Hey Gang
I just bought an MG1 locally and it is in great shape. This will be the 3rd one that I have owned so I’m pretty familiar with the synth. I very briefly tested everything out after bringing it home and everything worked fine. Yesterday, I decided that I was going to add a 1/4 jack on the back for audio out. (I’ve successfully done this twice). I removed both boards, removed the rotting foam, performed the 1/4 out mod then put everything back together. The mod works fine but now there is something wrong with the filter. Everything is very muffled as if the filter is nearly closed. I am getting no mid or hi frequencies. I didn’t touch the internal trip pots and I’m not sure how what I did could have any effect on the filter. I don’t think it is a problem with the sliders failing because the all seem to be working. What have I done? Please help 
eric
The filter cutoff slider is working fine ? How can you tell if the sound is muffled ?
I strongly suspect that the foam removal operation wasn’t 100% successful, and that some sliders and/or switches are defective maybe ?
I guess I can’t be 100% sure but all the sliders and switches work fine other than everything being very muffled. I just blew some compressed air in all sliders, it didn’t help.
Sometimes, many times in fact, some of the tar that this foam has turned to, gets inside those sliders and switches, and this sticky mess is enough to create contact problems inside those.
But, assuming that the synth was working fine before any work was attempted on it, it might also be a bad contact on one of the connectors, or cracked solder joint somewhere from removing and reinstalling the boards ?
Well, I guess I’ll pull the boars back out to do a visual inspection of the soldier joints. Thanks for your input, I’ll post my findings.
Does the problem happen on the main out and headphone out? Did you verify that all of your connectors are connected properly? The 3 pin connector on the right corner of the lower pcb can slip out pretty easily. Try reflowing the solder joints on the filter slide pot. Does the filter pot do anything when you move it?
I already resoldered those pins and removed, disassembled, cleaned, reassembled and resoldered both the cutoff and res sliders. I am starting to wonder if some black gunk that I have missed is shorting something out inside.
As for the filter slider, it works but the range of the cutoff is way off. I’ll take a video of the whole thing tonight and post it.
It’s certainly a possibility. It’s no wonder that foam is called the FOD (foam of death).
Here is the video. Not the best quality but I think it will do. Notice how the filter makes a poping sound. Any advice?
Do you have any pots handy you can use to swap out the slider? Even if it’s just a standard pot to test it.
I can hear two things wrong with the filter in that video: first the reduced range of the cutoff (obviously), but also the “peak emphasis” (resonance) not working right either. That filter is not self-oscillating when the peak emphasis is at maximum position.
Check all your connections (especially misaligned connectors or partially disconnected), and also check for broken wires (especially the ones coming from the transformer).
Since everything was working fine before you opened it to work on it, then it surely must be something that happened during that time. Check if the voltage regulators are producing the required voltages (measuring with a multimeter). 7812 = +12 Volts DC, and 7915 = -15 Volts DC.
I have limited experience in the testing area but I do have a multimeter handy. Can you walk me through this? I really appreciate your help.
Be careful not to touch the wires coming thru the back of the MG-1 and going to the transformer, nor the wires of the glass fuse on a holder, that’s the primary side and it’s the household power from the wall outlet !
With your multimeter set on DC voltage, put the black probe on the negative side of the big capacitor (that canister that looks like a C cell battery, follow the - line on it to that side), and the red probe on the right leg of the 7912 regulator (those three legged things screwed to a small black heat sink). Your reading should be around +12 Volts.
Now move your red probe to the right leg of the 7915, your reading should be around -15 Volts.
As complementary info, on my own MG-1, after I had worked on it for some mods, one of the wires broke inside one of the connectors without me noticing it, until I noticed something wrong with the operation of the synth. And until I gently moved those wires, every connector seemed okay. But then one of the wires popped out of one of the connectors, and I had found the culprit.
Thanks for the walk through. Everything looks good so far. I measured +12. / -15.4
Yep, those numbers look right.
You could always try yo undo your output mod and see if everything’s back to normal ?
I tried reversing the mod but it didn’t fix the problem. One thing of interest to note is that after I reversed the mod, I powered up forgetting to connect the four plastic connecters to the bottom board. I thought for a second that everything was back to normal because the synth made a normal (not all muffled) tone. After a few seconds, I realized what I did, powered off, reconnected everything, powered back on and everything was muffled again. I’m not sure, but could this point to the top board being the problem?
There are 6 connectors on the bottom pcb. If you are talking about the four that are close together (next to the power section on the left), those are the power connectors. It wouldn’t make any sense that your synth would work without those connectors. Maybe the caps held enough of a charge for a moment?
Can you verify that you didn’t accidentally cut a lead from the headphone out jack? Is the ground to your master volume still soldered to the + end of C8 on the bottom pcb? Just ruling out connections that are easy to break.
Well, I’m making some progress! After studying the service manual I decided to test the voltage of some of the ICs. I noticed that IC 10 seemed to be dead. Since I had a few 353 ICs left from a previous MG1 project I soldered a new one in. That seemed to fix the muffled filter problem but I am still having the no self oscillation issue. I’m half way there.
I checked all of this and this is not the issue. Thanks for your help.
Hey Gang
I am still having trouble with the MG and wanted to post an update. First, here is a new video of the problem.
As you will notice, the filter only allows audio through it when the cutoff slider is towards the middle. The peak emphasis has some but very little effect, you can’t really hear this in the video.
The following is some things that I have tried / tested.
*All wires from the bottom board to the top board have been tested at the soldier joint on the top board. All values are correct and the grounds are connected properly.
*I performed Falcon’s Minimoog Filter mod which replaces the capacitors and some of the resistors in the filter section.
*I replaced all of the electrolytic caps besides the 2 main power supply caps. (didn’t really think that this was the problem, but those caps are old)
*I replaced transistors D25, D26, & D28.
According to the service manual IC9 and Q1017 - Q1022 might be defective. I have the transistors on hand but I don’t have that IC. Also, does this sound like it may be a problem with the sliders or switches and not filter itself? Any other testing that anyone would recommend?
Thanks to those who offered suggestions previously.
Eric