What kind of diode is the CR26 in the schematics? There's also a code on its body: M 02 00 48 but couldn't find anything. Is it some zener being reverse polarized? Thanks for your help!

 

6 6v can also be writen 6v6.

i dont know what the 6v6 is.
but i installed a 6v2 today and it's a little glass zener diode.

dont know if that helps

 

I can't read the text clearly, but I interpret that as a 6.8V zener.

This might be of interest, check this table out.

http://forum.allaboutcircuits.com/attachment.php?attachmentid=7081&d=1234137616

 

I agree, it's got to be a zener. It sets the reference voltage for the transistor constant current source that charges the batteries, that current being (Vz-Vbe)/R23 (or R23 in parallel with R24 if switched to "FAST").

 

Sorry for poor quality, however it's 6.8 v right. I need to change this component because it's shorted. Thanks for confirmation.
The voltage R22+CR26 should give the voltage on C21, C21=R22+CR26, right?

I can't read the text clearly, but I interpret that as a 6.8V zener.

This might be of interest, check this table out.

http://forum.allaboutcircuits.com/attachment.php?attachmentid=7081&d=1234137616

the link outputs a table about Resitor Standards?!

 

It looks like a 6V8, and indeed the voltages add up. There's a strange ground arrangement in that circuit, where the 0V reference is taken between the two batteries, so that means the voltages written on the base and emitter look very odd.

Ideally you want to know the voltage across C21 so you can work out the dissipation of the zener by finding the current through R22, and hence the current through zener CR26, and hence the zener power dissipation. It's not clear from the circuit what the voltage across C21 is, except that it's (hopefully) less than 65 V, so it's probably best just to measure it; it won't make much difference if you do this while the zener is shorted. Ignore the transistor base current if you like, and the answer for the zener dissipation will err on the side of caution.

Zeners tend to fail short-circuit if they're over-dissipated, or receive a bit of a transient current surge. It doesn't hurt to pick one that has a power rating a fair bit higher than needed on average.

 

The voltage R22+CR26 should give the voltage on C21, C21=R22+CR26, right?

Yes. But voltage across C21 can readily be worked out from the schematic.

Its about 46V. (Cr25+battery+Q2 base_voltage+zener)