Vintage Synth Repair

On Thursday 15 June 2006 09:21 pm, easterleggs wrote:
> Thanks for the replies.  I understand, in theory anyway, the idea of a
> Norton amp having current-differencing properties as distinguished
> from the op amp's voltage-differencing properties.  Extending that
> logic, I tested for current difference at the inverting and
> noninverting inputs of each of the four stages of the LM3900.  All
> measurements were very close to zero.  Should I assume this chip is ok?

I've really very little experience with those chips,  but figure I should 
probably google around and get to know them at some point.  I know that on 
the schematic that shows up on the datasheet I linked to the inputs are very 
different from any other op amp that I've ever seen.

> Looking to the VCA as the last suspect in the chain.  I tested the
> voltages at the ins and out of the CA3080.  14.43V going in at each
> input.  14.14V coming out.  The power supply for the Avatar is
> +15V/-15V.  Is the fact that the output voltage is so close to the
> power supply voltage a red flag?

The 3080 is also a current-operated device.  Most particularly the control 
input,  pin 5.  I know that signal levels going into it need to be *way* 
lower than much else in most of this equipment,  and that the output is a 
current as well (the 3094 is a variant with a buffer on the output),  and 
that the control input *must* have some nontrivial resistance in series with 
whatever's feeding it because it's only a diode drop or so above the negative 
supply rail,  and forcing too much current into that pin will trash the part.

> I have a scope, but I've never used it other than to get familiar with
> it.  How could I test these components with the scope?  What would I
> be looking for?

A book I used to have once that was called "101 things to do with your 
oscilloscope"...?  Seriously,  a scope is a great tool to have.  But it's 
voltage-operated,  and may not show anything at all at some points around 
these chips,  even though there's signal there.

For example,  you won't see anything that makes any sort of measurement sense 
at the control input of a 3080,  but if you go across the resistor there to 
the other end,  away from the chip,  you should see something there.  
Similarly with the other pins,  depending on how it's configured.

One trick I used to use to check op amps (though I don't know if this applies 
to the 3080 and don't think it applies to the 3900) is to short the two 
inputs together with a small screwdriver.  The output at that point should be 
halfway between the power supply rails.  If it's still (for example) stuck 
down near the negative rail (the most common failure I've observed),  then 
the chip is bad.

Oh,  and if any of you guys are looking for datasheets for any of these parts, 
feel free to look around at my parts pages:

http://www.classiccmp.org/rtellason/parts-index.html

where there is no registration, cookies, limits, or other BS hassles about 
getting what you need.


-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin