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You want to look at the signal after the BBD and see if the BBD is correctly biased (there is signal coming through, and the signal is symmetrical, and not distorted on one side).

You will need a signal generator, that produces a sine wave or strum your guitar while measuring. With a sine wave it is easier, because the wave is symmetric and periodic. Try 0.25V at 600Hz.

Ground probe of the oscilloscope should go on ground and the other probe goes at the first BBD input. If you get signal at the input, it is working until this point. Then, check if there is signal at the output of the BBD. It will have lots of clock noise. If there is no signal adjust the bias trimmer until the signal is visible. You will have to do the same for the next stage.

The gain trimmer is used to get unity gain between stages. 

As said, you want the black probe at ground level. I use the input jack ground for readings.

Your readings are strange indeed, if you did if from pin 4 to pin x, why is IC3P4 -14,8vdc?  Shouldn't it be  p4 to p4=0vdc?

The 4558 is a normal dual opamp, so the readings are done as for a normal OpAmp.
Check a video of how to read circuit voltages and it will be the same. I guess the extra load of the voltmeter is negligible here.

Hi,
have you tested if the volume drop occurs at every setting?
It could be variations in the BBD gain, but at such low delay times it shouldn't be a problem.

Also, check the caps. Sometimes a large cap somewhere can make things way quieter, like using 100nF instead of 100pf at IC1B.
The transistors name/orientation also. If you used an equivalent transistor it could have swapped pins.

The best way to find a problem in this case is using an audio probe, and tap at some different points of the circuit to see where the volume is dropping the most. I would definitely check levels before and after the BBD.

Hi,

Bias seems OK.

According to Howard Davies for the EH7850: "TRIM 5 (GAIN 2):  Unity between NE570 pins 14/15 and 7.
Set the 2nd gain trim last looking at pins 14 and 15 of the NE570 and setting this trim so that the signal is equal in level to that at pin 7 of the 570. If this can't be done, go as far as the trimmer permits and re-adjust the first gain trim to achieve it.
"
I would use this value, since it will give you unity over the compander.
I guess you could also adjust for unity between IC1_A pin 1 and IC5_B pin 7, both sides of the blend knob, so that you have a perfect blend when the pot is at 12 o'clock.

Another aspect is, if you want the pedal to oscillate at max. feedback. You can adjust gain 2 so that there is oscillation, or not. Your choice.

Nice,
now you got the 1590LB itch, do not scratch it, otherwise you will wake up some day with lots of those.
Using stereo open jacks in this enclosure is tough, kudos to you.
There are some jacks that take less space and have a nice flat surface where you can put your PCB on.

I really like the huge knob. Kind of reminds me of a safe.

Depending what you are trying to achieve you need to have a signal or not.

To bias the BBD use a sine wave, increase the gain so that the wave distorts in the BBD and adjust the bias so that the wave is roughly symmetric. This depends on the delay setting, so either choose your preferred setting or set it with the pot half-way. You want to have the same headroom on both sides, using the distorted wave helps, because it definitely uses all the headroom available. The clock noise right after the BBD is normal. I prefer to measure at the input of the opamp, so that it is away enough from the output and some of the noise is gone. Do the same for the second BBD.

The gain between BBDs should be adjusted so that the second BBD sees the same voltage as the first.

To adjust the balance at the output of the second BBD I do it without signal. You will see the clock noise and you can minimize it by adjusting the trimmer. I am such a fan of this thing, that in my build I added  a trimmer right after the first BBD too. I don't think it helped though, the noise level seems to be the same.

The gain stage after the second BBD is adjusted so that you have unity gain between the two sides of the compander, if I am not mistaken.
It is basically summarized here, although here you have different BBDs. You will have to try different delay settings until you find one where most of the delay time is usable. For the noise balance, the highest delay time is suggested, since everything above 20k is not audible anyway.

I would also suggest the multiplex,
combining two chips makes it sound very interesting with the different echo possibilities.

I also built the magnus modulus and added a wah. The LFO can be used for modulation, tremolo and auto-wah with some tweaks. There are some settings where it gets too fast though.

I would suggest implementing a LFO with different shaped waves, offset and ratios. It might add some weirdness to the repeats. Therefore you would require some additional changes to achieve that.

