Korg Lambda chorus/phase trouble

[ImperatorDX]

Recently I bought a Korg Lambda. It is in nearly pristine condition (for a synth from late 70's) apart from the chorus/phase section which is giving me some problems.

Basically, there are two sections on the synth: percussive and ensemble. They can be routed through the chorus/phase independently. The effect circuit seems to work fine with the percussive section, resulting in a clear and deep phasing and chorusing effect accompanied by a noticeable boost in volume.

Unfortunately, when I switch the ensemble button on, the diode lights up but the effect on the strings and other ensemble instruments is very faint, almost inaudible. When I move the joystick, I can hear the change in speed of the chorusing/phasing but the effect is very weak and quiet. There is also no boost in volume when I engage the effect in the ensemble section.

I sprayed some contact cleaner inside the switches but it didn't change anything. I also removed and reconnected the white sockets with cables - it still works alright on the percussive section and fairly weakly on the strings. The electronics in the chorus/phase section (and inside the whole synth) look very clean as if it had just left the factory.

Do you think it could be a simple problem or something more serious like busted BBD chip (MN3010 dual 512-stage low noise 14d)?

The sound is beautiful and can get really fat even without the internal chorus but it would be nice to solve this problem.

Chorus/phase unit schematics:


Here are some pics of the chorus/phase section in question, I'd be grateful if anyone familiar with vintage electronics could have a look if there aren't any apparent issues.

Chorus/phase switches


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Chorus/Phase circuit








Thanks

[rhino]

thanks for including the schematic.

you'll need to do some audio tracing. Op=amps IC2,3,5,6 are all suspects. Down the list are C205 and FET Q7.

I've never seen a BBD fail on its own, so that's last of the list.

[ImperatorDX]

Thanks for your comments, rhino. During a closer analysis of the photos I took, something unusual drew my attention. It looks like two resistors near the BBD chip are trying to initiate a separatist movement or something. Have a look at these photos:









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I marked the two connections that I believe should be there but for some reason they are broken. What do you think? Is it normal? Every other resistor in the synth except these two is connected on each of their two ends (that's how resistors are supposed to work in a circuit, I suppose). Do you know any reason why the connections here are cut off apparently in two places for each resistor? In both cases a whole section of the lead is missing. Is it some kind of a failure mode or does it look like human intervention? If done by someone, is there any reason for that? Also does the dark discoloration on the flat yellow capacitor in the area look suspcious? I apologise for the crossfire of questions but I've got very little experience with electronics.

[edit] I've just determined that the broken parts mentioned above are two 750k resistors A and C. The interesting bit is that resistor A is in the Percussive chorus/phase signal path which works perfectly well. Since the connection is broken in the same way for the Ensemble section and the result is weak effect output, it looks like something else must be affecting the chorus. I really wonder what the purpose of those two symmetrically broken resistors is.

[rhino]

on the schematic, they are shown marked out. looks like they were part of the switching circut that drives the LEDs
seems to have no part of the audio line. still suspect one of the op-amps.

[ImperatorDX]

Thanks for your opinion and suggestions. Could you explain what you meant by audio tracing, please? Those JRC 4558DV op-amps seem to be shared by the Percussive chorus/phase section as well and they process the signal from it correctly, but who knows...

[rhino]

also called signal tracing. you need a small amp and a probe to pick up signals from different points in a circut.

the dual op-amps can fail on either side independently... one side of each handles the precussion, the other side the ensemble.

do you have any friends with electronic knowlege?

[ImperatorDX]

Thanks rhino. I don't know anyone knowledgeable of electronics but I believe I can learn whatever it takes to fix this problem on my own and with assistance of the helpful chaps like you. As I now know what audio tracing is, I'm going to learn how to do it (don't want to electrocute myself or destroy the working synth) and will give it a go. Cheers

Just in case, I hope those op-amps aren't some isotope of unobtainium.

[ImperatorDX]

While waiting for the new op-amps to arrive, I've found a photo of a different Lambda. Here the chorus/phase circuit looks different from mine. Unlike in my Lambda, the two resistors I mentioned before look complete here and also I've got a black lead soldered to the resistor R206 [edit 0 Ohm resistor not R206] near the 'J4' inscription, which is not present in this Lambda. The black cable in mine leads to the green PCB board with pots and sitches from the Percussive section.

Another Lambda




My Lambda



Does anyone know what could be the purpose of that black cable? Also the two cut off resistors are still boggling me.

[edit] I think it is a substitute for a jumper. 'J4' suggests that too.

[Mooger5]

Also the two cut off resistors are still boggling me.

Those cut off resistors are in paralel with R107 and R108. Notice A is in the Percussive section and C is in the Ensemble section. The Percussive section is running fine, so neither resistors seem responsable for any malfunction. They could be a later "refinement" to the bias circuit.

What is the light-blue component near D3?

[ImperatorDX]

I think this light blue component is capacitor C2.

[Mooger5]

OK, C2 is common to both Perc and Ens but nevertheless it´s still an old tantalum cap. And it´s attached to the negative rail of the power supply. Those can cause shorts when they fail, so I´d replace it with an electrolytic 1,5uF/25V, or a new tantalum.

[ImperatorDX]

Hmm, it seems like a good idea but I'll leave it for a while if it works fine. There is a little caveat in this project - I've got near zero experience with electronics. I'm learning in parallel to trying to solve this problem. The op-amps have arrived, now I'm going to practise soldering and desoldering on whatever old broken electro-c**p falls into my hands before tackling the synth's circuitry.

If the problem persists, I'll do the capacitors.

[ImperatorDX]

Yesterday I replaced IC 2,3,5 and 6 with TL072 dual op-amps. It was a moderate success. Most importantly, I managed not to break anything and the desoldering and soldering process went smoothly. My joints look similar to the original ones, which boosted my confidence with the soldering iron, desoldering pump and solder wick . The sound of the Ensemble chorus/phase has improved a bit so that the effect is now usable and it adds some nice extra movement to the strings.

However, it still isn't the same as the in-your-face chorus of the Percussive section. When engaged, it doesn't increase the volume of the Ensemble section in the way it does with the Perc part. Could it be the result of some old capacitors or would you suggest starting with the FET Q7?

[nathanscribe]

Hint: when you replace ICs, not just op-amps, any of them, you may be better off putting them in sockets - presuming there's room. Solder the socket in, and then you can pull and replace the chips much more easily.

[rhino]

sounds like you're on the right track.
next change Q7
then, if still no joy, chaneg caps C205,206 and 211

EDIT: i think i'd cange the caps first