ERica EDU sequencer

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Jumbuktu
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ERica EDU sequencer

Post by Jumbuktu »

Just finished building the Erica EDU series 5-step sequencer from the DIY kit. It works, except there are two small issues:
- The CV out is belowthe required voltage range. There are trimmers to adjust 0-5v or 0-2.5v (depending on where the range switch is positioned) but I can't get the 5v output above about 4.6v
- The clock input doesn't get triggered by most of my clock sources. I can get it to work by adding a voltage offset.

Both problems suggest I have something wrong with the voltage supply section of the circuit. Any suggestions? The schematics are towards the end of the user manual, which is here: https://www.ericasynths.lv/media/SEQ_MANUAL_v3.pdf
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KSS
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Re: ERica EDU sequencer

Post by KSS »

Are you sure the CLK isn't working according to its specs? What do they say about required CLK voltage?
The CLK input circuit is a bit odd. I'd prefer seeing OA1A set up as a comparator. Then it's easy to choose whatever actuating level you want.

I didn't read the whole document, but it says they discuss every design choice. Have a re-look at the parts of the text that involve the CLK circuit.

For the 5V output
Check that R33,34,36 are correct values and in the corrct positions.

Barring these, it's likely to come back to the most often seen mistakes in DIY. Poor soldering and misplaced components or swapped-misread values. <--This last can be a mfg problem if parts come to them poorly marked or sorted.

Edit: I did go ahead and read the whole thing. They do a pretty good job of it but do leave out some important points. Two things that stand out are ignoring the voltage drop of diodes -but their output Vdiv values show they include it- and completely ignoring any discussion of the required CLK input level. Since they used a unity buffer on the CLK input instead of a comparator, the CLK here does require a higher voltage than is typical for other modules. That's not really a fault, but I'd say it's a poor design choice when most Euro modules will accept 5V or even lower as valid.
The reset input here is similarly requiring a higher voltage than is normal for Euro modules.

It's generally 'safe' to assume in most synth formats that *triggers* will usually be near the supply voltage. But gates can be all over the place.

If you really want to understand this circuit -and many other synth circuits, download a free copy of The CMOS Cookbook from its author Don Lancaster's website tinaja.com. Where you'll find on pg 19 the expected voltage value for a high and low input. 70 and 30% of supply voltage. Chapter 6 is all about CMOS counters and the 4017 chip itself is detailed on page 84. Many other CMOS chips are also described in detail. Pg numbers given reflect the 2nd edition with blue cover.

Using the CMOS book's 70% and accounting for the 0.6V drop of the series diodes, the expected voltage level to CLK the 4017 will be 7V8. The RST might benefit a little from the differentiating capacitor producing a spike and so may be lower.

The real lesson here is knowing what the original designer intended. Erika and their development partner let you down a little in not calling out these needs directly.
Last edited by KSS on Mon Jun 20, 2022 4:15 am, edited 1 time in total.
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Re: ERica EDU sequencer

Post by KSS »

What voltage do you see at TP8?

That output circuit is again a little odd. But ultimately pretty simple. It's a two legged voltage divider followed by a non-inverting 'unity' buffer. Choose one leg or the other. The best designers will plan for the trim to be centered, so use 2k5+51+68 as Rtotal and 68 for the other parameter in your voltage divider calculation. See if this aligns-makes sense using what you measure at TP8.

Crappy trims can lead to problems also. I think Erica is using the Spanish ACP pots which are fine quality and shouldn't be a problem. But it's still good plan to keep trims in mind as potential failure sources. Especially if you've been working the wiper back and forth many times. ACP CA6 or CA9s have more life than many, but it's still often far less than one assumes if you only think of them as small pots. Mfrs are always looking for ways to save money and setting a short life for trims means they save.
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Re: ERica EDU sequencer

Post by Jumbuktu »

KSS, thanks so much for the detailed explanation. I'll check out the voltage at TP8 tomorrow.
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Re: ERica EDU sequencer

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Ok, measured the voltage at TP8 - 9.8v.

By my calcs, that makes the maximum possible voltage at the 5v ouput to be 4.42v (68k / 56k). That is consistent with what I was measuring. Actually a bit lower.

