To: 'Analogue Heaven' analogue@hyperreal.org,
From: "Caloroso, Michael E" CalorosoME@corning.com,
cc:
Subject: [AH] DIY: Analog synth (Memorymoog) tuning/connector fix (long)
Date: Mon, 19 Jul 1999 14:42:46 -0400
I've had numerous inquires on the details of this, so I've decided to lump all of it in one post for the archives. The focus is on the Memorymoog, although this info applies to any analog synth.
----------------------------------
The Memorymoog has a history of problems with tuning. If your unit has to be calibrated more than once a year, or the calibration values get noticeably worse each year, or the tuning changes from day to day, or the monophonic mode is out of tune with the polyphonic mode, or the tuning changes with any movement or impact (just hold a note and hit the case with your fist), that is the result of oxidation on the connectors inside the Memorymoog. There are almost 100 connectors inside these beasts, which aid in servicing but are a frustrating source of malfunction. In my experience, I've found that the most reliable equipment either have the fewest connectors or the connectors are high quality.
The connector contacts in the MM are tin-plated. Over time, oxidation forms on the contact surfaces in the form of an impedance, which 1) varies with humidity and temperature, and 2) is non-uniform over the contact surface, where simple movement changes the impedance. The consequence is that you lose voltage across these oxidized contacts.
The polyphonic control voltages that drives the voices cards travels through three boards (six connectors), while the monophonic control voltage goes through four (eight connectors). If *ONE OR MORE* of these connector contacts has oxidation, the control voltages are degraded. Note the emphasis; at worst there will be six points where the polyphonic CVs are losing voltages across the connectors. This is the primary cause of the MM's tuning problems. To add to the problem, these critical voltages are carried over ribbon cables via DIP16 plugs, and the mating sockets in the stock MM are poor quality for analog signals. You can verify this with a DMM; probe the signal at the DUMX board and at the voice card where it terminates, if you measure any voltage drop then you're seeing contact loss.
Power busses are equally vulnerable, if the voltages aren't stable no amount of calibration will help. TTL logic signals (d*g*t*l) with their built-in interim zone between high & low are insensitive to these losses and generally don't have a problem.
As a temporary fix, the contacts can be cleaned by periodically removing/re-inserting the plugs as to "scrub" the oxidation off, or they can be shined up using an eraser.
Ultimately the contact surfaces will corrode beyond cleaning. I am the original owner of my MM, and it took less than ten years for this to occur. I was now faced with having to replace connectors.
During that time, I had acquired an ARP Pro-Soloist which predates the MM by ten years. It had laid dormant, yet was perfectly in tune and 100% functional when I powered it up. This is an instrument that is over twenty years old yet at half its age was in far better health than my MM. My interest piqued. Closer inspection revealed that the connectors used in the ARP were a combination of tin plating and gold plating, IE the pins were gold plated while the contact inserts in the mating plug were tin plated.
Before starting ARP, Alan Pearlman designed and built amplifier assemblies for the space program and the military. I also have many years of experience in military electronics. The military demands reliable equipment, and the standards mandate that all connectors contacts have gold-plating; tin plating is not acceptable. Al's experience with military electronics carried over to his synthesizers and is a big reason why his instruments were more reliable.
Connectors with tin-plated contacts are rated for ten years, while gold-plated contacts are rated for TENS of years. Gold also does not oxidize.
My first impulse to fix my ailing MM was to replace *every* tin-plated connector contact and pin with gold-plated equivalents, but there was a problem. The gold-plated contacts for the AMP MTA plugs used in the MM couldn't be found in any catalog and were available only from AMP in lots of 10K, and AMP only produced a stock when the demand justified the expense. You also need a special insertion tool for the MTA plugs.
However the ARP demonstrated that you can get years of reliability at reasonable cost by combining gold/tin platings on connector systems. So I decided to focus my efforts on replacing only the connector pins and DIP sockets on the PC boards and leave the plugs as is. This was more cost-effective in parts and in labor, and the parts are easier to locate. I also minimized my work by targeting only those connectors which carried any analog signals, since logic signals weren't a concern. You can find this info in the master connector chart on the MM's schematics, which you'll need for this work. If you need schematics you can get them from the world famous Mark Glinsky's manual manor at http://www.magicnet.net/~mglnsky/msg4.html.t years of reliability at reasonable cost by combining gold/tin platings on connector systems. So I decided to focus my efforts on replacing only the connector pins and DIP sockets on the PC boards and leave the plugs as is. This was more cost-effective in parts and in labor, and the parts are easier to locate. I also minimized my work by targeting only those connectors which carried any analog signals, since logic signals weren't a concern. You can find this info in the master connector chart on the MM's schematics, which you'll need for this work. If you need schematics you can get them from the world famous Mark Glinsky's manual manor at http://www.magicnet.net/~mglnsky/msg4.html. They're a far better image than any online schematic.
They're a far better image than any online schematic.
I also nailed the CEM3340 VCO chip sockets on the voice cards, just to be safe, because the sockets carry critical control voltages.
