Clean my clock?

[armdnrdy]

I'm having a bit of trouble understanding what trimmer TR2 is doing in the Boss VB-2 clock circuit.

There is a set up description on BYOC that states, "The 500k adjusts the clock. You want to set this so that the vibrato sounds as full as possible without sounding too noisy.

I would imagine that C20 and R34 set the clock frequency so...is TR2 some sort of clock bias? Any ideas on adjustment?

[armdnrdy]

Here's a similar clock section from the CE-2 showing a fixed 150K resistor in place of the trimmer.

[Mark Hammer]

My semi-educated guess is this:

Vibrato is the sort of thing whose "musicality" is a product of restraint, rather than excess.  Between the various sources of clock rate, and the tolerances of the components involved, there are many ways/reasons for the amount of "boing" to be too much or too little.  At first, when I saw the rather large range of the trimpot (500k + 47k fixed resistor  do you even KNOW what the appropriate value ought to be Mr. Boss?), I was a little perplexed.  But then, as I started to consider the range of possible clock speeds, and the range of tastes, having a 10:1 adjustment of a bias voltage to nail the right amount of boing seemed entirely plausible.

[armdnrdy]

Thanks Mark!

Reading your post made a light bulb go on in my head.  

I've messed with clocks and BBDs but always with a fixed clock. (BBD tester)

Seeing that the Depth control is upstream from the clock...the Depth would control the amplitude of the clock signal.

The trimmer TR2 would increase/decrease the amplitude of the clock signal making the "boing" more or less pronounced.

So...to adjust TR2....I would imagine that one would put the Depth control all the way CW and adjust TR2 to where it sounds full but just shy of introducing clock noise due to the increased amplitude through the BBD output filter.  

[DrAlx]

I'm having a bit of trouble understanding what trimmer TR2 is doing in the Boss VB-2 clock circuit.

There is a set up description on BYOC that states, "The 500k adjusts the clock. You want to set this so that the vibrato sounds as full as possible without sounding too noisy.

I would imagine that C20 and R34 set the clock frequency so...is TR2 some sort of clock bias? Any ideas on adjustment?

Actually R34 does not set the clock rate.
The clock rate is set by a few things.

1) C20 of course.
2) R32 and TR2. These are the path for the main current source that charges C20 (while D2 and D3 are off, and pin 5 is low).
3) Q5 is a switch for the discharge path of C20.  When C20 is fully charged, pin 7 of the MN3102 will be set high causing pin 5 to go high shortly after. This makes Q5 turn on which causes D2 to turn on and discharge C20.
4) As soon as C20 discharges, D3 switches on and Q6 quickly "pre-charges" C20 to a level determined by the control voltage from the LFO.
At that point Q6 becomes isolated from C20 (D3 switches off) and charging of C20 continues via R32 and TR2.
So the job of Q6 is to sort of provide a variable starting level for the charging of C20.

If you compare this to the simpler sort of clock in the Deluxe Electric Mistress, you'll see TR2 serves the same
sort of role as the clock trimmer in that circuit.  
The EM clock is easier to understand because the LFO control voltage clearly sets the end level of the charging process rather than the start level.


In response to your original question about how to set this thing, I would set the Rate to maximum (since large amplitude vibrato is more of a problem when the rate is high) and set the Depth to minimum.  The whole point being that you should be able to at least get a useable sound when the Rate is hig.
If you set the rate high you will most likely not be setting the Depth high ('cos it will sound horrible) and would most probably be keeping it low (which will sound better).
So I would say that the clock rate should be fast enough to make sure that the above combination of high rate and low depth sounds OK.
If it doesn' sound OK, then setting the depth to max will definitely not sound good.
I am basing this on how I've set the clocks for various flangers that I've built.  

[armdnrdy]

Thanks for the explanation DrAlx.

So..if I understand this correctly, if one were to delete D3 then you would be left with a free running clock with the rate adjustable by T2?

[DrAlx]

Thanks for the explanation DrAlx.

So..if I understand this correctly, if one were to delete D3 then you would be left with a free running clock with the rate adjustable by T2?

Correct.

[armdnrdy]

Thanks for the reply Alex.

