Need help with Moog/Realistic MG-1 repair

[patrickvf1976]

I have a Realistic MG-1 synth that is in mediocre condition at best. The issues I'm dealing with aside of corroded key contacts is that some notes don't play as loud as others and the poly section bleeds through when the poly slider is set to zero. When looking at the guts I noticed that IC U11 is defaced and U12 also looks kind of funky. Could this be the problem that causes poly to bleed through? I also suspect that some of the resisters near the poly slider might need to be replaced, but at this point I'm just guesstimating, so I'm hoping before I end up doing exploratorive surgery, that someone could help me isolate the problem area and perhaps even provide me a solution. As for the varying volumes could there be an issue with one of the divide-down chips or should I rather take a look at the row of resistors on the power/poly board, or could it even be caused by the corroded key contacts themselves? Any suggestions?

[patrickvf1976]

Ok, let me rephrase the question - What could cause the poly section of my Moog/Realistic MG-1 to bleed through when the POLY slider is set to 0, and what could cause it to play some notes not as loud as others? I need some answers, suggestions, etc as soon as possible. I want my synth to be fixed soon.

[brock]

I don't really know anything about the MG-1 but looking at the info on this page http://www.sdiy.org/philgallo/mg1x.html and the schematics linked at the bottom of the pages I would bet your problems are corrosion/wear and corrosion.

All the tone sources have a common summing point and all the volume pots have a ground termination for "off" so if you are getting bleed through it's because the pot is corroded or worn, or the ground connection is messed up.

The varying volume of different keys is probably due to the corroded key contacts. The envelopes are generated by the organ signals which go through the contacts. If they are corroded they probably have some significant resistance which will attenuate the signal, reducing the envelope amplitude, thus reducing the output. I don't know what kind of contacts the keyboard has but I have an old Wurlitzer combo organ with corroded j-lead contacts, some of which seem impossible to fix. I hope you have better luck with that than I've had.

[patrickvf1976]

Thank you very much. Yeah I agree that corrosion could be a factor. The metal parts of the inside casing has rust all over and some of the key contacts do have a thick layer of green stuff on them. I'll try to give it a good clean. What can I do about that slider? Would some DeoxIT clean it good enough to stop the bleeding? I have also noticed that the top sides of teh pc boards look discolored as if they suffered water damage at some point in time. Or it could come from that notorious foam material that somebody apparently removed. Either way the inside looks like that poor thing was sunk in a lake lol

[brock]

Green stuff on the contacts might not be the worst thing in that it's something you can probably fix. The contacts are probably brass plated and they are oxidized like the rest of the machine. Soap and water and a bit of work will probably fix that. I wouldn't use anything abrasive or chemical to clean them unless they are solid brass.

The slider might be tougher to fix. DeoxIT might work but I've found when the problem is at the end of travel it's usually wear or a solder problem. I'd start by measuring the resistance of the wiper to ground and the ground lead to ground to see where the high impedance is and see if it's a solder or pcb trace issue. If it's the slider contact then DeoxIT might help. If it's worn it won't make any difference. If the restive film is worn but not completely broken you might be able to make it work by reversing the slider, which would only work if it's mechanically symmetrical and I can't tell that from the pictures I've found.

[defog]

Replace the sliders.. All of them. I've repaired about 10 of these synths in the past 10 years, and every single one of them had issues with at least a few of the sliders. If it is bleeding when set all the way to zero, the track is doubt worn out. Sometimes they just have cold solder joints from the pots being board-mounted, but more often the slider is actually just worn out. The same thing happens with the SH-101, Juno 106, and some of the other slider-pot synths (ARP).

Also, no matter how hard you try to get that black goo that melted all over it off, it is bound to cause issues eventually, especially in the form of corrosion in the IC sockets for the CA3046 ICs.

Whenever I repair these synths, the first thing I do is grab a ton of q-tips and rubbing alcohol and clean the goo off of the components as best as I can, then I re-flow solder over the entire circuit board (upper & lower boards) and replace all of the slider pots and standard pots. Chipforbrains (technology transplant) sells kits for these that include all of the slider caps as well. They're a tad pricy, but you won't find those slider pots that fit properly easily anywhere else. And if you do find one and replace it, another slider will eventually go on you and you'll be right back where you started.

http://www.technologytransplant.com/forums.html?cPath=30_62_23

As for the divider/organ IC, that thing is not an easy source and usually only can be found through another dead MG-1. I've seen some retrofits and even installed one on a repair I made, but it never worked like the original one (some keys would not trigger the organ). If the actual IC is going, you might as well give up on the poly section (even though that's what makes these things beasts in my opinion). Let me know if you have any other questions with your repair. Good luck!

[patrickvf1976]

I found orange/brown spots on the solder points at the low end of that particular slider and I wonder if that could cause the slider to work properly. I'll clean it up and see if that fixes it. If not, then I'll replace it. About the divider chips - I was only mentioning them because I suspected that they would be causing the lower output on some of the case, but as brock mentioned that corrosion on the contacts could affect the output level of the notes and behold the contacts with the most gunk are the ones that produce the quieter tones. Gonna clean that and if that doesn't work then I'll take closer look at other parts (resistors, ICs) I don't think that failing divider chips would cause quieter notes on some keys across the keyboard, an entire octave yes, but individual keys? Well I'm gonna get working on it and let you know how it turns out. Thanks for all of your advice!

[defog]

[patrickvf1976]

I didn't get much of a chance to mess around with it but today I gave it a shot trying to remove some of the corrosion/oxidation.. goo.. with some 99 isopropyl aclohol and qtips. I wasn't able to get much off of the contacts, at least not without scratching it off, which is exactly what I was trying to avoid since the ends that actually touch the wire are very delicate. Is there any other method that I can use to get some reasonable results? I keep hearing about this DeOxit stuff but I'm not sure it that will work that much better than alcohol and qtips.