Behringer UV300/Boss VB2 Rate Mod?

[ashcatlt]

I posted this exact thing over on the DIYStompboxes forum, but I wanted to post it here too. Maybe y'all will offer some different perspective.


I've got a Behringer uv300. It's essentially a workalike for the boss vb2, and from what I can tell the two circuits are pretty similar. Can't find a schematic for the Behringer unit, but here's the Boss:

The thing is just plain too fast. It's lowest Rate setting is too fast for most of the things that I do, and I'll never need it to go as fast as it does at the top end.

There are two trimipots inside the Behringer unit, which had me excited, till I started turning them. One seems to control the depth somehow. Turning it away from the center-ish position where it was from the factory made the effect disappear with the depth knob all the way up. The other didn't do anything I could tell by ear in my completely unscientific test. I put them both back about where I found them.

I read this post where somebody rehoused one of these and added a switch to put a cap in parallel with "C15", which seems to correspond with C24 in the boss scheme. Now, this is all surface mount components and I am not at all confident in my ability to do something like that without screwing the whole thing up!

What I could do (at least, I'm more confident) is cut lug 3 of the (PC mounted) Rate pot in half, and solder a resistor between the two halves. Seems to me like this should just shift the whole range of the pot in the slower direction, no? Is there something I'm missing here where that won't just work?

[JohnH]

A couple of thoughts:

The rate pot is a reverse audio taper (type C), which suggests that much of the action is at the low end of the resistance. If you add a resistor in series, then Id agree that it should slow down, but it would not then be able to cover the range that it currently does also, ie if you used 250k, it would start at a fastest rate the same as its current slowest rate, and the pot might then halve that rate again, but not in a linear way.

Adding to the caps, I would have thought that both C24 and c25 are part of the rate control, so adding to both might be better. If they both got doubled by adding the same in parallel, then the rate knob will still work evenly, just all rates would be halved. For me, that would be an easier fix, given a very small soldering iron, just to dab a couple of small caps across the existing ones. If the new cap leads are trimmed and pre-tinned with solder, they should attach with less than a second of soldering heat.

Interesting - can we watch your thread on DIY S to see what the experts say?

Those Behringer boxes have some good scope for fiddling with. I have their chorus, and it sounds very good indeed, and I dont think the electronics on the board are any better or worse than any other modern circuit board. Its just that all the jack connections, battery access etc and mechanism are cheap. I might do such a re-house on it!

cheers

John

[ashcatlt]

Yeah, sorry, here's the link. So far only one response over there, which seems to agree that both caps would want to be adjusted. I don't, know. Just going by what dude said in his re-housing post. I think he said that he had originally intended to do both, but found it unnecessary.

Thanks for your reply, BTW. I just really don't want to mess with the smd parts or the circuit board in general. Also, those caps are on the bottom side of the board, and I'm not sure how much clearance there is down there before you hit the enclosure.

It's going to be a little tight to work where that pot is, but there's plenty room for a resistor to hang out. I'll be perfectly happy if the whole range is skewed in the slower direction.

[sumgai]

ash,

Sad to say, but I don't think playing with additional resistance in that line is going to give you the desired results. Working out the formulas, that 0.047 is pretty much maxed out, and even achieving 2Hz is a bit more than I would haved anticipated.

According to the images you linked, the board has plenty of room available for a soldering iron to maneuver around. Shunt that cap (C24 in the Boss diagram) with a 0.033 cap, and see what happens.

But of course you're welcome to try it anyways! After all, you can always go back in and put everything to rights, should the extra resistance turn out to be futile.

HTH




sumgai



BTW, lapicide alluded to "Uma Floresta". That would be Ben McCombs, a fairly well known name in the surf instrumental world. Small one, eh?

[ashcatlt]

Okay. Following from the responses I've gotten it looks like adding resistance is not the best answer. Seems it will just limit the range too much. Luckily, I've got a local friend who works on computers for a living who's willing to do it for a couple beers.

[sumgai]

ash,

My previous answer was pretty late in the day(night), even for me, so I didn't go into it very deeply. And I still won't now, but I thought I touch the base one more time for you.

Your assumption about the resistance is correct, but for more reasons than you might know. In the very long run, this pot is part of a voltage divider - look at the other side of that junction of C24/C25, and notice the 1MΩ resistor, which is going to the +9v rail. While the voltage divider itself can be fiddled around quite a bit, thus changing the charging times (that's what the pot is doing), the real fly in the ointment is that there's an additional junction between the two 1MΩ resistors (R47/R48), which goes back to the same reference voltage* point as the Rate pot is connected to. That's going to insert a steady-state voltage into the equation that will also act to put a limit on the minimum time that a cap has to charge up and discharge.

Shunting another cap across that first one is the easiest way I can think of to circumvent that limitation. Fiddling with other components will likely do the job, somewhat, but why bother - the math is messy at best, and it's the same amount of physical labor in any case, so I'd vote for the KISS principle.

HTH




sumgai



*R31/R49 form a voltage divider, ostensibly half of the positive rail voltage, and that's buffered by U5 (to maintain current capacity).

[flateric]

Theres a thread on Talkbass forum about slowing the UV300 down:
www.talkbass.com/forum/showthread.php?t=543004
They say its capacitor C15 that slows the rate down. Now I don't kow if the numbering on the Talkbass pcb photo corresponds with the numbering on your schematic but have a look thru the link and see if that helps

[ashcatlt]

flateric - when I first read your post, your link showed up as purple, indicating I've already followed that link because it's the same as in my original post.

These circuits are darn near identical as far as I can tell. Behringer's C15 = Boss's C24. Everybody (several people I trust from past experience) has told me that I should also parallelize what shows as C25 on the Boss schematic which is C12 on my unit.

The only question left is whether 1Hz will be slow enough. That's like a quarter note at 60BPM... You might not think that very fast, but for the new LT thing, we've got the delays set to like 800ms, which is about 75BPM, and this is typical.

For this particular time out this thing is going to follow a very short delay with quite a bit of feedback and then go into a DOD death metal. It's going to be completely horrible. I'm afraid quarter notes might be a little too busy for this application. Luckily, I can get something very similar in PodFarm2, and it will go really nice and slow, so... I'll be back.

[sumgai]

ash,

Modding both caps will ensure that the rise and fall wave shapes will be in close sync. By modding only one cap, you create a situation where the overall rate is set at X, but the rise time might be much quicker (or much slower) than that of the fall time. The most extreme example of that would be a sawtooth waveform, like this:



While in audio we'd probably not want something that drastic, there are times when a quick-cutoff might be desirable, as versus a long, slow drop in level. Conversely, the opposite is exactly how many effects achieve that "slow gear" effect, where the signal sounds like you're riding the guitar's volume control on every note. Worthy of experimentation, to some folks anyways.

It's up to you of course, just thought you'd like to know.

HTH




sumgai