SAD1024 clock noise help

[four_corners]

IMHO, there's a very narrow range of good settings for a string ensemble triple chorus, at least for use on a string section. There may not be much setup information for the chorus as the LFO always runs so the BBD clocks are a moving target but the LFO speeds should be measurable. Is it the same chorus as the Korg Delta with a 4069 chip in it? Post the schematic and we can cogitate.

Basically the ensemble tri chorus in my Trident isn't balanced with the 3 choruses evenly spaced/phased, so it sounds uneven. It is probably the same chorus that is in the Delta, as this is a 4069 > 3- 4013 > 3- MN3004. To be honest, it looks very very similar to the BBD circuit for the flanger. There are trimmers for each 3004 chip, but it is just to bias out any distortion, not the actual phasing.

It seems like placing 3 trimmers either somewhere on the 4069, or somewhere around the dual transistor before each 4013 to change the clock speed would allow me to balance them to 0, 120, 240 degrees from each other.



So yeah, actual pots wouldn't really be needed if I can just add trimmers that can get me to an even ensemble.

[four_corners]

IMHO, there's a very narrow range of good settings for a string ensemble triple chorus, at least for use on a string section. There may not be much setup information for the chorus as the LFO always runs so the BBD clocks are a moving target but the LFO speeds should be measurable. Is it the same chorus as the Korg Delta with a 4069 chip in it? Post the schematic and we can cogitate.

(ALL IMAGES IN THIS POST ARE SCHEMATICS OF THE POLYSIX, NOT THE TRIDENT. TRIDENT ENSEMBLE SCHEMATICS IN PREVIOUS POST)

Looks like the Korg Polysix has almost the same circuit as the Trident for the ensemble, except the Polysix does have a trimmer before the dual transistor.

Here is the second Polysix delay stage...



Basically, right before the first base of the dual transistor it has a trimmer that wipes between ground and -15V. It also has an extra 4558 circuit coming into the dual transistor that is actually a Speed/Intensity pot, but that feels like a bigger mod than just adding a few trimmers to actually make the ensemble sound correct.

Here is the full Polysix effects schematic if you are currious...

[anotherjim]

x6 CD4069U inverters forming x2 ring oscillators of x3 inverters each.
One oscillator will run slow @0.6Hz or thereabouts. The oscillator runs faster around 6Hz. These speeds are the "classic" Solina settings.
Each oscillator creates x3 sinewave output that differ in having 120deg phase shift between them.
The slow & fast LFO outputs are mixed together with resistors into the dual transistor control. I would expect the larger mix resistor is carrying the faster LFO as this needs less depth. That 750R resistor to 0v could be a trimmer. 2k or so.

The frequencies of the LFOs depend on  R & C values in the ring circuit around all three inverters. The high-value feedback resistors are for auto-biasing the inverters to behave as linear amplifiers but also set gain with the input resistor. The feedback capacitors and input resistors set the frequency. Too much gain & it will distort. Too little and the ring fails to oscillate.
There are "TP" testpoints for each oscillator so the service manual might have something to say.
You can "tweak" the LFO frequency with a trimmer pot as a variable resistor in one of the inverter inputs, but the range will be small.
A practical issue is that with x3 BBD chorus units, it's difficult to set up by ear as they tend to mask each other.

[four_corners]

A practical issue is that with x3 BBD chorus units, it's difficult to set up by ear as they tend to mask each other.

After messing with it a bunch, I came to this realization as well. I was able to mute some of them by trimming the bias for one of the chorus' to "mute" it so it would just be the other two for the time being, but even then it was hard to determine what really needed to be messed with, as each chorus is already 2 oscillators before even dealing with the phasing. So I'm just going to leave it as-is as far as that.

One last question (I promise!).

If I wanted to add a "mix" knob for the Ensemble, so that I'd be able to dial in the amount of wet/dry, would I essentially need to add a pot somewhere along this red path (I would guess 1 lug and the wiper act as the variable resistor, and the last lug to ground maybe?)? Not exactly sure where on this path it should be placed, but I can see that the audio signal splits in 2 before one side goes to the audio input of all the MN3004's and the other goes In/Out of one of the two SA571's, but does not immediately mix with the remaining 3 outputs (the 3 MN3004 delay signals). Seems like the dry signal and choruses mix after that at the k30a transistors/4558 opamp (I think).

Thanks!

[anotherjim]

It seems there are two sounds that Q28 switches between using the K30 FETs, Vibrato or Ensemble. It seems like you can't have both. You can reduce the intensity of Vibrato for a "dry" sound, but not the Ensemble.  A further wrinkle is that the Vibrato always feeds (and that only) the Ensemble input.
The easiest idea is to replace the JFET inputs to the mixing opamp with a balance control. Essentially a pot with wiper lug to 0v and an end lug to each feed. Q26 and Q27 would be removed and Q27 drain-source connected with wire. The Balance pot outer lugs go at the left side of R206 and R208. The pot would be linear 100k (or W taper if you can get it). BTW, keeping the JFETs and controlling their gates with a single pot to sweep between the sounds is attractive, but not so easy to arrange a proper, smooth control.

