Moog Prodigy Filter not working

[Elgo]

Hello all,

I received a Prodigy as part of a trade - the original Prodigy without the CV/Gate jacks.

The fuse was blown, and I replaced it. I've been able to get it to the point where everything works, except the filter.

The only effect the filter controls have on the sound is a volume increase/decrease, but no resonance.

Has anybody dealt with this problem before? I'm assuming the filter section was damaged by the blown fuse. Should I break out the multimeter? Will I need to purchase a new Prodigy filter?

Thanks for your help,
Elgo

[Kevin Lightner]

There's not really a single part that is the filter.
It's made up from several separate parts.

When resonance is out, it may be the resonance (emphasis) pot, an opamp or OTA chip or something physical like a broken trace.

[Elgo]

A response from Kevin Lightner himself! Thanks for the info, Mr. Lightner. I am clearly no electronics expert, but I am going to do my best to get this beast back up and running eventually.

One other problem is that the ground is broken off the power cable - I should probably replace the plug with a grounded one before I continue. This is one thing I've done before on my Six-Trak.

Could I damage the Prodigy by continuing to plug it in without a ground?

[Kevin Lightner]

A ground is safer and may result in quieter operation, but electrically and sonically it may be fine without one.
Sometimes they're there for sound electrical reasons and sometimes they're there to adhere to laws and regulations.
I don't think you'll damage the instrument without one, but there's the possibility of shock hazard or even damage to the sound system.
My rule is: if a unit was originally furnished with a 3 prong grounded plug, it's probably best to have one.

[thealien666]

If there is no resonance (emphasis) at all from the filter, then it's probably another half blown 4558 op amp (this part is known to fail often in old synths). Replace the IC identified as U16 on the main board, and there's a good chance that filter emphasis will work again...

[Vsyevolod]

A three prong grounded plug exists to circumvent the possible danger of the neutral wire coming undone somewhere in the unit. The third wire, known as 'ground' will then send the hot voltage to ground rather than through you to ground. So yes, as Kevin said, it's not going to affect the sound and especially not the filter though it's a very good idea to fix it for the sake of safety.

Stephen




.

[Elgo]

Many thanks to everyone for your input and advice.

I will definitely replace the plug with a three-prong before I continue.

I found a 4558 dual op-amp for the Prodigy at Syntaur, so I will order it. Should I check the PCB traces with a multimeter before I replace the op-amp?

As you can probably tell, I'm not too well-versed at synth repair, although I have done a few things here and there with a soldering iron on my Six-Trak and Opus III.

[thealien666]

Might I suggest this place instead ? (much less expensive)
And you might also want to put a socket in, first (if the original chip is not already on one). Much easier to install new chip without heat damage, with a socket.

http://www.digikey.com/product-detail/e ... -ND/658330
http://www.digikey.com/product-detail/e ... -ND/821765


EDIT Oh, and this might help if you don't have one already. It's an inexpensive desoldering iron. Works great, for the price.
But, as Kevin wisely said, it might be something else wrong with your Prodigy. But replacing the 4558 isn't too costly and might do the trick. And yes, checking traces on the PCB is always a good idea. Especially since the fuse was blown, there might have been a short somewhere and damaged traces as a result.

http://www.radioshack.com/product/index ... Id=2062731