2600 repair suggestions needed

JDMcEachin jdm at synthcom.com
Mon Jan 13 03:05:31 CET 1997


At 02:46 AM 1/12/97 -0800, Robot wrote:
>Okay, I just got a scope from a gratious engineering family member who had
>it sitting, collecting dust in his garage.  So I thought I'd like to tackle
>my bum S+H on my 2600.  I traced though some of the things that might be
>going wrong.  The noise generator works fine and makes it to the pin in on
>the 4015 module just fine. The internal clock works fine and is correctly
>converted to a pulse trigger on the rising edge for the pin in on the 4015.
>No output though.  The service docs don't have schematics of the individual
>modules as I presume ARP had originally never intended for techs to work on
>modules.  They probably believed some module replacement deal would work
>under warrantied parts, etc.  But alas now that it is a million years later
>and ARP has deceased and I don't know the best plan to aproach it.  The
>sevice docs I have simply say replace the module if it's not working.  DUH!
>I need to fix it though. So far I have pulled the 4015 out and am planning
>to start by replacing the caps as I haven't a very good way of determining
>which if any caps are bad.  My DMM does capacitance but it only seems to
>work properly when the cap is removed from the PCB.
>
>Any help or suggestions on the best approach to fixing the poor sample&hold
>module for my 2600 will be very appreciated.

The first thing I would suggest is replacing ALL of the opamps.  Before I
did, it was "use the 2600 a few months, have it break, wait a year to get
around to trying to fix it, replace an opamp, use the 2600 a few months,
etc.".  When my 2600 S&H failed, I decided it was time to take on the task.
I replaced the LM301s w/ LF411s, and clipped the comp caps (on the end of
each opamp, between pins 1 and 8, not needed because the 411 is internally
compensated).  Plugged it in and it sounded great - my S&H was working
again, and I didn't have to spend any time tracing the circuit.  Of course,
20 minutes later the speaker amp blew.  Oh well, at least 2600s are easy to
fix. You may want to beef up the filtering on the power supply or replace it
- it seems to be prone to transients that cause active components to blow.

Replacing the opamps will take more time than money, if you know how to
solder. Doing so makes the 2600 a much better sounding synth.  If you want
to be able to turn up the spring reverb w/o turning up the hiss, then do so
right away.  You won't regret it.  I've written about this many times
before; check the archives.

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




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