Korg Lambda repair and modification

The Korg Lambdas I have encountered have each exhibited problems:

Fault: A very thin, almost silent, Chorus preset; all other sounds were fine.
Solution: A loose capacitor in the filter bank for that preset, which was a simple solder job.

Fault: A loss of Percussion presets and a thinning of the Ensemble presets in Normal Octave mode; in Up Octave mode everything was fine.
Solution: A faulty CMOS switch (IC1 on KLM-184, a 4066) which failed to pass the divided-by-two clock to the first TOG, thereby effectively muting the first oscillator in that mode.

One case proved more problematic, however, and I have documented the process for reference.

Two main jobs were carried out on this machine:

  1. Tidying up a previous bad repair job
  2. Troubleshooting a bad oscillator

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Tidying up a previous bad repair job

The main PCB in the base of the Lambda is home to the TOGs, divider/keyer circuits, and associated per-key envelope generators. The lines of diodes/resistors/caps that form the envelopes can be seen quite clearly stretching along the board, and the nine divider/keyer ICs can be seen poking up, mounted vertically on daughter-boards. Each of these smaller boards holds a single divider/keyer IC and some zero-Ohm jumpers.

Korg Lambda KLM-185 main board

Korg Lambda KLM-185 main board

In this example, two of these daughter-boards had been damaged – someone had gouged away the tracks on the PCBs and bridged the gaps with wire, but solder was everywhere and the PCBs were in less than ideal condition. Rather than simply tidy up the existing mess, I chose to replace one of those boards with a fresh PCB. The manufacture was carried out by Jim Harris, who kindly documented the process in the video below.

The results are very satisfactory. I always install replacement ICs in sockets for ease of future repair. Turned-pin sockets are preferable as I find they assist in seating the IC firmly, but here a flat-pin type is used as it was the only one to hand.

Korg Lambda KLM-182 front

Korg Lambda KLM-182 front

Korg Lambda KLM-182 rear

Korg Lambda KLM-182 rear

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Troubleshooting a Bad Oscillator

When I initially tested this particular Lambda, it was exhibiting thinner sounds than expected. It became quickly clear that one of the oscillators was not sounding. The beat-frequency indicator LED B was locked in one state, revealing the culprit to be the third oscillator (Miii, as per the schematics).

There are multiple possible causes for a dead oscillator in the Lambda: faulty clock (including its output buffer logic), faulty octave switching, faulty wiring and dry/cracked solder joints/tracks, faulty TOG, badly seated ICs, corroded sockets, dirty switches, or even failed capacitors pulling down the power rails. There are probably more, but these are some options that occurred to me during testing.

In fact, everything seemed OK except the TOG itself, so I ordered a replacement vintage part and installed it. Despite initial success (all three oscillators worked, hurrah!) the new-old TOG failed after a few minutes. A pattern emerged of it working for the first few minutes after switch-on from cold, but then failing – and the machine had to cool down again for the TOG to function once more. It occurred to me there could be another faulty component bringing this TOG down, but isolation of the TOG from the other parts and testing it again showed the TOG itself to be at fault.

These ICs have been obsolete for many years, and are expensive to replace, if they are to be found at all. Stocking up with a batch of S50241s, of unknown provenance and with unknown remaining lifespans, is not a viable option.

How not to build a Top Octave Generator

I initially resolved to build a replacement using CMOS logic to fulfill the same function as the TOG – effectively it is a logic device, taking a clock signal and dividing it by fixed amounts to give outputs such that the pulse trains on them represent the frequencies of an octave scale.

Although the frequencies outlined on the S50240 datasheet are specified to give a certain pitch for a certain frequency clock, the Lambda’s keyboard begins at F, not C. To minimise the circuitry used, the TOGs are clocked so the outputs are pitched to match the octaves on the keyboard; thus, the nominal C output becomes an actual F, etc. This requires a higher speed clock than the recommended 2.00024 MHz, somewhere closer to 2.5 MHz to raise the overall pitch by a few tones.