Probably the least invasive way to get at hidden sounds in vintage gear is to take direct connections outside the box either by fitting extra jacks in the back or a multicore to a breakout box. That approach also offers the possibility of external access to the effects and filters.

Oh, and those MN3004 chips are something else you really don't want to fry.

[four_corners]

It seems there are two sounds that Q28 switches between using the K30 FETs, Vibrato or Ensemble. It seems like you can't have both. You can reduce the intensity of Vibrato for a "dry" sound, but not the Ensemble.  A further wrinkle is that the Vibrato always feeds (and that only) the Ensemble input.
The easiest idea is to replace the JFET inputs to the mixing opamp with a balance control. Essentially a pot with wiper lug to 0v and an end lug to each feed. Q26 and Q27 would be removed and Q27 drain-source connected with wire. The Balance pot outer lugs go at the left side of R206 and R208. The pot would be linear 100k (or W taper if you can get it). BTW, keeping the JFETs and controlling their gates with a single pot to sweep between the sounds is attractive, but not so easy to arrange a proper, smooth control.

Probably the least invasive way to get at hidden sounds in vintage gear is to take direct connections outside the box either by fitting extra jacks in the back or a multicore to a breakout box. That approach also offers the possibility of external access to the effects and filters.

Oh, and those MN3004 chips are something else you really don't want to fry.

Ah okay, that makes sense.

So basically this...



But what about the diodes that were after the JFET gates? Do they also just get wired to those resistors (D12 to left side of R206, and D13 the left side of R208)? Or are they not needed now? Seems like the mechanical switches for the "Ensemble Off" and "Vibrato Off" would be effected otherwise.

[anotherjim]

These diodes are there to make the JFET switches behave, so without they are redundant but can stay there.
The 100k R56 should probably go, its function pretty much exists via the new pot.
The existing selector switches aren't going to do anything now.
A full ensemble chorus doesn't really benefit much from a dry mix. Without the ensemble, the raw "string" sound with some attack and release can make a nice "spook organ" sound that's different from anything else. Some cut-down ensemble units only had 2 BBD so did add a dry mix. They usually made up for it by feeding stereo so you get a bigger sound from it.
If I was making one, I'd opt for the balance pot since it's simpler, a bit more versatile and cheaper than this switching method but you already have the switches. It might be better to leave it be - but it's your toy.

[four_corners]

These diodes are there to make the JFET switches behave, so without they are redundant but can stay there.
The 100k R56 should probably go, its function pretty much exists via the new pot.
The existing selector switches aren't going to do anything now.
A full ensemble chorus doesn't really benefit much from a dry mix. Without the ensemble, the raw "string" sound with some attack and release can make a nice "spook organ" sound that's different from anything else. Some cut-down ensemble units only had 2 BBD so did add a dry mix. They usually made up for it by feeding stereo so you get a bigger sound from it.
If I was making one, I'd opt for the balance pot since it's simpler, a bit more versatile and cheaper than this switching method but you already have the switches. It might be better to leave it be - but it's your toy.

Knowing the frustration I already went through with the Flanger, I think I’ll take your advice and leave it as is for now. It is already a pretty unique synth being able to mix the strings, brass, and synth section.

Thanks again to everyone here that helped. It’s funny how much more long term knowledge you gain by messing something up! So long as it isn’t so messed up it isn’t fixable

[anotherjim]

That synth comes from a fascinating time before all digital took over. You're lucky to have it I think. They and their like were the stars of a million porno movie soundtracks, or so I bin told. Oh, yes, yes, yes...

[four_corners]

That synth comes from a fascinating time before all digital took over. You're lucky to have it I think. They and their like were the stars of a million porno movie soundtracks, or so I bin told. Oh, yes, yes, yes...

Haha, I wouldn't doubt it. It has a very "majestic" quality to it.

Of course, I go to put on all the knobs and attach the pcbs last night, I play it a bit, go to bed, wake up, and the thing won't turn on, ugh. Looks like the +15V regulator happened to crap out on me. I feel like it is just a coincidence and not that I fried some crazy thing, as it is a 42 year old synth and I was turning it on and off a lot over the last week.

The negative regulator is fine, it is just the positive. It is reading +25V on the input, and a handful of mV on the output.

I can barely find anything about the specific regulator, but I assume I can just use a 7815. Might as well swap the negative regulator while I'm at it.

[four_corners]

nevermind! got it.