ASM1 LFO influance?

[Magnus Danielson]

From: e <edgarfov at zahav.net.il>
Subject: Re: ASM1 LFO influance? 
Date: Sun, 20 Aug 2000 01:12:21 +0300

> hi all
> 
> i read some re:'s  about the problem iv presented.
> iv been offered to put somewhere a reference part 
> which i dont know realy where and what  is it to place it.
> 
> another offer was to stuff a large cap over the lower leg of the divider.
> (where is that?)
> which of them is the best??

Both these are relating to the voltage divider used in the VCO reset curcuit to
give the voltage where the ramp should be given an reset pulse. This divider
is going from +15V to GND through a 20k (R19 in online schematics) and a 10k
(R20) resistor. The LM311 comparator uses the voltage supplied as the reference
level. The 100 pF capacitor is the DC blocker for the sync signal which is then
laid on top of the DC signal, so that the reference is dropped for synking.

Anyway, what Rene proposed was just to hook at capacitor (say 10 uF) right
across the lower resistor (R20). If the +15V is jumpy from the LFO, then this
could be a cure... but! this way you would also kill the sync input. If you
don't mind doing that, well, there you are. This approach can be used to be a
quick tester, if it doesn't help you a single bit, the bellow strategies will
not help you either.

An alternative approach would be to replace the 20k resistor (R19) with two
10k resistors and then have the capacitor be hooked from between them and down
to ground. This would not kill your sync signal.

What I proposed was to use some sort of voltage reference chip, this is really
like overdoing it, but if you want to do that, then take a 10V reference chip
(REF-10 for instance), hook it to the power lines and then use the output to
feed the R19 instead of having the +15V powerline feeding R19. This would
require R19 to be of 10k instead of 20k in order not to change the voltage one
is after.

Would these directions be helpfull for you?

> my asm dirfs... and its too bugy to be ignored.
> 
> btw
> another fact that cause the vco to drift is when i tweak the filter fc
> position
> iv noticed that ther is a small range that also cause  the vco to drift..
> 
> i feel my boat is leaking from everywhere .. 

Um, and you have big caps and a strong enougth powersupply?

> tnx and sorry for the small saint of despair..

Well, I would be kind of worried too if it happend to me.

Cheers,
Magnus