MG1 Filter Problem

[e_just]

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

[thealien666]

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 ?

[e_just]

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.

[thealien666]

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 ?

[e_just]

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.

[facon]

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?

[e_just]

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.

[thealien666]

I am starting to wonder if some black gunk that I have missed is shorting something out inside.

It's certainly a possibility. It's no wonder that foam is called the FOD (foam of death).

[e_just]

http://youtu.be/vS6ij5QGNQ4

Here is the video. Not the best quality but I think it will do. Notice how the filter makes a poping sound. Any advice?

[facon]

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.

[thealien666]

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.

[e_just]

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.

[thealien666]

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.

[e_just]

Thanks for the walk through. Everything looks good so far. I measured +12. / -15.4

[thealien666]

Yep, those numbers look right.

You could always try yo undo your output mod and see if everything's back to normal ?