behringer uv300/boss vb2 rate mod ?

[ashcat_lt]

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?

[Mark Hammer]

Vibrato is one of those things that is hard to detect if the rate of change is slower than a certain amount.  I'm not proposing that Behringer (or even Boss) nailed that minimum modulation rate flawlessly, but it may be that you won't be able to get it much slower.  Having said that, you may want to supplement both C24 and C25.  It loks too much like some sort of twin-T resonant circuit for me to think you could get away with changing only C24.

Try tacking on a .01uf cap in parallel with C24 and C25 to see what that does.

[PRR]

> Seems to me like this should just shift the whole range of the pot in the slower direction, no?

AND reduce the range.

This is a wacko oscillator. Frequency is square-root function of (inverse) VR1+R54. The stock 255K-5K (a 54:1 range of resistance) gives apparently 2Hz to 15Hz range (a 7.5:1 range of frequency). Indeed 7.5 is (nearly) square-root of 54.

Adding say 250K in series gives 1Hz to 1.4Hz. That seems narrow.

25K in series is 1.8Hz-5.4Hz. This also may be narrow.

As Mark is telling you:

The DIRECT way to shift it all downward, without losing range, is to increase (that drawing's) C24 AND C25. (Changing just one will change frequency but also mess with the stability... at some point it just quits.)

> this is all surface mount components

Maybe wise to find another victim.

Is there PCB trace visible on what we are calling C24 C25? Scrape the varnish off the copper. Tin the copper, one trace at a time so you won't melt _both_ sides of the SMD caps and they won't slide off. For HALF rates, get small leaded 0.047u caps. Trim to maybe 1/2", put an L-bend in the tip of the lead, tin well. Set the lead on the trace and touch with iron.

Or you could outboard-build the sine-ish LFO of your preference, 4V peak, break the top of VR2 "Depth" pot, and feed it in.

[ashcat_lt]

Thanks guys.

I guess the added resistance would give me very fine control of the rate, but...

I was tempted by the "build your own LFO" idea.  One could even install a switched jack and use an outboard LFO - like maybe one of my moogefoogers.  Would an expression pedal (like the DOD FX17) do anything here?  Kind of like a whammy?

Anyway, my tools and nerves won't let me do the cap thing.  Luckily, my friend works on computers for a living, and is willing to do this for a couple beers.

[Taylor]

I was tempted by the "build your own LFO" idea.  One could even install a switched jack and use an outboard LFO - like maybe one of my moogefoogers.  Would an expression pedal (like the DOD FX17) do anything here?  Kind of like a whammy?

The pitch shift is only a result of a varying delay line. See here. So you only get a pitch change when the delay time is moving. If you were to freeze your LFO right when it's making your guitar sound flattest, it wouldn't stay that way - it would return to pitch in a second. This is why Mark is saying that you can only go so slow before the pitch change is inaudible.

Adding an expression pedal would give you some very fleeting pitch shifts. Start with heel down, then toe down, you get a pitch shift, but it will return to normal very quickly. So you can't get a constant pitch shift like you would with a Whammy.

I think that "fake whammy" is the basis for the Dano "Shift Daddy".

[zmix]

Ten years later, reanimating this thread..

I got a Behringer UV300 and also thought that the range of rates was too fast, so I added a 0.1µf cap in parallel to C12 and C15, [C24 and C25 on the BOSS]  but due to the nature of a serial delay line, the slower the modulation rate the less intensely the pitch is modulated, so I added a 270kΩ resistor in parallel to R5 [R47 (330kΩ) on the BOSS] which is connected to the wiper of the modulation depth pot.  (for an equivalent resistance of about 150kΩ).

Modifying the depth this way increases the range considerably.

[eliobou]

That's funny because I put capacitor in this pedal yesterday and find this thread while thinking that the minimum speed is now great but the depth is not enough.

so I added a 270kΩ resistor in parallel to R5 [R47 (330kΩ) on the BOSS] which is connected to the wiper of the modulation depth pot.  (for an equivalent resistance of about 150kΩ).

You mean R42 on this schematic right ? How is the depth modified at higher speed ? I don't fully understand how it changes the range.

[ElectricDruid]

Welcome Eliobou!

so I added a 270kΩ resistor in parallel to R5 [R47 (330kΩ) on the BOSS] which is connected to the wiper of the modulation depth pot.  (for an equivalent resistance of about 150kΩ).

You mean R42 on this schematic right ? How is the depth modified at higher speed ? I don't fully understand how it changes the range.

No, I think he does mean R47/330K (Boss schematic above).

The pitch shift increases with increasing rate-of-change, so faster LFOs give a deeper apparent pitch shift (for what is actually the same modulation amplitude).

[RickL]

I'm reading the resistor in the Boss schematic as R42 as well. R47 on that schematic is immediately above IC5, connected to R48.

[ElectricDruid]

I'm reading the resistor in the Boss schematic as R42 as well. R47 on that schematic is immediately above IC5, connected to R48.

My apologies, you're right. I mis-read it, and R47 is indeed where you say it is. It's a bit small and not too clear.