Opus 3 troubleshooting

[mrclayton]

I have an Opus 3 that has dead brass & vcf sections.
All sliders and most of the switches have been replaced.
When I turn the brass section on, it makes a click
sound when I push down a key. Any ideas where I should
start with this? I'm handy with a soldering iron if I know
which parts need replacing. Its not obvious to me in this
case.

[Portamental]

I wish I could help but it's hard to diagnose over the web. The Opus3 is a fantastic instrument, worth putting a few dollars into it, if you can't solve the problem.

Good Luck with it!

[thealien666]

Does the "organ" section still produce sounds passed thru the VCF section ?

[mrclayton]

No. Although the organ section does work,
The vcf controls still do nothing. The modulation
section on the left does work.
Everything seems to work okay except Brass makes no sound
And vcf controls do nothing to any sound.

Thanks
Clayton

[thealien666]

No. Although the organ section does work,
The vcf controls still do nothing. The modulation
section on the left does work.
Everything seems to work okay except Brass makes no sound
And vcf controls do nothing to any sound.

Thanks
Clayton

Sounds like a failure of parts of the power supply board, that feed the brass and vcf sections maybe ?

[mrclayton]

Sounds like a failure of parts of the power supply board, that feed the brass and vcf sections maybe ?

When I was poking around on the control board with a multimeter looking for any obvious failed components, there still seemed to be voltage running through the entire brass & vcf sections. If the problem was from the power supply board, would there be no voltage running through the vcf/brass sections?

I noticed you are from Canada as well (thealien666). Anywhere near Toronto? If so do you know a good synth repair guy around here? There is one guy that I've heard of but apparently he is always backed up with work.

[zaiusz]

Temporarily shorting pin 8&9 on U3 (40106) should trigger the brass if there is a component causing the ic to be stuck low. It could also be just a dead U3 as well. The heart of the brass control and triggering is the 40106. Really could be anything though... even a dead op amp in the mixer section.

[thealien666]

I'm in the Montreal area, sorry.

It's strange that the VCF section doesn't seem to work... This is a relatively simple audio circuit.
You said that all the sliders and switches have been replaced, but did the synth ever worked correctly after these replacements were installed ?

[zaiusz]

mrclayton, check your pm inbox.

[kaseijin]

Sounds like you need to start replacing IC's on your control board. Truthfully, they have a limited lifespan anyway, and should probably just all be replaced. The 4000 series logic IC's are known for failing, though when I had a similar issue with the VCF, the culprit turned out to be one of the op-amps.

Mouser is a good source for parts.

[mrclayton]

I replaced the 40106 (u3) chip and it did nothing.
There are other chips I am suspecting after going over
the circuit description in the service manual.
It is very detailed and explains what each part does.
I am waiting for more parts that I ordered from Jameco.
They were the only site who seemed to have all the parts I need in stock.
To be safe, I ordered replacements for most/all of the opamp chips, plus
the voltage buss switch chip u4. Altogether 13 chips and, while I'm at it, a replacement
capacitor for the power supply. If none if this works, I've only wasted around $20 and I'll bring
it to an experienced tech.

[mrclayton]

All along I have suspected the opamp chip for the brass section (u25). The first supplier I ordered from for the 40106 chip didn't have any in 3080's stock. Now that I found some, I changed u25 first and tested the keyboard.
Everything seems to be functioning now, brass section and VCF.
Since I ordered several of them, I might as well change them all to avoid future problems.
I'm super excited. I have had this keyboard for about 4 years only half functional. I was missing the best part.
Is there any way to easily add trigger & cv outputs to control other synths?

Next I want to rebuild the wood panels with maybe walnut, since mine are pretty beat up.
Has anyone attempted this before? The sides look like they're glued on pretty good from the inside
I'm getting some good Ideas for that project at http://www.synthwood.com.
Thanks for everybody's advice.

[thealien666]

So it turned out to be the OTA buffer amp at the output of the filter... Nice troubleshooting.

Since you have the synth opened, it would be a good idea to replace electrolytic and tantalum capacitors (especially in the PSU). They're both prone to failure with time, and this synth is 30 years old.

[EMwhite]

Wow, I'm psyched for you. I an Opus 3 and love it. But it's a bit flakey and on it's way out. Despite my cleaning out the sliders and painstakingly removing the evil slime from the boards with q-tip and alcohol, metal scraping tools, blow torches and dynamite, they are still just a bit rough and have some static.

So at some point I'll need to buy the 'kit' from that guy in Asia Pacific and do a ton of soldering. (I assume that's where you got your sliders or did you find an alternate source on Mouser, etc? I wish I had a list of actual part #'s).

But my Power supply is going bye-bye, I have to power it up then quickly off and on again in order to get it active. This works every 2 out of 3 times at the moment. Likely to get worse and likely the caps. But I'm really glad for your triumph, you must be pleased.

The Opus is among the most under rated Moogs out there. Very unique and the Chorus is wonderful.

Another MoogerPerson turned maker is building up a Low-Pass, I might just look into building up a Chorus from the Opus schematic and drop it into an MU panel. Well, now I'm rambling...

[thealien666]

...I might just look into building up a Chorus from the Opus schematic and drop it into an MU panel. Well, now I'm rambling...

Hahaha, good luck on that... The chorus circuit uses 3X SAD512D analog BBD chips which are long discontinued. The circuit might be modified to run with 3X MN3007 with doubled clock rates... Well now, I am rambling also...