MG1 Filter Problem

e_just · Post by **e_just** » Tue Feb 03, 2015 11:40 pm

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?

facon · Post by **facon** » Wed Feb 04, 2015 12:30 am

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.

e_just · Post by **e_just** » Wed Feb 04, 2015 11:26 pm

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.

e_just · Post by **e_just** » Wed Feb 04, 2015 11:27 pm

facon wrote: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.

I checked all of this and this is not the issue. Thanks for your help.

e_just · Post by **e_just** » Sun Feb 15, 2015 10:47 am

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.

http://youtu.be/YHJDwbggap4

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

thealien666 · Post by **thealien666** » Sun Feb 15, 2015 4:39 pm

Personally, and that's just me talking, I would have tried to fix the filter until I would have resolved the issue with it before attempting any modifications. Because now, not only did the mod not fix the original filter problem, but it might even exacerbate the issue. Who knows ?

Let me thank you for the update and sincerely wish you good luck.

facon · Post by **facon** » Tue Feb 17, 2015 9:21 am

You can rule out the slide pots by de soldering them and using a different pot in their place. You could try jumping the emphasis pot with a wire/clip to see if bypassing it entirely gives you the effect you would normally have when fully up.

I would verify voltages on all the ICs and transistors in the filter circuit if you haven't done so already.

e_just · Post by **e_just** » Tue Feb 17, 2015 9:03 pm

facon wrote:You can rule out the slide pots by de soldering them and using a different pot in their place. You could try jumping the emphasis pot with a wire/clip to see if bypassing it entirely gives you the effect you would normally have when fully up.

I would verify voltages on all the ICs and transistors in the filter circuit if you haven't done so already.

Thanks! I have checked the transistors in the filter section and they seem fine. I also checked voltage on most of the ICs but still have a few more to check. I have some (8) sliders and all of the switches on the way, I'm not sure that they are the problem but they still need to be replaced. I should have a lot of things ruled out by this weekend.

e_just · Post by **e_just** » Fri Feb 20, 2015 9:59 pm

The problem ended up being a bad IC9. I replaced it and now the filter is back to normal, I think... Now the audio is distorted. The distortion effects all audio, not just the 2 OSCS. I guess I now have a bad VCA? It never ends

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