SAD1024 clock noise help

[four_corners]

Hey everyone,

I know this isn't directly a stompbox building question, but it seems right up the alley of some forum members here.

I'm currently working on fixing up an old Korg Trident, and having an annoying issue with the Flanger circuit. I'm getting a very audible clock ticking when I turn the Flanger on. The Flanger totally works correctly, but the clock noise is very frustrating.

From everything I've seen, the circuit has a lot of similarities to the Ross Flanger. The Korg Trident Flanger and Ross Flanger both use a SAD1024, four 4558, and a 4013.

Does this sound like an issue with the SAD1024 or 4013? Is there maybe some way to suppress the clock ticking with a resistor or capacitor running out of one of the SAD1024 pins?

I've attached an image of the Flanger circuit (sorry for the bad scan... its the only schematic available online for the Trident Flanger).

Thanks for the help!

[ElectricDruid]

If it's a regular "ticking" noise that responds to the LFO rate, then it's the LFO that's causing it. That's IC11, bottom-left. If it's a high pitched whine, that's the clock breaking through.

From your description, it sounds like LFO ticking. Are the VR3 and VR4 trims set correctly? It might be that over time it's drifted out of range and that now it's pushing the clock further than it wants to go. That could cause a periodic thump.

HTH

[four_corners]

If it's a regular "ticking" noise that responds to the LFO rate, then it's the LFO that's causing it. That's IC11, bottom-left. If it's a high pitched whine, that's the clock breaking through.

From your description, it sounds like LFO ticking. Are the VR3 and VR4 trims set correctly? It might be that over time it's drifted out of range and that now it's pushing the clock further than it wants to go. That could cause a periodic thump.

HTH

Definitely is a regular "ticking" noise that responds to LFO rate (not high pitched), so I'll try messing with IC11. I'll desolder, add a socket, and try something like a LM358 or TL022? Basically a lower power op amp?

[ElectricDruid]

Yep, lower power op-amp might help.

Definitely have a fiddle with the trimmers first though. Start with the easy stuff. You never know...

[four_corners]

Yep, lower power op-amp might help.

Definitely have a fiddle with the trimmers first though. Start with the easy stuff. You never know...

Seems like it is either the trimmers (like you mentioned), or something else, as swapping to a lower powered op-amp didn't change the ticking, nor did a higher powered TL072 raise the ticking.

The most frustrating thing is that the Service manual gives instructions to trim the clock width and clock center, but they mention monitoring at testpoint 6 or 7 on the Flanger board, and for the life of me cannot find these testpoints! They aren't marked on the top or bottom of the board, or anywhere in the service manual. Any idea from the circuit schematic previously posted where I might be able to probe to monitor the frequencies? Here are a few more images too...

The only other weird thing is VR5 and VR7 don't seem to do anything, and aren't listed in the service manual as to what they do. I find it odd that they do not do anything at all, but who knows.

(PS - Your shop is great! I've bought some stuff in the past).

[anotherjim]

I think I see TP's on the schematic.
TP6 is out of IC7 pin 12.
TP7 is out of IC7 pin 13.
These are the two clock phases and should have identical frequencies so either can be used.

[four_corners]

I think I see TP's on the schematic.
TP6 is out of IC7 pin 12.
TP7 is out of IC7 pin 13.
These are the two clock phases and should have identical frequencies so either can be used.

Thank you so much! Was staring at this so long looking for those. I have just enough electronics skills to get very frustrated in a lot of situations! If I knew any less, I'd never mess with anything, if I knew more, I'd figure some of this out much sooner, haha.

[four_corners]

I think I see TP's on the schematic.
TP6 is out of IC7 pin 12.
TP7 is out of IC7 pin 13.
These are the two clock phases and should have identical frequencies so either can be used.

Hmm, maybe those aren't the TP's? They are both identical, but I'm getting really high frequencies on my scope out of them both, like 500-800 kHz, and it only shifts around a bit when I mess with the trimmers. The weird thing is that they both run directly into the SAD1024, so it makes sense that they would be the test points for the clock.

[anotherjim]

You should get high frequencies but not that high.
You're looking at the output of IC7"b". IC7"a", the lower numbered half, is I think the clock oscillator and will be running pin 2 at twice the frequency of TP 6 or 7 at about 200kHz.
What are you measuring the frequency with? Some test equipment can show an "alias" of the real frequency is set on the wrong time range which can happen if it's allowed to auto-range.
You say the effect works but if the clock really was that fast, I don't think the SAD1024 would work well.
If the clock frequency really is what you measured then it could be something wrong with C46 or the circuit from Q18 down to Q19.

