Looking for VCA chips for early prodigy! Please help!

[samaepstein]

just wondering if anyone out there has the line on VCA chips for an early model(without the cv's) prodigy. any info on this would be mighty appreciated! the old girl has been shelved for way too long due to an unusably thumpy vca. thanks for your time!!!

[psound74]

I may be talking out of turn since my experience is based on the Model D, but is there an adjustment pot for the VCA on the Prodigy? (On the Mini there are actually TWO VCAs, if you get thumping, there is an adjustment pot for one of them to rebalance; it's in the service manual, so it seems like it might be common...)

[samaepstein]

thanks for the info. i wish i could tell you the answer, but it rests in the hands of a great tech here in the atlanta area. he spends a lot of time on the road with different larger projects that might keep him from being able to find these parts so i was just trying to see what i could see to help the process along. thanks again for the feedback.

[Kevin Lightner]

There is a VCA offset trim in the Prodigy, but no actual VCA chip.
It uses the common 3080 and opamp configuration.
No rare chips. The 3080s were stopped being made in that package as far as I know, but many are still available around.
I still have some as do most techs in the biz.

True- the Minimoog does have two VCAs, but only one will cause popping under normal circumstances.
The other is used by the loudness input jack, which is normally used with a CV pedal as a volume pedal.
There's a very simple way to adjust this 2nd VCA, btw.
Feed an audio signal into the external loudness jack and trim it until the sound disappears.

[samaepstein]

thanks a lot kevin, i have forwarded on that response. i know my tech (marty from atlvintagekeys) has been at this for a long time and probably knows this, but he told me that basically his parts machine and parts resources are coming up dry for the particular part he seeks. these were his words "Hi Sam, I'm still trying to locate VCA chips- Moog went through a number of re-designs of the Prodigy line as parts became unavailable, so there's 4 different versions of the VCA chip in question. These are difficult to find- there are no more unused parts, so I have to locate ones removed from other machines.
-MG"

he has been great with previous repairs so i generally trust what he says. also, i had an mg-1 that i gave to him to try to get in input>filter jack installed, and though there are many examples of how to do this on the internet, he said he was unable to due to the fact that it was an early mg-1 that had some strange wiring shortcuts in it and though he tried several ways, he couldnt get the mod to work. does this sound like anything anyone has run into before? thanks again!

[Kevin Lightner]

Just for reference...

The US model of the Prodigy is model 336A
The export model is 336BX

On Prodigy's above serial number 4160, the VCA balance trimmer is R122.
On lower serial numbers, it is trimmer R129.

To adjust, you basically just want to hear the VCA.
Turn off all sound sources.
Filter wide open, resonance (or emphasis) down. No filter modulation or filter env.
All env controls down on the VCA should cause a thump in the audio output.
You may have to turn up your amp loudly to hear it clearly.

Adjust the trimmer for your serial range above (R122 or R129) for the quietest thump or click.
That's about it.
Also, don't forget to turn back down your amp or you'll be deaf when you turn back on the oscillators.

[samaepstein]

just to make sure this doesnt change anything, i only get the thump as i take my finger off of the note.

[Kevin Lightner]

just to make sure this doesnt change anything, i only get the thump as i take my finger off of the note.

It is difficult to provide absolute info regarding a specific condition I've not seen 1st hand.

To better duplicate this condition, you might try setting the VCA env controls to all knobs down, except for the sustain control, which should be maxxed in this case.

But as a general answer, the VCA balance trimmer should also improve or eliminate thumping upon a key-up condition.
It wouldn't hurt to try anyway.
You might not totally eliminate the thumping, but there's a good chance it will be minimized.
If not, the instrument may require some actual component-level tech work.
The good thing is that these parts are fairly common if it comes to that.
If my health improves, I would be happy to help out, but again, you may only be looking at a minor adjustment and not a true repair.

Also, I visited my site to look at what I have up there for the Prodigy and was embarrassed to find that the schematics I have up there are very bad.
If I can, I will scan both the old and new versions of the original schematics at a higher resolution and will upload them.

In the meanwhile, my best wishes on getting the issue resolved.

[ribnib]

Hi!

I also have a thumping or spike problem. It seems to be affected by Attack and Decay/Release. I can cindof move the "thump on" with attack time. The "thump off" only appears when the key is released prior to the tone decaying out with the decay time.

I adjusted the VCA balance trim, which on mine is R120, and did gain a little more signal to spike ratio. But it is quite menacing leading me to think there is something wrong. I don't have sufficient knowledge to start testing in there, but I thought I'd put it out here and see if any of you have a suggestion. Im thinking its something to do with capacitor c44 which plays a role in the whole ADSR thing. ( i think thats c40 on the prior 4160 models)

Also there is quite a bit of noise when osc. are turned down and only thump is heard. If I hold the "note" it hisses.

Any thoughts would be much appreciated!

[samaepstein]

i too have this noise accompanying my thump. oh may 2010 breathe life into her again!

[ribnib]

Does anyone know if a voltage difference on the power supply could be causing this thump? I'm measuring 11,9 negative and 11,6 positive voltage over the capacitors c39(-) and c38(+) to ground. The audio output is registering a spike of around 0,2 volts. Is this purely coincidence?