Polivoks grounding hum

[OriginalJambo]

Recently I opened my Polivoks' keyboard assembly to fix a key that had came loose. However, when I put it back together and turned it on I noticed I now have ground hum present in the audio signal.

There is a cable coming from the power supply transformer to the keyboard assembly that's screwed down and I'm pretty sure it's the grounding wire. That's firmly in place but I still seem to be getting hum.

Anyone know how to deal with problems like these? I can take pictures of the insides if that'd be any help. I do realise not many people here will have any experience with Soviet synths like this.

[RawBC]

I wouldn't classify myself as an Electrical Engineer by any means what-so-ever. However, I have taken apart many a piece of gear, hmmmmm...

Open and close the case very slowly. Is there anything like wiring that would come in contact with either the power supply transformer or the ground wire? Sometimes I think all is prim and proper before closing a case or chasis, and low and behold there is a dangler potentially crossing something it shouldn't. The problem is those last few millimeters before it closes up.

What is the condition of the wiring inside? How about the transformer?

When you opened it up, did a connection or a solder point come loose?

That happened to my SP-202, I opened the case and ever so slightly loosened the connection for the internal mic. At first glance you couldn't tell at all, I actually had to poke the solder joint to see that it wasn't completely connected anymore. When I was really young, I inherited a Casiotone and opened it up one day, and when I did instead of listing straight up, I went left to right and cracked the board that had the AC adapter plugged into and had no power until I jiggled the jack. I was only 13 when I did that, so age hopefully grants wisdom (not always, in my case.)

The Plivoks is 220V? In the US we have 3 prong to 2 prong adapters for our 110/120 outlets that defeats the ground. Have you tried something like that to see what happens?

I guess those are starting points, maybe? I hope it helps. I am not too familar with Russian synths, but a few folks here are, hopefully they read this and point you in the right direction. Good luck!

[Solderman]

This exact problem has surfaced after I replaced a pot in my Polivoks. There are two big 2000 microfarad capacitors attached to the power regulator card X8, and one of them now reads 3 microfarads. Guess I'll be replacing it soon, but I have to wonder what caused the cap to fail.

[Zamise]

Not sure I can be a whole lot of help either, but I have a Maestro I've done some work on... It picks up a radio station when I first turn it on... Anyhow, only thing I can think of is perhaps try another outlet or move the synth to another area of the room or house see if that helps any, thinking they just aren't shielded very well and perhaps something else near it may be causing the hum.

[Solderman]

Sorry Zamise, I should clarify, the problem was indeed one of the power supply smoothing capacitors, which failed suddenly. I've already replaced it and the hum issue has stopped.