[puretube]

Since the LFO also influences the bias, I`d look there.
IC12 combines the manual bias-adjust voltage from VR6 with the intensity-pots wiper-voltage.
(Have you tried slowly moving VR6 yet? Just by listening to find the sweet spot without distortion, and maybe with less ticking ...
Located next to IC12 is an electrolytic cap C51: does it look healthy?
And the other, bigger caps in the near area of the LFO & BBD as well?
I`m referring to your PCB-image, and the schemo on page 11 of this:
https://www.vintagesynthparts.com/wp-content/uploads/2018/01/Korg_Trident_MK1_Service_Manual.pdf

Found pic of the flanger-pcb (topside), too:
https://static.gearspace.com/board/imgext.php?u=http%3A%2F%2Fwww.pepemusic.de%2Fpics%2Fsbz%2Fkorg_trident_flanger.jpg&h=314ebf7a396a24fc5be3b2f651fcbd3e

[four_corners]

You should get high frequencies but not that high.
You're looking at the output of IC7"b". IC7"a", the lower numbered half, is I think the clock oscillator and will be running pin 2 at twice the frequency of TP 6 or 7 at about 200kHz.
What are you measuring the frequency with? Some test equipment can show an "alias" of the real frequency is set on the wrong time range which can happen if it's allowed to auto-range.
You say the effect works but if the clock really was that fast, I don't think the SAD1024 would work well.
If the clock frequency really is what you measured then it could be something wrong with C46 or the circuit from Q18 down to Q19.

The clock frequency definitely isn't what I'm measuring, as the effect rate has a nice range when actually using it, from very slow sweeping flanger sound at 0, and a fast sort of vibrato/trem phaser sound at 10.

Here are the 2 pins on IC7 I'm using...


I was using the auto trigger on my scope, but maybe it is the alias you are talking about. All I know is there is no way I'm anywhere close to sub 1 kHz regardless of the Manual pot or trimmers position, as zooming out that much looks like a solid gate signal essentially.

[four_corners]

Since the LFO also influences the bias, I`d look there.
IC12 combines the manual bias-adjust voltage from VR6 with the intensity-pots wiper-voltage.
(Have you tried slowly moving VR6 yet? Just by listening to find the sweet spot without distortion, and maybe with less ticking ...
Located next to IC12 is an electrolytic cap C51: does it look healthy?
And the other, bigger caps in the near area of the LFO & BBD as well?
I`m referring to your PCB-image, and the schemo on page 11 of this:
https://www.vintagesynthparts.com/wp-content/uploads/2018/01/Korg_Trident_MK1_Service_Manual.pdf

Found pic of the flanger-pcb (topside), too:
https://static.gearspace.com/board/imgext.php?u=http%3A%2F%2Fwww.pepemusic.de%2Fpics%2Fsbz%2Fkorg_trident_flanger.jpg&h=314ebf7a396a24fc5be3b2f651fcbd3e

Here are a few images of my actual board, as well as a close up of IC12. The caps look fine to me for the most part, unless I'm missing something.








If IC12 is generating the LFO, this is what Im getting on the 2 outputs (pin 1 and 7), they both change frequency with the Speed pot.





I've messed with IC6, turned it down as much as possible before it starts breaking up which does help a tiny bit.

VR2 definitely has the most effect on the ticking, as it seems to be the effect level. I can turn it down to juuust before the effect cuts out, and it reduces the ticking a ton (which is great), but then VR1 isn't enough to bring back the resonance/feedback enough(the effect should self-oscillate around '7' on the dial).

I wonder if replacing VR1 with a higher value trimmer would allow me to bring back in the feedback more while
keeping VR2 down low? When I'm looking at the component side of the PCB, turning VR1 counter-clockwise brings in more feedback, and clockwise removes feedback. Does this mean that a higher value trimmer would just be attenuating the feedback more, and not really help me out? If the latter, R84 looks to be a 47k resistor right before VR1 (which is a 100k trimmer), I guess I could reduce that to maybe a 10K resistor to essentially gain -37k of resistance below the 100k trimmer. Maybe at this point though I'm putting a bandage over a different issue. Another option is just changing the main front Feedback pot (VR9) from a 10K to a 50K potentiometer or something like that.

