[sdiy] New long term project

[Tim Parkhurst]

On Mon, Aug 10, 2009 at 6:53 PM, David
Ingebretsen<dingebre at 3dphysics.net> wrote:
>
> In the schematics, the matched NPN pair (2N5172, I still have a boatload of
> them and the PNP's Nyle used from when I was building an SP in 74, but alas,
> no FET's) is a pair of discreet transistors which were bent over and a
> thermistor stretched across the both of them.
>
>        1a.     Can this pair be replaced by an SSM chip? What
> characteristics would be important when trying to see if I can substitute
> the SSM chip?

Yes you probably can replace the pair with an SSM2210, or an LM394.
Both are high quality NPN matched pairs. The reason for the matched
pair and the tempco are for temperature compensation in the expo
converter. You'll probably get better tracking and long-term
oscillator stability with a modern matched pair. BTW, Futurlec still
carries the LM394 for about two bucks. A great bargain.

>
>        1b.     Is it your experience that making this substitution will
> somehow change the character of the oscillation? I hated matching transistor
> pairs back then, and don't think I'll like it any better now but will do it
> if it will preserve some audio quality.

Making this sub will probably improve the tracking and temp stability.
I can't imagine that it would change the 'character' or sound of the
oscillator.

>
>        2.      Next, as I lamented, I don't have the old 2N5163 FET. I have
> found some cross references, but am unsure again which parameters I need to
> compare to match them. Can I just trust the cross reference charts and
> blindly substitute the one recommended?

I haven't seen the schematic, but my guess is that it's used to
discharge the timing cap for sync purposes. As long as the sub is
pretty close in performance, I would bet it wouldn't make any
difference.

>
> For what it's worth, this is not for profit, or sale. I just want to say I
> finished what I started as a 17 year old so many years ago.
>
> Thanks
> David
>
> ***********************************
> Historical note:
>
> I don't know what I'll do with it other than look at it and feel happy :)
>

You don't need any reason other than that. Personally, I think that's
the best reason ever for building something. :-)

> ************************************
>


Tim (hysterical note) Servo
-- 
"Sire, the church of God is an anvil that has worn out many hammers."
- H.L. Hastings