I cropped the spec section from the Boss VB-2 instructions manual that I found on the web.

Note the spec for delay time listed as 4ms.



I'm quite sure that Boss didn't employ women in their factory to plug a guitar into each VB-2 that came off of the line and turn the trimmer (TR2) until it sounded "right" to their ears. There had to have been a factory alignment procedure.

So...even though I could "tune" this by ear, I always like a starting point for reference.

I started thinking about that 4ms and did the math to find the frequency to achieve that delay time with a 1024 stage device.

Stage ÷ T = 2*F

1024 ÷ .004 = 256000

256000 ÷ 2 = 128000

So to achieve 4ms from a 1024 stage BBD, the clock frequency would be 128kHz.

Does this make sense? or am I barking up the wrong tree with the 4ms?

[DrAlx]

Does this make sense? or am I barking up the wrong tree with the 4ms?

That sounds OK but I don't know how you'll measure that with a scope.
The 4ms delay will happen when the LFO control voltage is at it's lowest.
I'm guessing you'll need to use the slowest Rate sweep (2Hz) but even then the clock waveform will be moving around quite quickly.
I'm not sure how they would easily do  trimming at the factory unless there's a way of setting the LFO to give its minimum control voltage and not have it move around.
Maybe they found they could just put TR2 in roughly the correct position (say half way) and get away with it.
I mean how many people are going to tell if the max delay is much longer or shorter than 4ms ?  4ms sounds like a hell of a big delay for vibrato, even with the LFO at 2Hz.
As I said, for vibrato you want to be able to access the smaller delays when Rate pot is if high in order for it to sound good. 
As an example, the Tri-Vibe pedal is a vibrato design based on phase-shifters rather than BBDs, and the effective delay times produced by the phase-shifters are MUCH smaller than 1ms but it still gives a nice vibrato.  So I don't know why Boss listed the maximum delay in the spec in the first place.  It would have made more sense to spec the minimum delay since that is the thing that is harder to achieve with BBD.

[amz-fx]

The factory adjustment procedure was to set the pedal in Bypass (OFF), and adjust VR4 so that there is a 120kHz clock on pin 4 of IC3.

Then turn the pedal ON, and input a 0dbm 200Hz sine signal into the input and adjust VR5 for symmetrical distortion of the sine.

Yes, they probably had people in the factory doing this calibration on each pedal.

regards, Jack

[armdnrdy]

That sounds OK but I don't know how you'll measure that with a scope.

In my last post I started to include how the clock could be adjusted without interference from the LFO but then deleted it as too much information.

Jack is correct. The VB-2 incorporates a mode switch which gives the choice of "Latch", "Bypass", and "Unlatch." In Bypass mode no effect is obtainable when the pedal is pressed. Latch turns the Vibrato effect on and off when the pedal is pressed. Unlatch applies a ramping vibrato (ramp time adjustable with "Rise time" control) when you press the pedal.

So...the clock can be set when the pedal is in "Unlatch" mode. The VCA keeps the LFO signal from reaching the clock when the pedal is not pressed.

On the original VB-2 the switching is accomplished using the mode switch (2PDT) and a BA634 flip flop. A more user friendly configuration can be realized using a 3PDT stomp switch, (bypass and extra pole for indicator) a SPDT toggle, (ramp, auto vibrato) and a momentary S.P. N.O. footswitch to activate the ramping vibrato.

Here is the factory schematic:




At Jack:

Thanks for verifying the frequency! Did you get the info from the VB-2 service manual? I searched the net but came up empty!

"I'm quite sure that Boss didn't employ women in their factory to plug a guitar into each VB-2 that came off of the line and turn the trimmer (TR2) until it sounded "right" to their ears. There had to have been a factory alignment procedure."

This reminds me of a story told by Ritchie Blackmore.

"The design I liked, (Marshall) but the sound was awful. So I went back to the factory and said, "Look, I want this changed and I want that changed." There would be women there assembling things, and I had the amp boosted to 400 watts. So I would be playing away right in front of all these people and they'd be trying to work. I'd go, "That's not right, more treble," and they'd take out a resistor. I had to play full blast or otherwise I couldn't know what it was going to sound like. The people hated me."