Chorus 690 clock distorted output, not pulse

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Zack-stomp
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Post by Zack-stomp »

Hi. I’m trying to fix a Chorus 690 that came to me broken. The main problem (until/unless I find more next) is that the clock stage doesn’t output properly. Instead of a clean pulse wave that the SAD 512 wants, it outputs a nearly off signal with blips of voltage.

I’ve tested most every component in this area and it all looks good. The best I can tell is that the CD4001 works properly, but the incoming signal is too high? I see a reverse sawtooth wave that peaks instantly at about 13VDC then slides low to about 7VDC going to the logic chip.

With 15VDC supply to the CD4001, isn’t it changing about 7.5-15V to output high and less to output low? If so, that could be why I see it only flip on briefly and nearly always stay off. I’ve traced through the gates of the chip at pin 3, 4, 10, and see that pattern invert as expected.

Beyond testing components, I’ve also replaced the CD4001, the 2N4124, and the LM324. No difference. I tested the diode, and it tests fine as a diode, but I didn't replace it.

Also of note, the expected 3.3K resistor to ground on the 2N4124 was a 2.7K resistor. I replaced it (and tested with other values +/-1.5K or so) and didn’t see a difference.

This page has the schematic:
viewtopic.php?f=1&t=30415&p=278885&hili ... 90#p278885

I’ll attach an annotated schematic with my measurements reflecting most/all notes above. The clock stuff is in the upper right.

Any thoughts? Thanks!
Zack
DOD_690_schematic.pdf
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Fender3D
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Post by Fender3D »

That pulse waveform is correct for SAD512 since it has a clock divider which will provide the 2 out of phase clock signals internally..
It's the same waveform you'll find in MXR micro flanger or chorus of the same period.
You might as well swap 4001 chip just in case, but I really think this is another dead-SAD time....

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Post by Zack-stomp »

Fender3D wrote: 12 Aug 2020, 15:43 That pulse waveform is correct for SAD512 since it has a clock divider which will provide the 2 out of phase clock signals internally..
It's the same waveform you'll find in MXR micro flanger or chorus of the same period.
You might as well swap 4001 chip just in case, but I really think this is another dead-SAD time....
Thanks, I did swap the 4001 already.

Shouldn't I see the pulse clock regardless of the status of the SAD?

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Post by plush »

Zack-stomp wrote: 12 Aug 2020, 17:44
Fender3D wrote: 12 Aug 2020, 15:43 That pulse waveform is correct for SAD512 since it has a clock divider which will provide the 2 out of phase clock signals internally..
It's the same waveform you'll find in MXR micro flanger or chorus of the same period.
You might as well swap 4001 chip just in case, but I really think this is another dead-SAD time....
Thanks, I did swap the 4001 already.

Shouldn't I see the pulse clock regardless of the status of the SAD?
Yes, unless the SAD is shorting the clock to ground, which can sometimes happen to dead SADs.

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Post by Zack-stomp »

plush wrote: 12 Aug 2020, 19:12
Yes, unless the SAD is shorting the clock to ground, which can sometimes happen to dead SADs.
[/quote]


Thanks. But what if I tested with the SAD removed? I did, with the same results.

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Post by plush »

Zack-stomp wrote: Thanks. But what if I tested with the SAD removed? I did, with the same results.
If you re getting the same modulated pulse from clock, then it's ok.
Try running the clock circuit in any simulator, you'll find out that everything is as it should be.

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Post by Scruffie »

So it's been determined the clock is fine but where are your voltages? Could be a problem in the audio path.

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Post by Zack-stomp »

plush wrote: 13 Aug 2020, 06:14
If you re getting the same modulated pulse from clock, then it's ok.
Try running the clock circuit in any simulator, you'll find out that everything is as it should be.
[/quote]

Okay, thanks. So even though the SAD512 docs say it needs a clean pulse wave, this "on" blip should do it, eh? I'll test more with that in mind.

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Post by Zack-stomp »

Scruffie wrote: 13 Aug 2020, 11:51 So it's been determined the clock is fine but where are your voltages? Could be a problem in the audio path.
Power supply voltages are all as expected and clean (15V) where tested. I welcome more thoughts/questions, but will test a bit further with the idea that the clock stage is okay. Thanks.

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Post by plush »

Zack-stomp wrote: this "on" blip should do it, eh? I'll test more with that in mind.
What do you mean "on" blip" ?