Seems like the LFO is inducing some noise with the pulses.
Why use a 4558 as LFO? They are the worst ICs for LFOs. Test it with a TL062.

I never understood why they used a 24V supply for a pedal that basically runs at 15V. Were regulators that bad in the 70s?

Thanks for sharing!

Well,
I tried it again. This time with the MN3005 at 15v and a jack for tap tempo.
First some guts.



I almost hat to add a 2 mm spacer between lid and enclosure, but I could reduce the height of the voltage doubler and switched jack.

I don't think the tap is something usable with a 600 ms delay, but it was worth the try. I guess the next one will be a "normal" one.

Hi,
check if the the voltage are correct. As said, the bias should be around 3.9v. The tl062 worked better in my builds, with less voltage variations in the bias.
You can also adjust the circuit of the LFO to increase the sweep, but watch out, or it might get unstable and just stop the LFO, so make some tests with different values with small changes.

Very nice!

I wanted to buy a key holder and put an amp inside. That would be nice.

Looking at the schematic shows that pin 7 of IC5 is disconnected, with both inputs grounded. It will not make a difference. So the circuit should be OK.

Can you also measure the transistor pins?

Something I just realized is: You are powering it at 10v. BBDs are very sensitive to bias. When I say very sensitive, I mean less than 1V can make a huge difference.

First: The Bias of the BBD is still at 4.8V (pin 3 IC2) and the output is at 4.2v (pins 7 and . What does the document say about using other voltages? There is at least one change that is necessary (R13). I only ever seen this pedal powered with 9v or 12v, never 10v. You may have to change R13 to something between.

Second: The bias of 4.8v is probably for 9.4v (as stated in the document). That means: 10.3/9.4*4.8 = 5.3v.  I would start there. If you cannot get there, try reducing 82k. Some people even suggest that it is not necessary and may only reduce the ability to change the bias.

Do you have an oscilloscope?
If you do not have one, turn the bias trimmer until there is no flanging on one side, and then turn it the other way around until there is no flanging. Mark this two points, there is a high chance that the ideal bias is exactly in the middle. You can then fine tune with your ears.

Flanging is more pronounced with the feedback on max., but then you cannot hear the original repeat clearly. Try with the feedback at min. too.

Trimmer T1 also changes the feedback. See how it sound when it is at the lowest setting. You don't want the loud oscillations now. 

Is the filter matrix position working? I think it is easier to test everything with the LFO setting, and with the rate pot at half way, where the flanging occurs slowly. 

The clock trimmer should be adjusted so that you have a weird chorus like sound at half of the way and a normal sound on the other. In one direction it will go deeper into chorus territory and have lots of hiss, too much to the other side and you end up not having the flanging, because the clock is too fast and you are into that territory where it sounds kind of dry.

I think in filter matrix mode there is no LFO, so you are at a fixed clock setting. Try measuring pin 3 of IC4 or pin 7 of IC6.
By the way, do not use the voltages in the document as a reference for setting the trimmers (you can multiply them by the same factor 10.3/9.4 I used, to see how they should look like)

Last but not least, BBDs can be noisy. Hiss is normal at some settings.

PS: I was writing this, when I saw your update. I think the noise is incorrect bias+wrong adjustment of the trimmers and not a bad transistor or leaking cap.

The voltages are all over the place.
Are you powering it with a 12v or a 10v supply? Is that 10V reading without load?

The 5.2 makes sense if you are powering it with 10.4v.

First try to get the voltage at the voltage dividers right. That means the correct VR, VB, VC and VD.
If you are using 10.4v they all are going to be a little higher than the readings.

IC7B Should have 4.5V at pin 5, without that VB will not work. It is weird that you have 5.2v at pin 6 and 7, but not at pin 5. VB is 0 at every other pin that you measure. But IC7 B, pin 7 is at 5.2v. That is weird.

It is probably a could joint near IC7b, which results in the weird voltages.

Without powering the pedal, test the continuity of every component in that region (use the schematic for the resistor numbers, lower right corner of the schematic has the different supplies).

If you get the voltages there right, the voltages at the chips should be right, and the chips correctly biased.

Let us know if you find something.