MV1C battery recharge disable sanity check...

[Kujako]

I've been trying to add a coin cell battery holder to my Omega consolized MV1C. First I tried a small PCB with a battery holder and a diode, but I measured ~5v at the battery coin cell holder. So I figured the diode may be bad. I then disabled (in theory) the recharge by removing R36, D5 and D6 from the underside of the backup area and switched to a simple coin cell holder. But I still get ~5v at the battery holder (well closer to 4v due to voltage dropping)... so what am I missing here?

Not finding any shorts or continuity where I wouldn't expect it.

 

[Kujako]

Tried replacing D4 to no effect. But with it removed I read zero volts at the battery, as I would expect. So I'm getting voltage back through the diode.... but I'm struggling to remember if that's expected or not. I'm not remembering how a schottky diode behaves in this situation. With a zener I recall that voltage drops in relation to current draw. So if this were a zener I could understand getting close to 5v through the diode with no draw on the circuit due to no battery.

 

[Kujako]

Current operating theory is that the D4 Schottky diode leaks voltage but at such low current that it's not an issue. But I would love someone to confirm before I close this up...

 

[ack]

I see the same thing on a MV1C I have. Additionally when putting the battery in, the voltage changes to the battery voltage instead of the leaked.

 

[Kujako]

I see the same thing on a MV1C I have. Additionally when putting the battery in, the voltage changes to the battery voltage instead of the leaked.

Thanks for confirming. As an additional check I tried with a cheaper less sensitive multimeter and I only measure around 20mVm.

(because who doesn't have a bunch of old multimeters around?)

 

[Big_P Big_p]

I see the same thing on a MV1C I have. Additionally when putting the battery in, the voltage changes to the battery voltage instead of the leaked.

Great thanks for confirming that @ack. After a night of brain overload that was what I thought was happening.