Korg Lambda chorus/phase trouble

[ImperatorDX]

Thanks rhino, I've already ordered 2SK30A-GR transistors from Hong Kong (they are hard to find) but the delivery can take up to a month. I'll probably also order the caps from a quicker shop and will replace whatever arrives first.

[ImperatorDX]

The caps have arrived. The only thing is that the 1uF/50V turned out to be smaller in physical size than the originals. Does the size matter or should I go ahead?

[rhino]

capacitors have gotten smaller over the last 20 years, primarily because of improvments in insulation (between the layers). Look inside a radio or tv from the 1950s or 60s.

[Mooger5]

Don´t forget to respect the polarities (sorry if you already knew about it but it´s always good advice).

[ImperatorDX]

Thanks, I appreciate any advice.

[ImperatorDX]

Haha, this chorus/phase unit is really stubborn. I replaced the capacitors C205, C206 and C211, paying attention to the correct polarity, but it didn't impress my Lambda much. Still the same, I can hear the effect going on the Ensemble part and it still sounds beautiful but very, very subtle compared to the chorus on the Perc section. I hope the Q7 will be its last bastion

[ImperatorDX]

I replaced the Q7 transistor but still no improvement. I tried not to fry the component and gave it some time to cool down between the takes. I touched it with the hot iron only for half a second at a time. The effect is audible until I detune the oscillators so that they overpower the faint output of the chorus/phase circuit. I think the problem lies somewhere else but I don't have enough knowledge and equipment to diagnose it.

Anyway, I think the chorus/phase for strings on this synth adds more vibrato than the actual chorus or phaser and using the oscillators themselves yields enough gorgeous phasing and chorusing sounds that there is no need to risk causing any accidental damage by further tampering with the interiors, unless it's something easy to do.

I'm also getting an Electro Harmonix Flanger Hoax to go with the Lambda for more flexible and spectacular analogue modulation effects. Thanks for your help.

[Mooger5]

That´s too bad, but don´t give up so soon. The more hypothesis you eliminate the closer you get to the solution. Have you learned how to use a DMM?

Rhino, what if there´s not enough voltage/current reaching Q7 for it to fully open? That would explain the faint output. Aren´t CMOS chips known to partially fail (where some multiple sections work, some don´t)? Now that we know Q7 is good, IC7 or IC8 might be the culprits. What do you think?

[rhino]

Good call. Also, we need to look at more of the schematic: there must be some kind of 'mixer' section to combine the signals after the chorus.

[ImperatorDX]

I've got the schematic of the guts after the chorus/phase:

[rhino]

Great. Thanks.
Next official guess is IC16.
AN829 is a dual tranconductance op-amp. Good luck finding them. Some newer IC may be interchangable.
EDIT;
One place that seems to have the AN829 in stock is http://www.utsource.com

Three others of the same kind (did not bother to look at specs, pin-out, etc.

CA3280
LM13700 - different pin-out
NE5517 - different pin-out

EDIT#2; Casio and Roland used another style "BA-something" that was an inline package. May be able to solder new longer 'pins' to one and bend them to fit the 14-pin outline of the AN829. Aint vintage FUN?????

[ImperatorDX]

Thanks rhino. Yeah, vintage at its finest Still, I totally love to play this old Lambda. Currently there are some An829s on ebay, shouldn't be a problem.

[Mooger5]

A common fault in vintage equipment is dry solder joints A quick reflow will fix it. Also check the wiring around R109 (the dual pot for the ensemble section). And don´t forget the replacements for IC 7 & 8 (CD4013 & CD4069), and sockets. Those are cheap and easy to find.

Good luck.

[ImperatorDX]

Yes, I'm going to try replacing the other ICs you mentioned. I did the reflow of all joints in the chorus/phase board when I changed the Q7.

Now I've got another problem. It's been there since I got it but I thought it was normal. The strings envelopes don't work properly when all the sounds in the Perc part are switched off. They retrigger and cut off like the paraphonic VCA/EG. I think it's only the oscillator that joins in when I deactivate all the Perc sounds. It is especially annoying with long attack/release times. When I switch any of the Perc sounds on, the strings get thinner but the envelopes work all right, so I play with the Perc volume all the way down and one of the perc tabs switched on.

I'm trying to figure out the schematics for the VCA and EGs but they are discrete and there are two of them for each key and the problem is rather global.

[edit]

I invetigated this EG thing today ad it seems really complicated. It actually isn't simple envelope retriggering problem. Depending on the attack and release setting I could get perfectly good polyphonic sound without retriggering, but with some other settings there is a sound similar to the paraphonic effect of the brass patch. Maybe this is due to some aural effect rather than the technical glitch. Damn, it's hard to put finger on what's wrong... if it's actually wrong.

Strangely enough, I even managed to control the brass decay time which technically shouldn't be possible on Lambda. When you engage brass + choir with maximum attack time and switch on any tab in the Perc section (lower the Perc volume down all the way), you can use the Perc decay to affect the brass in Ens part.

Also check the wiring around R109 (the dual pot for the ensemble section).

This may be a good shot. I purposedly worked the Ens volume slider a little bit and the chorus/phase suddenly became more audible. Yay! Thanks for that tip. I need to have a look inside soon.

[Mooger5]

Sorry, I´ve never played a Lambda, so I don´t "see" much of what you´re describing. If it´s not a retriggering problem, maybe it´s the unit that has single-triggering? If it´s the case it´s a good feature. The user manual must say something about it.