The need to divide by 3-digit numbers in the low hundreds, some of which have no common factors, suggested the 4040 CMOS counter. My initial sketch is presented here for reference only, and in no way represents a functional circuit:

Erroneous TOG replacement using 4040 CMOS

Erroneous TOG replacement using 4040 CMOS

The clock, buffered by a 40106, pushes the 4040 along on each positive edge. The 4040 is a 12-stage (divide-by-4096) ripple counter, which means it has enough stages to divide by several hundred, but suffers the disadvantage of long propagation delays – each stage only flip-flops after the previous one, so there are several consecutive toggles to go through before a stable and desired output is obtained. The principle of using a clock divider is that when the correct number of clock pulses has been received (as derived by AND-ing together the correct combination of output bits), the counter is reset and an output pulse sent. This triggers a flip-flop to give a 50% duty-cycle output at the correct frequency. In theory, the result is a pulse train with a frequency of clock/n where n is the divide-by for a given pitch.

Experiments were sadly not successful. To illustrate one example, the note A requires division of the clock by 358. The total propagation delay using the 4040 to get through enough cascaded toggles is 65+(30*7) = 275ns. As the clock period is approximately 200ns, it can be easily seen that this is too long. In practice, I found this particular example gave a note around E – the incoming clock pushes the 4040 along before that count’s toggles have settled on the correct combination, erroneously triggering the next division faster than the desired division can be completed; a partial set of toggles is carried out and spills over to the next count – the resulting output is therefore at a higher frequency than desired.

Although there are ways around this, pragmatism demanded a tighter solution.

How to replace an obsolete Top Octave Generator the easy way

Time constraints eventually led to the purchase of a custom-built TOG replacement from FlatKeys, which is a small SMD circuit housed in a compact enclosure, and connected to the original IC position by a ribbon cable. It came configured as a 50240, which is identical to the 50241 except for the output pulse width – not an issue here as the Lambda further divides the TOG outputs at the divider/keyers. The enclosure fits nicely down the side of the main PCB, and the ribbon plugs into the TOG’s socket. It works perfectly, and there is no obvious difference between the original and replacement tones. It responds to pitch bend and modulation as expected. Though it would have been nice to make an entirely DIY circuit myself, simplicity proved the greater benefit.

Korg Lambda FlatKeys TOG replacement

Korg Lambda FlatKeys TOG replacement

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4 responses to “Korg Lambda repair and modification”

  1. Bigg Dady says :

    Excellent explanation with many good work is there.Thank you and keep your work !

  2. agatonsachs says :

    Thanks for the in depth explanation of the Lambda and the extensive work you’ve done on the repair issues !
    I have just bought a Korg Lambda, that was a dream for me in my teens (some 35 years ago) but sadly could not afford in those days. I was just baffled by the rich sound and the mesmerizing pw/chorus magic that you couldn’t put your finger on what was happening. Anyway, the Lambda us on the way to me via usps and I just wanted to know what to look for first when testing the keyboard except for listening of course 😉 Is it especially to listen for any thinning of sounds as you mentioned? Du you recommend a capacitor replacement like in the power supply or overall? Or should I just let it be if it seems to work?
    In the retro hifi dorums there are different viewpoints, some recommend cap replacement (because of a duller sound with age) and some say let it be as long as it works cause the old stuff had better quality.
    Thanks,
    Andy in Sweden

    • synthnerd says :

      Hi Andy, glad you like the page! My advice would be that if it works, just let it be. If any of it develops a problem, then fix it, but my experience with things like cap replacement is that they’re a waste of time unless something is already wrong.

      To test it, I’d leave it switched on for a few minutes to warm up, then go through each of the sounds in turn, making sure the controls all work and that the oscillators all make sound. The Percussive sounds only use one oscillator, and the Ensemble sounds use two or three. However, when you mix sounds from both sides, the Ensemble sounds lose the first oscillator to the Percussive sound, if you get my meaning. So to properly test the Ensemble sounds, turn off all the Percussive sounds first. Make sure the three tuning pots all have some effect. You can hear the three oscillators quite distinctly when they’re detuned.

      Sometimes the large switches get dirty and don’t connect properly. Give them a few clicks back and forth. Also test all the different audio outputs. If you use headphones, you should hear percussive and ensemble panned to opposite sides, until you put the chorus/phase effect on, which makes everything stereo.

      Hope that helps! Good luck and enjoy!

      • agatonsachs says :

        Thanks ! I have another question abot the Lambda.. can I email you? I have a pocture I’d like to add..
        My modulator joystick doesn’t spring back to the middle position, just in the left – right direction i e pitch bend. Not in the chrous phase direction if you see what I mean..

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