From jdm@synthcom.com Mon Jan 13 09:22:58 1997 13 Jan 97 12:22:18 +0500 12 Jan 97 21:14:45 +0500 id CAA54843; Mon, 13 Jan 1997 02:01:43 GMT id CAA26673; Mon, 13 Jan 1997 02:01:36 GMT Date: Sun, 12 Jan 1997 18:05:31 -0800 From: JDMcEachin Subject: Re: 2600 repair suggestions needed 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 From jdm@synthcom.com Thu Jan 16 08:57:08 1997 16 Jan 97 11:56:53 +0500 16 Jan 97 03:31:13 +0500 id IAA78780; Thu, 16 Jan 1997 08:16:45 GMT id IAA84150; Thu, 16 Jan 1997 08:16:41 GMT Date: Thu, 16 Jan 1997 00:20:47 -0800 From: JDMcEachin Subject: Re: 2600 repair suggestions needed At 11:25 PM 1/15/97 -0600, Mark Pulver wrote: >The LF411 (and the dual version the 412) is a full production National >Semiconductor part. It's carried by Mouser, DigiKey, Jameco, etc, but may >NOT be carried by your local folks just cause it's cheaper for them to >stock the junk 741's. > >Mouser doesn't list the part in their current catalog, but call 'em - (800) >346-6873 -, they have it. Be careful with DigiKey... They list the part, >but don't get confused (like I did!) and think that $6.30 is a normal cost >for one of these. I was looking at the mil-spec part number. A real cost >for the LF411 is about $1.50 and the LF412 is about $1.60... Mouser has instrument grade OP-O7s for US$0.96 (I think) - I'd go for those, they've got better noise specs (I've got this thing about paying more than $1 for an opamp). I wonder why the cost of 411/412's went up so much???? 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 From jocke@netcontrol.fi Thu Jan 16 08:59:17 1997 16 Jan 97 11:58:53 +0500 16 Jan 97 07:18:39 +0500 id MAA37956; Thu, 16 Jan 1997 12:01:06 GMT id LAA52696; Thu, 16 Jan 1997 11:55:53 GMT Date: Thu, 16 Jan 1997 13:55:28 +0200 (EET) From: Joachim Verghese cc: synth-diy@horus.sara.nl Subject: Re: 2600 repair suggestions needed MIME-Version: 1.0 >Mouser has instrument grade OP-O7s for US$0.96 (I think) - I'd go for those, >they've got better noise specs (I've got this thing about paying more than >$1 for an opamp). The OP-07 is very good in most respects, but its slew rate isn't any better than that of the 741. Also, in the case of Robot's broken S&H module, it would be preferrable to use a JFET-input op-amp for the S&H's output stage, since the original circuit uses a dual FET + LM301 combination. If and when the dual FET (2N3958) breaks, you can just bypass it by shorting the G and S terminals, provided that the LM301 has been replaced with a JFET op-amp such as the LF411. In addition, the LF411 has low offset and high slew rate. > I wonder why the cost of 411/412's went up so much???? Can't think of any viable explanation - European distrubutors still sell it for around $1.50, small quantity. The LF411A, on the other hand, which has even tighter offset specs, is priced at around $10. -joachim