Vox Repeat Percussion Needed Explaining!

[danny114]

Hey guys,
In school right now doing a project for one of my classes and I've decided to build the Vox repeat percussion, but in a Wah encasing so the foot action controls the rate of the tremolo. The build is about to begin now, as ive just started making the breaks on my vero board so hopefully all goes well.

Anyway, for my project however I have to explain in much more detail the actually process of how the tremolo works, so I was really hoping could any of you help me with that? I mean explaining which components make the LFO, etc, and how does this tremolo essentially work with regards to the signal flow and all that. Simply put, HOW DOES MY TREMOLO CIRCUIT WORK? Im really confused and the sites/books im finding are more to do with tremolos in general as opposed the vox repeat percussion itself and its components.   

I'm young and im a beginner so sorry if i come across as a n00b, just really could use the help 

This is my schematic, and if you could help me understand how it works in some detail that would be GREATLY appreciated. Happy to compensate in whatever way i can for a detailed answer, as it would mean the world to me and my grades!!

[ img ] http://tagboardeffects.blogspot.co.uk/2012/08/vox-repeat-percussion.html [ /img ] I'm using just the original schematic not the 2nd, and 3rd modified ones.

THanks a ton!

[pappasmurfsharem]

I believe you just asked the smarter people here to do your homework for you   Then again we've all done it

On that note. If I knew how it worked I would explain it for you but alas I don't.

Also the layout =/= a schematic so you will potentially get more intelligent responses if you post a link to the actual schematic.

I'll be a peach and link it here for you =)

http://analogguru.an.ohost.de/193/schematics/Vox_RepeatPercussion.gif

It may be of interest to you to build something you can understand though, just from a principle standpoint  

This is SOME insight by RG in an old thread.

Upon thinking about it, I realized I needed to provide more info.

The UniJunction Transistor has two emitters, E1 and E2, and a base, B. The base B is connected to the junction of a resistor pullup to the power supply and a cap go ground. The emitters have resistors in series with them. How it works is that the base junction is off, reverse biased, until the base voltage ramps up above the voltage in the middle of the channel between the two emitters. The resistances between the base and E1 and the base and E2 and the resistors in series with the emitters make the voltage where the base will start conducting be a voltage-divider fraction of the power supply. When the base gets high enough, it conducts, and this current causes the lower emitter (E2) resistance to drop, so the base conducts until it almost empties the capacitor. When it quits conducting, the channel resistance comes back up, the resistance to E2 is reestablished, and the cap starts charging again.

The PUT does the same thing, but in a different way. It is a latching thryristor like an SCR, but it starts conducting when its gate is more negative than the anode. So to make this act like a UJT, you tie the base to a fixed voltage divider to set the trigger voltage. The anode is tied to a resistor/capacitor like the base of the UJT, and the anode to ground, or nearly so. The anode ramps up as the capacitor charges until it gets high enough to conduct into the gate voltage. The gate current turns on the thyristor/SCR action, and the cap is discharged through the anode/cathode until the current drops below the holding current, which is quite small.

So if you connect the anode of a PUT where the base of a UJT was, the cathode of the PUT where the E2 used to be, and connect the gate of a PUT to resistor divider to ground, the resulting circuit acts very much like the UJT used to. The gate is not the exact equal of the UJT's E1, so the circuit is different there.

[R.G.]

HOW DOES MY TREMOLO CIRCUIT WORK?

The input signal is attenuated by the series resistor from the input jack and the collector-emitter resistance of the NPN transistor from the input signal to ground. The attenuation is changed by the amount of current into the base of that transistor.

The other transistor is a simple gain stage to make back up some of the signal lost by the attenuation section. The PUT just provides a base drive voltage/current for the attenuator transistor.

[PRR]

It has probably been discussed *before*. See the button up above?

Why should we re-type upon demand, when computers save our words for us?

A particularly recent discussion with much of what you ask:

http://www.diystompboxes.com/smfforum/index.php?topic=103700.0

[danny114]

Thanks a ton guys!!! Really helped!

Another question if you'll don't mind! I've put the layout on breadboard and tried to test it however i got some strange results.

The signal of the guitar came through (a bit noisy, but still relatively clear), however when i turned the pot, no amount of tremolo effect was apparent what so ever.

Any idea why this is happening? And how may i troubleshoot this?!

Cheers guys. You're all going in my report

xx

[PRR]

> guitar came through ... no amount of tremolo effect was apparent

Re-read the thread starting around reply #11 (where Gavanti gets sound but no trem).

http://www.diystompboxes.com/smfforum/index.php?topic=103700.0

He had some REALLY expert advice from R.G. with several paths for debugging, also re-rigging with less trouble-prone devices.

[danny114]

New issue though now unfortunately now the guitar signal isnt even coming through :@ (((((