[puretube]

Looks all good.
Now what do you see at pins 1 & 7 of IC12?
Pin 1 should show some smooth trianglish LFO-wave without any "pips" (DC-jumps).
Pin 7 too, with some added constant DC from VR6.
My suspicion was that some "extreme" LFO-voltage peak creeps into the BBD-input, thereby unbalancing the bias to beyond the sane limits, and thus causing ticks.
The question about the big electrolytics was to exclude an obvious possibility of power-supply "tick-modulation".

[anotherjim]

Auto-triggering the scope is fine, but if you switch the timebase setting either way you may find it snaps into a better, and more believable measurement. Around 5us/div for 200kHz and it looks like you had it on 500ns/div if I'm reading it right. Aliasing is a nuisance on digital scopes - you don't get that with analogue CRT ones.

[four_corners]

Auto-triggering the scope is fine, but if you switch the timebase setting either way you may find it snaps into a better, and more believable measurement. Around 5us/div for 200kHz and it looks like you had it on 500ns/div if I'm reading it right. Aliasing is a nuisance on digital scopes - you don't get that with analogue CRT ones.

Ah, that makes sense, as this is my first non CRT scope, so still learning some of the details. I'll try that out.

[four_corners]

Looks all good.
Now what do you see at pins 1 & 7 of IC12?
Pin 1 should show some smooth trianglish LFO-wave without any "pips" (DC-jumps).
Pin 7 too, with some added constant DC from VR6.
My suspicion was that some "extreme" LFO-voltage peak creeps into the BBD-input, thereby unbalancing the bias to beyond the sane limits, and thus causing ticks.
The question about the big electrolytics was to exclude an obvious possibility of power-supply "tick-modulation".

Since my scope images I posted seem to match your descriptions, do you have any other suggest on where I should look?

Btw, I want to thank you all again for the help, I know everyone has lots on their plate, so it means a lot that you've gone out of your way to help me out with this.

[anotherjim]

A little 'scope thing, nothing to do with the fault, but if you use x10 probes, have you tweaked their compensation trimmers? Only your clock waveforms look badly rounded off. This could either be the CMOS logic switching slow or the probe compensation.

[four_corners]

I thought it couldn't hurt to upload a little audio of the ticking to make sure it indeed is the clock sound.

The recording is a little weird, as the synth is in the garage right now away from an audio interface, so I just held my iphone up to my headphones, lol. I'm not worried about the noise floor you can hear, but the ticking.

I'm messing around with the speed pot to give an idea of the sound.

https://www.whyp.it/tracks/49667/clocknoise?token=VKv0N

[four_corners]

A little 'scope thing, nothing to do with the fault, but if you use x10 probes, have you tweaked their compensation trimmers? Only your clock waveforms look badly rounded off. This could either be the CMOS logic switching slow or the probe compensation.

Regardless of what the scope is showing, if I trim the range and center frequency trimmers to a decent sounding Flanger, would the clicking still be this audible? Neither of the trimmers are at one extreme or the other, and I'm done tons of combinations by ear to just get a good sounding Flanger, some with the frequency range more narrow, some wider, and the center frequency in multiple places, but all this seems to just effect the width of the flanging frequency range.

If the clock waveforms truly are that high (500-800 kHz), would the flanger still sound decent, or would it sound audibly messed up? The actual modulation speed seems to be fine, but this is separate, correct?

Thanks again for your help, I really appreciate it.

[anotherjim]

I just had a thought, if the clock waveform on the scope changes frequency smoothly with the deepest LFO modulation, then there probably isn't anything wrong with that side of things.
I think the thought of others was that the clock for the BBD chip was stalling as the LFO waveform reached a peak of its swing so the BBD stops sampling the audio for long enough to introduce a "black spot" occurring at the same rate as the LFO speed.

The SAD1024 BBD chip can be clocked up to 1.5Mhz and I'm wondering about the numbers in the service information. That seems more like the setting for a chorus effect than a flanger to me. Flangers need to reach a fast clock speed at one end of the sweep in excess of 500kHz.

LFO ticking is most often a problem caused by the fast switch of the squarewave part of the LFO causing a glitch picked up by the audio circuits often via the power supply connections. In old equipment, this can happen when power rail smoothing capacitors have degraded, nearly always these are the electrolytic variety. Many equipment restorers replace them as a matter of course.