. The diode that provides charging just gets removed.

Then you can just stick a new watch battery in it every five/ten/whatever 
years.

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================



> On 3 Jun 2021, at 15:29, Richie Burnett <rburnett at richieburnett.co.uk> 
> wrote:
>
> Hi all,
>
> I'm currently servicing a Cheetah MS800 digital wavetable synth and want 
> to change the old NiCd Varta Mempac to a brand new NiMH Varta Mempac 
> before putting it up for sale.
>
> The old blue NiCd battery hasn't started to leak yet, and is still holding 
> it's charge but I know that old NiCd Mempacs are regarded as a ticking 
> time-bomb in equipment of this age (>.<)  I wouldn't feel comfortable 
> selling it without addressing this issue first.
>
> The existing 100mA 3.6V NiCd trickle charges from the 5V supply rail 
> through a 1k resistor when the unit is powered up.  Should I be looking at 
> changing this trickle charge current when changing the battery type to the 
> 150mA 3.6V NiMH replacement?
>
> I did a search online and found very little solid information about this. 
> Everything from "The NiMH Mempacs are a drop in replacement" to "you need 
> to decrease the trickle charge current because NiMH is less tolerant of 
> being overcharged"  The trickle charge current already seems low to me.
>
> I would be interested to hear your thoughts!
>
> -Richie,
>
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