[sdiy] Analysis of Buchla 259 principal oscillator...

[Magnus Danielson]

From: Aaron Lanterman <lanterma at ece.gatech.edu>
Subject: [sdiy] Analysis of Buchla 259 principal oscillator...
Date: Tue, 14 Feb 2006 02:10:49 -0500 (EST)
Message-ID: <Pine.GSO.4.60.0602140152130.20374 at bigzilla.ece.gatech.edu>

> 
> ...or, a (failed) attempt therein. I seek the wisdom of the gurus.
> 
> IC22 is a 311 comparitor; it appears as though a couple of resistors form
> a "pullup resistor" effect to 5V. (15V through 10K to output of 
> comparator, and 4.99K from output of comparator to ground).
> 
> Yet, the notation shows a square wave with a "4.29 V" by it.
> 
> Question #1: I'm guessing there's stuff happening in the caparitor that 
> cause it to not pull up all the way to 5V? How do I find out what the 
> difference is? It is not something I can seem to find on the 
> datasheet.

See IC21? This is a normal TL082, and normally the input is at 0V. So, you
have 5k//24k and not 5k at the bottom of the divider. If you now use that you
see that you hit 4,39 V rather than 5 V. So that should explain itself pretty
well.

> but either way, that's hardly a triangle centered around zero! There's 
> just a line with a triangle on it, suggesting that's the output - I don't 
> see any "zeroing" circuitry elsewhere...

Details, details... what is a little bit of DC among friends?

> Question #2A: Is there some DC shifting circuitry for the triangle in the 
> 259 I'm missing? It's a typically Buchlaesque diagram, drawn in Buchlaese, 
> so that is one possibility.

Good point. Maybe it didn't make it to this schematic?

> Question #2B: Is there something daft in my analysis? What is it that I'm 
> missing?

A sane social-life? (You know you had that one comming, didn't you?)

> Oh, wait, there MUST be something daft in my analysis. Notice the 
> resistive divider of two 100K resistors, R140 and R141, which is feeding 
> 7.5 volts to + terminal on IC20, the integrating op amp - so vo, it looks 
> like, is supposed to be centerd around 7.5???

Yeap!

> Help! I've fallen in a Buchla design and can't get out!

Poor Aaron, I can't help you about the DC shift, something is fishy there, I
grant you that, but I think the square-output issue is solved.

> - Aaron
> 
> P.S. I thought the 20.7K might be 26.7 or something, as it's very smudgy - 
> but if you use that, you get, vtri = -2.8 for v- = 0, vtri = 3.1313 for v- 
> = 5, and vtri = 2.2285 for v- = 4.29, again not really centered)

You can't give up, can you? ;O)

Cheers,
Magnus - looked at the schematic more than once...