Incoming Project: DCR320

[MattyT]

Synthbuilder: Can you suggest a good starter scope?
Waldenpotato: I used the front panel design program, opened the fdp file for the overlay, pre loaded a 1 ru case from front panel express database, deleted all the panels I didnt need, and copied and pasted the entire overlay layout onto the 1 ru front piece. Hope that is clear enough.

[Synthbuilder]

Can you suggest a good starter scope?

I'm not up to date of what's good at the moment. But the Rigol DS1054Z gets good reviews. I use a Tektronix TBS1022.

[waldenpotato]

pre loaded a 1 ru case from front panel express database, deleted all the panels I didnt need, and copied and pasted the entire overlay layout onto the 1 ru front piece. Hope that is clear enough.

I didn’t realize they had preloaded 1u cases. I’ll have to check that out. Thanks.

[MattyT]

I got an o scope but have run into some issues:
1) when going through the instructions for “tune” i cant get it lower than 86k hz
2) finished the rest of the calibration and found the following symptoms:
-Input level from DAW needs to be heavily attenuated so as not to peak red LED. But cranking up the input so the unit is peaking, doesnt sound distorted
-most of the setting so the o scope in the calibration instructions didnt show me a full wave, so i adjusted. Not sure if this is a problem. Mainly had to zoom out
-null calibration was really spiky looking even at its best setting
-wave forms for off calibration were a little fuzzy
Let me know your thoughts

[MattyT]

Correction. Peak sounds distorted in clean mode, classic mode handles more input volume

[MattyT]

Ive managed to measure and mark the .26hz and .56hz locations on the rate knob. Is there an easy way to find the delay settings for the presets with my o-scope?

[Synthbuilder]

Is there an easy way to find the delay settings for the presets with my o-scope?

I'm not sure it's easy but download the Service Notes for the SDD-320 (available from lots of places for free) and check the lower box on the left hand side of page 4.

[Synthbuilder]

when going through the instructions for “tune” i cant get it lower than 86k hz.

On both channels? If so, you may have the wrong capacitors in positions C13 and C14.

Put the TUNE trimmer in its middle position. What's the voltage at pin 1 of U26 with the delay pot at its minimum setting? And at its maximum?

I may have to tweak some of the resistor values. I've built two units and both behaved identically, but it could be there is some variation in some of the components that I have not allowed for. You're the first person to have built one that isn't me. Fear not though, it'll be a simple resistor change if it does need changing.

Input level from DAW needs to be heavily attenuated so as not to peak red LED.

This maybe true. The unit is designed to cope with a variety of input levels, from direct connection to an output of a synth (relatively low level) to the send output of a pro-audio mixing desk (high level). I wouldn't expect the input level control to ever needing to go to its maximum setting. However, if the gain is too much, then you can reduce the input gain by decreasing the value of resistors R172 and R173.

Peak sounds distorted in clean mode, classic mode handles more input volume

Classic can handle bigger peaks for most signals because of the compression circuitry. However, this isn't true for all signals. Try putting some very low frequency content through the unit and you'll find the classic mode distorts quite easily. This is because the compressor takes time to react to changes in the input level which makes it quite sensitive to sudden changes in volume. The original device has the same issue.

null calibration was really spiky looking even at its best setting - wave forms for off calibration were a little fuzzy.

Yes, the output of a BBD is a nasty looking thing. All we can do with the null trimmer is to keep the signal as clean as possible.

[Synthbuilder]

when going through the instructions for “tune” i cant get it lower than 86k hz

Another question if I may. What make of 4069UB did you use? I wonder whether that makes a difference. I used Texas Instruments in both of mine.

[MattyT]

The parts for c13 and c14 are too small for me to tell if they are correct. But i think i put in the 100pf on the BOM. Is there something else i can test before i pull the parts off the board and just replace them?

Testing at pin 1 of U26 results in -4.78v on max delay and 4.76v on min delay.

I wont mess with r172 and r173 unless i have to pull something else off the board. If i do want to increase them, do you have a suggestion for the next step up?

Hex inverter is the Texas Instruments CD4069UBE

Let me know your thoughts on these items

[Synthbuilder]

Is there something else i can test before i pull the parts off the board and just replace them?

There are four resistors on each VCO that determine the frequency. Check that:

R7 and R9 are 3K3. R16 and R17 are 6K8. R12 and R13 are 8K2. R6 and R8 are 330K.

