ABDX modulation

[gitaar0]

I have a ABDX with mn3005. the delay works well.
I can not get the modulation to work well.
I have a vtl5c3 for the vactrol now that replaced a ldr/led combo in hope of getting a better result.
The problem is that the delay cuts out when I turn the depth pot all the way up.
Changing r56 has some influence but it still cuts out. I have tried values in between 0-1m5 but I can not find a value that prevents the depth pot from cutting out the delay, when I set the depth pot in the almost full setting.
Changing the clock helps and then I can set a value for r56 that makes the depth pot work ok in the whole range but then when I turn the delay pot for a very short delay value and the depth pot full up the delay cuts out with the delay pot.

I have build an old Aquaboy before with the modulation add on board and had the same problem using an LDR/led combo. Never got it working well except by adding a trimpot in series with the depth pot. I could do this here to but it seems to me that that was no how it was designed. Any suggestions?

[midwayfair]

R56 is in parallel with the pot -- the reason it still cuts out is that it has no effect on the MINIMUM resistance of the pot (which is still 0 + the LDR's on resistance), only the maximum.

You might try just pulling R56. It's possible that your vactrol's "on" resistance is too small, and R56 is making it even smaller. From there, you should be able to find a clock setting that will work at maximum depth and minimum delay.

[gitaar0]

Hi Jon,

Yes I thought about that too and tried it, but when i do that and set the clock where it does not cut out I have a very short delay. I could add resistance in series to the depth pot to get a higher minimum resistance, but it was not designed this way I suppose....is that due to a variation in different vactrols of the same number or am I missing something else?  Any other ideas?

Thanks

[midwayfair]

is that due to a variation in different vactrols of the same number or am I missing something else?

That I couldn't tell you, unfortunately. I understand the LFO part of this, but not so much the BBD part. Issues with LFOs swinging too far and cutting out are something I *have* dealt with. There are a handful of guys here who are wizards with with BBDs, so maybe they'll chime in with some other advice.

[madbean]

Here's what I would do: tackle it from the LED brightness side. Socket R50 and R51 and increase their values, but keep them the same value. Start with 4k7 and see how that works. Making them the same value ensures that the LED swings full bright and dark, and the increased resistance will dim the LED a bit which should prevent the max depth from causing a problem.

[gitaar0]

Hi brian,

Ok, I fiddled with r50 and r51. With a value of 8k2 for each and r56 removed i can set the clock anywhere without having the delay cut out at any extreme of the depth or the delay pot and with a good range of modulation. So that works. Thanks.
It is a very different value then what is in the project so I am still wondering why it so far off?

[madbean]

R56 is basically a kludge I added since there was room on the board. If you are using the VTL5C3 you really should not need it, and I think I was not clear about that in the build doc. My intent in putting it there was to offer some tweaking for random photocells, where light/dark resistances are way out of spec from the vactrol.

However, I'll need to check about the two resistor values. It may be that I put different values in my prototype and forgot when I did the final type-up. I'll check my proto tomorrow morning and report back. I've also been working on a second build (on a production board) as time allows so that will give me something else to compare.

Ideally, I want to get the two builds demoed so I can show everyone a comparison of the v3205 vs. the MN3005.

[gitaar0]

Hi brian,

Did you have a chance to check those values?

Thanks

[madbean]

I checked. I used 1k for both. I think I will add a note to the documentation to socket these values. Are you running it a 15v or 9v for your MN3005?

[gitaar0]

hmm, that is quite different as I am running it on 15 volt

[madbean]

Okay, this is what I forgot to put in the docs. The higher supply means the voltage output on the LFO is greater, and therefore the Led it drives is brighter. When that happens, the delay pot gets pulled too far because the total resistance between the photocell and depth pot is too low when it swings to full brightness and the depth is at max. That's why higher values for the two led resistors should be used. I'll add this info to the docs.
I double checked on my v3205/9v build and the 1k there works fine. 6k8 to 10k is better for the 15v supplies.

[juansolo]

Put mine together today (15v MN3005) and had the same prob with the LFO locking up. I went with a pair of 8k2 resistors and it worked fine. Really nice sounding thing this! As usual, it's gone in the box of boards awaiting an enclosure, so it'll be a while before it appears in build reports.

[aballen]

hmm, I just built mine today, also running at 15V.  The modulation on one side can get pretty wacky, is this the same thing you guys are seeing?  I wouldn't mine my modulation being a little smoother.

[aballen]

I don't seem to have any 8k2, closest is 5k6 or 10k.  is 10 k too high

[madbean]

The square wave modulation is going to be pretty wild. The triangle wave is smooth.