2600 repair suggestions needed

[JDMcEachin]

>>But isn't this for use in a 2600 S/H? Aren't the LF series FET input,
>>low bias current devices? Both these are crucial to good S/H tracking.
>>
>>If the application is for a S/H unit, I'd make sure to keep it to a
>>good low input current, low offset FET input amp. Even an LF353 or a
>>TL082 wouldn't be that hot in this app. However, you could probably
>>still do better than an LF411.

Well, the FETs are in the S/H circuit itself.  I've got 411s in mine, it
sounds good, big deal.  Maybe I'll try a TL071 or 81 to see if I can hear a
difference.


At 07:55 PM 1/16/97 -0800, Robot wrote:
>
>Okay!  NOW I'm getting confused.  

You and me both.  :)  I've been trying to get a straight answer on opamps
for years.  It usually comes down to two things: 1) is the pinout correct?
and 2) what can you afford?  Some circuits require a low offset opamp, some
require a jfet or non-jfet input.  Some people know which are which, some
don't, and some of us are just lazy.  Most people pick an opamp that they
like and use it in almost everything, some of us just buy whatever is in the
junk box.  My only rule is - no 741's in the audio path (secondary rule -
don't pay more than $1 for an opamp).

I use a variety of opamps - LF411/12s, TL071/2/4s, TL081/2/4s, NE5534/2s
(numbers are 1/2/4 opamp versions).  I buy them in bulk or I buy them cheap.
I guess I've always been lucky in that what I use always works.

You can spend even more money and use AD711s in your 2600 - about $4 apiece.
Your call.

You know, I just looked it up and the OP07 does have a worse slew rate than
a 741.  Live and learn.  The OP07 would work in your 2600, you may (or may
not) hear a slight blurring in transients (like square wave edges).

>I'm NOT educated enough to follow all of
>this.  I JUST took my FIRST ECE class last quarter and I know how to use
>test equipment and I can figure out shematics but that's about it.  We only
>spent a few weeks on op-amps and only the 741 at that.  I'll try and order
>some op-amps from Mouser but I'd PREFER to finds them locally.  Yes, I'm
>replacing the LM301's as per everyone's suggestion on the Sample+Hold module.
>
>If someone could SPECIFICALLY suggest PROPER op-amps to use in place of the
>301's as I can't find them OR the LF411's locally, I'd like that.  Either
>that or tell me the specs to ask the local surplus shop to find a match for
>what I need.  I don't know even close enough to tell them on my own.

My choice LF411, NE5534, TL081, TL071, in order.  There are better opamps,
there are worse.  It all depends on your budget.

As for how many you need:

RingMod Board - 4

VCO Board - 2

ADSR Board - 1

Mixer Board - 7

V Processor Board - 8 

Total - 22 (sorry Mark - you can use 741s in the envelope follower)

Buy a few extra, they're good to have around.  No dual opamps like I
thought.  There are also 4 canned 301s on the Waveshaper Board, which I
haven't replaced yet.  For these, you want opamps in little metal cans.

>Thanks again, but PLEASE don't send a reply to me AND the list, as I don't
>need two copies.

What, you don't enjoy it when some lazy person does this to you?

;-}

>You may want to buy a 1.5M metal film resistor to change the gain 
>structure of the reverb.  I think you need a 300k on the output, but that 
>doesn't have to be metal film.

I was wrong - replace R276 (220k) w/ a 1M metal film, and replace R288
(2.2M) w/ a 560k metal film.  Hope you didn't order yet, Mark. :P

JDM
Synthcom Systems, Inc.
http://www.synthcom.com/

"What I was looking for was an instrument that you couldn't tell what it
was.  That means you have to listen to the tone without associating it with
your memories.  That was the idea when we started with electronic music." -
Klaus Schulze