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CEM3340 Rev.G zener voltage
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author CEM3340 Rev.G zener voltage
electrotech
Hi,
Just bought a couple of these to resurrect my old E&MM Spectrum synth.
I built a test VCO on strip-board to check the original IC's and discovered that one was faulty and this also allowed me to check the new 3340's. (I didn't use the Spectrum as it's currently in bits and may have wiring errors...)
I was surprised to discover that the negative zener voltage on the new IC's was 7.32/7.4V against my originals at 6.7V.
I've not been able to locate a datasheet for the Rev.G IC's so don't know if the zener voltage has been increased but according to the original datasheet it should be 6.5V +/- 10%.
I was under the impression that the new chips used the original design ?
I'd be interested to know what voltage others have on their new CEM3340 Rev.G IC's.
Apart from this they appear to work fine.

Cheers,
Andy
guest
thats almost within 10%! i know i saw different transistor Vbe's (by a decent margin) when i was comparing the 2, but ill have to check my notebook for the zener voltage, if i recorded it.
Synthiq
electrotech wrote:
I was under the impression that the new chips used the original design ?

I also got the impression they use the original design data but I think the zener voltage depends more on the bipolar process used. After all these years the original process may not be in use anymore so they had to settle for something similar but not exactly the same so some parameters can easily be slightly off.
electrotech
Thanks for the replies.
I hope that you find the zener voltage in your notebook guest.
According to my calculations the voltage is nearly 14% high in my 3340's ... which means that both of these new CEM3340 I've got are 'not to specification' going by the original datasheet.
Found some interesting info. in this thread :-https://www.muffwiggler.com/forum/viewtopic.php?t=162704
which states "They have been manufactured to the original specs ", although I gather that they've had to use a more modern manufacturing process, as Synthiq has stated above and this might explain the differences.
Given this, I still think it's very odd that Curtis/OnChip Systems haven't produced a new datasheet for this chip...

Also, I would still like to know if anybody else who has used this Rev.G chip has checked/can check the voltage at pin 3. (assuming it's fed via a current limiting resistor from a -12V/-15V supply).
If so, I would be grateful if they could post in this thread what voltage they measured.

Cheers,
Andy
guest
i dug up my notebook, but i didnt write it down. but i do remember some voltage being higher. i did a comparison of the new and old here:

https://www.muffwiggler.com/forum/viewtopic.php?t=169687

upshot is that the new one works better.
electrotech
Thanks for digging up your notebook guest and also for that link - very interesting !
I'm beginning to think that the internal zener diode is best ignored given the variations with it and a -5.0V reference like the one you used is the better bet.
guest
i found that using a +/-5V ref helped a lot. not sure if would bother with the other trimming stuff i was experimenting with. the variation is mostly an offset, so it can be fixed while playing by twisting the tune knob. i may have also reduced Rs (base resistor) to 1k and decreased the CV input resistors to compensate.
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