The voltage at TP7 is 9.9v.

The resistors were supplied loose in this kit, so I measured them with a multimeter rather than using the colour codes. I just checked R33, R34 and R36 using the colour codes and they look Ok.

As for bad soldering, yes, this has become a problem over the last couple of years. I turned 70 in April and my eyesight is getting bad enough that I generally have 1 or 2 bad joints with every project. I have to inspect the boards with a magnifying glass to spot the bad ones. I've done that, can't see any yet.
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Re: ERica EDU sequencer

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Ok, that 9v8 seems too low. What's the voltage at the 4017 power and GND pins? Please check and report the OPA rails at their pins too for comparison.

I took four old j-wires from an organ KBD and soldered them into a piece of stripboard in an old sawbuck, crossing X's style. They're gold plated and flexible. I then wired a banana to each pair -the stripboard connects them- and now have an easy way to check TH resistors and axial caps. Just drop it into place as if it were log about to be made into cordwood. press down if necessary and the meter gives the result. The next one is in the 'sawbuck' for measurement while the present one is being installed. Makes quick work of confirming R's and many C's too. Far faster than having to move the test leads to each one.

If you don't have J-wires, Optima brand gold-plated guitar string in .014 gauge is a fine replacement. I've been meaning to write this up for some time, as there are a couple other testing points for other size components on the same stripboard. Need to get a round TUIT for that!

I'm understanding more and more what it means to get older. Taking a cell phone photo of a PCB and then looking at it on a desktop big screen is my GOTO these days. The separation from actual PCB viewing seems to help as much as the magnification, Phone cameras are so good today that resolution is not a problem even at large magnifications. Only issue still is that solder reflections can be tricky. I always take at least an overhead and a quarter view or two to let shadows help me see the truth of the soldering.
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Re: ERica EDU sequencer

Post by Jumbuktu »

Hope someone else is reading these words of wisdom! I probably haven't got too many years left for building DIY stuff, but younger ones can learn from this thread.

That setup for measuring components in place sounds good. I'm a guitarist, and my main guitar needs restringing. I'll check the voltages when I can - hopefully there is a reason and it fixes the gate issue as well.
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Re: ERica EDU sequencer

Post by Jumbuktu »

Ok,
CD4017 Pin 8 to Pin 16 is about 10.41v.
TL074 Pin 4 (Vcc+) is 10.43v
TL074 Pin 11 (Vcc-) is -11.55v.
Across C5 in the power circuit is meant to be 12v, is actually 10.43v
Across C6 is meant to be -12v, is actually -11.55v.

Looks like one side of the power is the issue. I'll check the component values and soldering.
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Re: ERica EDU sequencer

Post by KSS »

:tu: Getting closer!

Which power supply are you using?

You might want to check the main power supply with each module removed one by one. To see if there's one pulling the positive side down. And then if not, to see what it's doing when unconnected to any modules. <--Noting that some supplies need some base level current draw to function correctly.
Treat utility modules as stars instead of backup singers.
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Re: ERica EDU sequencer

Post by Jumbuktu »

I'm using a 4ms Pod 64X for this module, but I have other racks I could test it on. There are two digital modules in the same case. I will do as you suggest and report back.
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Re: ERica EDU sequencer

Post by KSS »

Check that the brick feeding the pod has enough voltage and amps for the modules in the pod. Easiest first check would be the brick voltage output itself. They do go bad sometimes.
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Re: ERica EDU sequencer

Post by Jumbuktu »

Well, this discussion has been fruitful. Turns out my power brick is inadequate. It's not the one I was originally using, somehow I got them mixed up along the way. I'm not actually using an official 4ms brick and maybe I should be anyway.
With a 15v brick, I was able to get the case power up to 12v. The SEQ cv output now goes over 2.5v for the 2.5v switch and about 4.9v max for the 5v output.

There is still a difference within the SEQ module between the voltage across C5 (lower) as compared to C6. I might never find why, happy to accept that the module is working as designed and I have learned a lot about trouble-shooting and power supplies.

Thanks KSS
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Re: ERica EDU sequencer

Post by KSS »

:tu:
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