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plush wrote: 13 Aug 2020, 19:02
Zack-stomp wrote: this "on" blip should do it, eh? I'll test more with that in mind.
What do you mean "on" blip" ?
The clock output at pin 10 slams on to about 12 volts in a vertical line. Then immediately--same vertical line--drops a little below zero but then lands on 0. It holds at 0 for a moment. The signal looks like vertical lines to 12 volts repeating.

The logic chip inverts this as it seems it should, at prior stages.

The SAD512 documentation says it wants a pulse wave. This signal looks to me like it's trying to be a pulse, but drops off 12 volts instantly versus holding it briefly. I've worked on a handful of clocks like this and only seen working devices with clean pulse waves, but you may likely have more experience.

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Post by Zack-stomp »

Scruffie wrote: 13 Aug 2020, 11:51 So it's been determined the clock is fine but where are your voltages? Could be a problem in the audio path.
As far as audio path goes, I see it happily going into the SAD512 and then gets lost. The odd and even outputs of the SAD don't send a signal.

I just tried swapping in an R5106 that came from Analogman and is supposed to (usually, mostly, hah) be the same as a SAD512. Same results on everything.

I really think it's the clock but came here for advice so if you all think the clock is fine, I'll keep testing otherwise, thanks.

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Post by plush »

Zack-stomp wrote: As far as audio path goes, I see it happily going into the SAD512 and then gets lost.
5106 is not a drop-in replacement, it has lower voltage rating (up to 13v only), while 512 works with 15. You might have killed your 5106 which is VERY SAD (king of puns, ay lmao).

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Post by ppluis0 »

Zack-stomp wrote: 11 Aug 2020, 21:44Beyond testing components, I’ve also replaced the CD4001, the 2N4124, and the LM324. No difference. I tested the diode, and it tests fine as a diode, but I didn't replace it.
Hi Zack,
The CD4001 and their related components are arranged as a fixed frequency oscillator and the action of the transistor connected to pin 14 of LM324 among the leftmost components seen on the schematic is to slightly modulate it.

What about integrity of the 25pF condenser related with the 4001 ?
Place in parallel any low value condenser you have at hand and see what happens.

Cheers,
Jose

PS: I like the way DOD used the pins 1 (or 16) of the NE570 to engage or mute the effect in choruses, flangers or delays :applause:

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Post by Zack-stomp »

ppluis0 wrote: 14 Aug 2020, 01:47
Zack-stomp wrote: 11 Aug 2020, 21:44Beyond testing components, I’ve also replaced the CD4001, the 2N4124, and the LM324. No difference. I tested the diode, and it tests fine as a diode, but I didn't replace it.
Hi Zack,
The CD4001 and their related components are arranged as a fixed frequency oscillator and the action of the transistor connected to pin 14 of LM324 among the leftmost components seen on the schematic is to slightly modulate it.

What about integrity of the 25pF condenser related with the 4001 ?
Place in parallel any low value condenser you have at hand and see what happens.

Cheers,
Jose

PS: I like the way DOD used the pins 1 (or 16) of the NE570 to engage or mute the effect in choruses, flangers or delays :applause:
Thanks, Jose. Tried the capacitor, but the oscillator circuit and clock signal is about the same still. Going to reply to others in a sec, looks like they were on the right track...

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Post by Zack-stomp »

plush wrote: 13 Aug 2020, 21:36
Zack-stomp wrote: As far as audio path goes, I see it happily going into the SAD512 and then gets lost.
5106 is not a drop-in replacement, it has lower voltage rating (up to 13v only), while 512 works with 15. You might have killed your 5106 which is VERY SAD (king of puns, ay lmao).
Hah, that's unfortunate. I saw a few notes (from random people in retrospect) saying they swapped it in where needed, and I got careless about comparing properly. Thanks for the alert.

As you and Scruffie thoughts, I guess the clock is okay (?). I swapped in an actual SAD512 from a Moog Opus 3, and the pedal seems to work. I'm qualifying because it didn't sound quite right, but I can see the Odd and Even output on the SAD with the new chip and not on the original.

From here, I'll decide if it's worth the cost to source another chip before I investigate the rest of the pedal.

Thanks again for your thoughts! Glad to learn that even though the SAD datasheet says it wants an actual pulse that stays on a moment, these instant on/off voltages work.
Zack

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