----------------------------------
For the MM, you'll need to find a source for header strips with gold-plated pins on 0.100 centers and 16-pin DIP sockets with gold-plated contacts. These are not hard to find in catalogs or online. I got lucky and sourced gold-plated parts at a surplus shop. I recommend Augat DIP sockets if you can get them. You can buy header strips with 40 or so pins which can be cut into the quantity you need. Side cutters work well, although occasionally you'll lose part of a pin. Buy more than you need. All together, you'll need 42 DIP16 sockets, more than enough header strips for 305 pins, and header strips with right angle pins for 32 pins (you might get away with the straight pins but I don't know how it will clear when put back together).
If you can find them (and let us know where 'cause I would really really like them for mine), you'll want new headers for the power supply board. These are the larger pins and are 0.156 centers. You'll need a 15-pin strip and three 3-pin strips. I have yet to find a catalog (Mouser, Digikey, etc) that carries these.
The complete list of connectors to replace is:
Voice Cards A-F (x6): S11, S12, P13, P14, P15, P16, P17, sockets for CEM3340s (x3)
Common Analog: S21, S22, S23, P25, P26, P27, P29
Contour/Glide: S32, S33, P31, P34, P35, P36A-F (x6)
Digital: P40, P48a, P48b
DMUX (big one!): S53, S54, S55, S56, S57, S58, S59, P511, P512, P514, P515A-F (x6), P516, P517, P518, P519, P520
RSC: P63, P64
LSC: P76, P77, P78
Power Supply: P131, P132, P133, P134
Keyboard: 16 pin right angle
Note that the LAMM update from Lintronics does not include this work.
If you have an older MM, you may need the reliability updates that Moog Music has issued, the complete list of service bulletins are online at http://www.ne.jp/asahi/interlogic/oku/memorymoog/mod.html,es that Moog Music has issued, the complete list of service bulletins are online at http://www.ne.jp/asahi/interlogic/oku/memorymoog/mod.html,
and Cary Roberts has clearer images of 840/841/842 at http://www.retrosynth.com/docs/memorymoog/index.html. http://www.retrosynth.com/docs/memorymoog/index.html.
----------------------------------
And now, words for the DIYer:
This is not a job for the novice. You should have excellent soldering/desoldering skills and experience with double-sided PC boards. For an online primer see http://aupe.phys.andrews.edu/diy_archive/references/soldering.htmling/desoldering skills and experience with double-sided PC boards. For an online primer see http://aupe.phys.andrews.edu/diy_archive/references/soldering.html and http://www.synthtech.com/tutor/tutor1.html
and http://www.synthtech.com/tutor/tutor1.htmlhtml
The task of removing DIP sockets and header pins is tedious and can damage the PC board if you're not careful. A good soldering iron is definitely recommended. You'll need desoldering tools (wick or solder-sucker) to remove the parts. In most cases you'll have to physically destroy the DIP sockets to get them out. If you've never removed parts like these before, start with the voice cards since they are single-sided boards.
Before you install the new parts, apply flux to the pads so that you get a clean solder joint, and clean up the excess flux with flux cleaner. I use a brush to apply flux cleaner and a wadded paper towel to soak up the residue, using a towel fresh surface each time. Good lighting makes a difference here. If you don't clean it up, leftover flux will cause problems down the road like low-impedance shorts across PC board traces.
Wear a ground strap or do your work on a static-safe mat. You're going to be working with irreplaceable static sensitive components. The CMOS and CEM chips are safer in their sockets on the boards.
I used a Panavise with a PC Board clamp so that I can work with both hands free, which helped a lot. Some of the MM's boards are too big to fit this clamp.
I suggest that you work with one board at a time and confirm 100% functionality after rework before you move to the next board.
I also suggest that you label plugs with a permanent marker as you remove them, IE S11E or P16E for voice card "E". There are too many places to plug the wrong connector and too many connectors to get mixed up.
Remember that when you install a new part that you have to have proper solder fillets on the pads of *both* sides of the double-sided PC board. If you're good enough with the iron and apply the solder to the pin and not the tip of the iron, the solder will flow well enough to make a fillet on the other side by itself. Note that only the voice cards are single-sided.
Some connectors have a "key" so that you don't orient the plug the wrong way. The mating header simply has a pin (and PC board thru-hole) omitted where the "key" is. In you new part you can pull out a pin by heating it and pulling it out of the header strip as the plastic softens.
Do not handle any of the gold plated parts with tools. The plating on the pins is extremely thin (10 or 30 microns) and can be destroyed if you scrape it with metal tools.
----------------------------------
Here's another MM foible: failing range/scale trimpots on the voice cards cause tuning problems. On the resistive elements of the trimpots, the precise area where the wiper contacts corrodes over time and will no longer work reliably.
You can unsolder each trimpot, rotate them 180 degrees on the PC board, and resolder them. By rotating them you will have to set the resistance to its previous value, but it will be no longer around the corrosive area. You will have to perform major oscillator re-calibration when you do this, because they will be WAY out (hint: don't use the C-7 routine. Calibrate the oscillators by ear with a reference).
Of course you can only do this once. After that it's time to spring for 36 new trimpots.
----------------------------------
Altogether this can take over 40 hours of work, but the reliability improvement is worth it. In the two years since I have done this to my MM, *none* of the tuning problems have surfaced. I recently checked the calibration, and the oscillator range is slightly off but the scaling is *rock solid*.
MC
- BOYCOTT ROLAND! http://www.dennishoulihan.com/nnishoulihan.com/
Opinions (and mistakes) expressed herein are my own and not those of my employer.