I would be worth checking the frequency of the clock at pin 7 of U8. This should be twice the frequency found at pin 2, and consist of a series of narrow spikes rising to +9V.

Testing at pin 1 of U26 results in -4.78v on max delay and 4.76v on min delay.

That's what it should be, so the problem isn't around the CV summing circuitry based around U26.

I wont mess with r172 and r173 unless i have to pull something else off the board. If i do want to increase them, do you have a suggestion for the next step up?

The effect is linear. So halving R172 and R173 would halve the gain. You could try 47K to halve the gain (-6dB) , or 33K to third the gain (-10dB).

Currently, with those resistors at 100K, you should find the peak LED will just light with a +4dBu signal (1.78V pk) with the input level control a little under half way around.

If your input audio signal is differentially balanced the gain will be twice that of an impedance balanced or unbalanced signal.

[Synthbuilder]

when going through the instructions for “tune” i cant get it lower than 86k hz

Just had a thought. How are you measuring frequency? It just happens that 86kHz is twice what I would expect the frequency to be with the TUNE trimmer turned to its lowest setting. If you're using the scope's built in frequency counter be sure that 'trigger' has been set correctly because incorrectly set it can double trigger which makes the frequency twice as high as it should be. It may be worth checking the period of the square wave manually using the horizontal axis on the display.

[MattyT]

I believe my trigger is correct.
The resistors you called out are all appropriately populated
Here is another observation. Pin 1 of U8 is about 50k and is spikey. Ive included a pic
Pin 2 is more rounded saw tooth and double the frequency of pin 1. Pic also included
I measure nothing on pin 7

[MattyT]

If im measuring the delay correctly, i am getting similar pot positions as in the manual. So maybe the delay time tune is actually correct in my unit? See pic for how im measuring input vs output

[MattyT]

One other observation. I get a lot of bass loss on the output. Im guessing thats just how this effect sounds (i dont have an original to compare). Less bass loss on clean mode. Just checking to see if thats the case

[Synthbuilder]

Pin 1 of U8 is about 50k and is spikey. Ive included a pic
Pin 2 is more rounded saw tooth and double the frequency of pin 1. Pic also included
I measure nothing on pin 7

Are you sure that was U8? The V3102? Pin 2 and pin 4 should both be square waves at half the frequency of the pulse wave found at pin 7.

I'm not at work at the moment so I don't have a schematic, but those waveforms are the sort of thing that could be present on U3 or U4.

I get a lot of bass loss on the output. Im guessing thats just how this effect sounds (i dont have an original to compare). Less bass loss on clean mod

Yes, in the original you never get to hear the wet signal on its own. But it does have very little bass - the idea being that the stereo width is not affected by the Dimension D below 100Hz or so. The DCR320's clean mode more resembles the standard Roland chorus so should have a moderately decent bass response. Even so, the wet signal here is still a little bass light as the signal path has several high pass filters in it. This is to stop the modulation signal from leaking significantly into the audio.

You shouldn't be getting significant bass loss on the dry signal in either mode.

[MattyT]

I may have been testing u3 instead of u8... i feel silly for that. Everything is measuring as it should. The unit has some great sounds to it from the testing I have done so far, and I am glad i built it rather than just buying the Klark Technic repro.
Turn knob to dry and all the bass is present. So no issues there
Speaking of measuring, if i am looking at my delay times correctly, tuning the tune control to 43hz actually gets me that max delay of 12ms for setting 1. Unless you think that will cause a problem, I think I will leave it there.
Thanks for all of your help and sorry for the boneheaded oversight that caused this goose-chase.

[Synthbuilder]

Everything is measuring as it should.

if i am looking at my delay times correctly, tuning the tune control to 43khz actually gets me that max delay of 12ms for setting 1.

Remember that the modulation will also affect delay time. Setting the VCOs to around 50kHz is done so that once the LFO is added by the correct amount you should still get the desired maximum delay time for preset 1. At least I hope it does. Let me know if it doesn't.

And, if you want to emulate the Juno chorus then you need the delay time to be really small. If the VCOs are set to be too low via the TUNE trimmer then you'll not be able to get the smaller delay time needed for this.

That said, it'll not do any harm and if you like the sound with it set to 43kHz, go for it.