MG-1 Continuous Tone Problem

[e_just]

Hey gang
I'm new around here. I just got an MG-1 "fixer-upper" for cheap and I'm looking to get it working properly. I have done a little bit of modding and repair work, so I'm sure I can perform the repair once I know what is needed to be done. Here is the problem. When powered on both OSCs, the noise and bell tone sound as if a key was being held down. If a key is pressed, the pitch changes according to the key and then just continues to sustain forever. The poly section on the other hand, works like it should. Would anybody happen to know what might be wrong with my synth?
Thanks
eric

PS Here is a video of the problem
http://youtu.be/PMP2DXNmAMA?list=UUCA54 ... _ai_pKt3kQ

[thealien666]

Hello and welcome Eric!

The Realistic MG-1 is rather different than other analog monosynths, there is no gate or trigger buss bar on the keyboard of an MG-1. It generates the trigger (or rather a gate) signal from a circuit that, essentially, "listens" to the polyphonic section and generates a gate signal as soon as a note from the polyphonic section sounds. It does that using two op amps (U7b and U8) which are one half of a 4558 and a good old 741. It could be one of these two that has become defective, and keeps the gate signal "ON" all the time.

OR, it could be a problem in the envelope (or "contour" circuit)... Does it work with the "Tone Sources" switch in the "Contour" section to the "Keyed" position (resulting in full on, and full off when pressing and releasing notes) ?

[e_just]

Thanks for your help. I just ordered thoes parts. Hopefully that solves the problem. If it doesn't, I guess I'll just rebuild the contour circuit.
Thanks again

[Ledbetter]

I had a similar problem with a Prodigy. One of the key contacts was bent slightly. Straightening it fixed the problem.

[thealien666]

Ledbetter, if you had taken the time to read my first post, you'd know that is impossible causing that problem of continuous sound on a Realistic MG-1, as clearly explained...

[e_just]

I thought I'd post an update. I replaced all 741s on both boards and the 4558 on the poly board. I also replaced most of the resistors and caps on the poly board. The problem didn't go away
I think the next step would be replacing the switches. One thing that I noticed is that when the countoured cutoff is turned most of the way up the synth sort of works. The sustaining note is still there but its filtered out. The rise and fall time seem to work when I do this. Its almost as if the oscillators sustain but the filter works properly. Any help?

[atkbg]

Don't go in with a sledge hammer. I don't have the schem in front of me, but replacing all the resistors and caps is generally not the way to repair.

Have you checked the sliders (which are notorious for failing due to the black foaming goo). Are they within tolerance? Are there dead spots?

I'm not sure what you mean by the "filter works". Are you talking about the EG's of the filter or the just the filter it's self?

When you say the noise and bell tone, are those the only sources that produce a continuous sound?
If the filter is open, is that when a continuous sound is produced?

[e_just]

I know I went a little overboard but I figured that those old caps are probably pretty dried up and out of spec. As for the 4558 and 741, they were cheap so I figured if I'm replacing one, I might as well replace them all. The sliders all seem to work other than the rise time which does nothing. Also the Env contour switch doesn't seem to have any effect. I swapped out the contour switch with OSC range switch to see if the switch was the problem and got the same results.

As for the filter, with the freq 3/4 of the way up and when set to full on contour in sustain mode, the fall time works properly to close the filter. As soon as the freq is turned back up, the tone comes back. The continuous sound is with all sources besides poly.

Here is an updated video of the problem

http://youtu.be/PVCruu-nMYI

Thanks

Eric

[moogle]

I bought an MG-1 today off of Craigslist and encountered the same problem. I turned to google to figure out what was wrong and found this thread. After watching your video I brought the filter cutoff freq down a little bit and set the contoured cutoff to max and the tone went away.

So it seems that I'm experiencing the exact same problem that you are with your MG-1, thanks for posting the video and please update this thread if you find out more about the problem. I'm quite new to analog synths and am pretty clueless about this stuff.

Thanks.

[thealien666]

... I'm quite new to analog synths and am pretty clueless about this stuff.

First, welcome to the Moog forum.

Second, in reading your quote above, maybe it would be best to leave the fixing of your synth to someone who has a clue (in fact a lot of experience) about this stuff ?

But has for learning about analog subtractive synthesis, there are lots of good articles about it on the web, and in books.

[Leo Castro]

Hi guys, i'm having the same problem.
Could you find a solution?
I think the problem is in the integrated circuit MC14007UBCP or also called CD4007. But until I get the replacement, I won't know! While I'm going to take it out of the circuit to check it.

regards

[Davi1s]

I am also having this issue, I am getting continuous long beeps with short pauses.