Moog MG-1 - PSU Problems? - Page 1
Forum section: electronics / repair

author: sebajom
author_rank: Contributor
timestamp: April 07, 2015, 09:28:35 pm
content: Hi!I'm restoring a Moog Realistic MG-1 analog synthesizer that I recently acquired. It wasn't working when I bought it, but that turned out to be due to bad solder joints, so things are good now. Everything works beautifully. Except...When probing some signals with my scope I noticed that the ground of the synth - including the chassis - has a pretty large, about 27V p-p, 60Hz AC component (relative to mains ground; see attachment). Consequently all signals, when measured, has this rather large signal overlaid as well. The unit is NOT mains grounded: It has a two prong power cord ('three prong-style' with the ground pin plugged) which I believe is actually original and not a hack (see attached photo).I've included the PSU schematic from the service manual - I imagine that's where the problem lies. Initial probing didn't reveal anything completely unexpected (to me, anyway) though I admit this is not really my area of expertise.Any good ideas where to look next?

author: sebajom
author_rank: Contributor
timestamp: April 07, 2015, 09:29:52 pm
content: ...and here's the missing PSU schematic.

author: edavid
author_rank: Super Contributor
timestamp: April 07, 2015, 09:42:09 pm
content: The 2-prong plug is not original since the schematic shows a grounded plug. Replace it.

author: Seekonk
author_rank: Super Contributor
timestamp: April 07, 2015, 10:10:06 pm
content: Grounding is nice, but often musicians remove power line grounding to prevent ground loops in band setups. That is the reason many go to balanced audio transformer isolation.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 07, 2015, 10:37:26 pm
content: When you made your scope measurements, what did you connect the scope ground to? If you connected it to the chassis, that might not be right. It has to be connected to circuit ground (i.e. the triangle symbol on the schematic). Are the two connected to each other? I don't see anything on the schematic that shows that.Ed

author: sebajom
author_rank: Contributor
timestamp: April 07, 2015, 10:56:17 pm
content: Quote from: edavid on April 07, 2015, 09:42:09 pmAre you sure? While I agree that the schematic seems to indicate it should be grounded, none of my other half a dozen synthesizers are mains grounded either (all two-prong plugs, like this one), and the power cable and connections all look very original.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 07, 2015, 11:05:20 pm
content: Quote from: sebajom on April 07, 2015, 10:56:17 pmI agree with edavid. I didn't understand what you meant by the ground pin being 'plugged' until I looked closely at the picture. It wasn't 'plugged'. It was broken off!Ed

author: sebajom
author_rank: Contributor
timestamp: April 07, 2015, 11:45:53 pm
content: Quote from: edpalmer42 on April 07, 2015, 11:05:20 pmOk, you're probably right. I'll swap it out for a proper cable.Thanks everyone!

author: Someone
author_rank: Super Contributor
timestamp: April 08, 2015, 05:21:50 am
content: Quote from: sebajom on April 07, 2015, 10:56:17 pmSome times the grounding was different depending on which country it was sold in, and the schematics do not always cover all the cases. Even up into the 80's it was common to have professional equipment that was mains connected without grounds.

author: SeanB
author_rank: Super Contributor
timestamp: April 08, 2015, 05:35:35 pm
content: You can see in the photos inside the unit that it has a 3 wire power cord, with a ground lead in it, and the plug has had the ground pin removed. Thus you need a new plug on the cord ( Hubbel 3 pin type) or a new cord. The hum is from the transformer capacitance between the windings, and cannot be fixed unless you use a new transformer with an interwinding shield connected to mains ground. Even then as the chassis is required to be grounded you will still have the need to ground the audio side, unless you get some audio transformers to isolate the inputs and outputs.

author: edavid
author_rank: Super Contributor
timestamp: April 08, 2015, 05:57:54 pm
content: Quote from: SeanB on April 08, 2015, 05:35:35 pmLook at the schematic - it already has a shielded transformer! Should be fine as soon as the shield is actually grounded.