vox repeat percussion

[Sir H C]

I would have to say that Q2 should be an NPN with the collector up top and the emitter at ground.  As it is, I am having trouble figuring out how that device can get the tremolo happening.  For the FET implimentation, is it a P-channel jFET used?

[RickL]

I built this a couple of weeks ago with pretty much the same results - sound through it but no volume variation. I socketted the bipolar in the LFO section and tried both PNP and NPN in both configurations (i.e. swapped emitter and collector) with no luck. With the transistor out of the socket shorting the emitter/collector sockets does cut the sound out so it would appear that the LFO itself is not working.

I also tried changing the orientation of the UJT with no success. I don't know enough about how UJT oscillators work to know where to look next.

[darryl]

Seems like we all have been doing the same thing, I have seach and found the picture Toothpaste for Dinner had put up of the Pedal. The picture show the right amount of parts and the Transistors look right. If there was just a better board picture I could figure out what wired wrong. I will keep going, I hate when I close and it is still not working.

Darryl

[R.G.]

Here's my cut on that schematic.

1. The UJT is a sawtooth wave generator. It causes a ramp from low voltage up to a big fraction of 9V at C2.
2. The sawtooth turns on Q2 as it ramps up. I believe that Q2 should be NPN as well, but I also believe that it should be inverted - i.e. emitter up - as shown in the schematic.
3. When the sawtooth blasts back to near ground, Q2 is turned off, and the signal flows through R8 and R9, losing 2/3 of its signal level, but being amplified back up by Q3.
4. As the sawtooth ramps back up, Q2 is turned on very gradually by the increasing bias through R4, R5, R6, and R7. This gradual turn on makes Q2 act like a variable resistor, and increasingly shunt signal to ground until it is quite a bit smaller than it would otherwise be.
5. The peak of the sawtooth is reached and the cycle starts over at 3.

I don't have a UJT handy, and my simulator doesn't have a model for one, so I just used a 0V to 8V sawtooth waveform. I then messed with the values of R4, 5, 6 and 7 to get the transistor to turn on and off correctly, and found that it does indeed give a repeating increase to a peak voltage, then linear drop, and increase again.

IMHO, it won't work as shown, unless you get the right UJT and the right transistor for Q2.

It's not too hard to redesign it to work, but it will be more complicated, as the UJT would be replaced by a Schmitt trigger sawtooth generator, probably an opamp.

[Dan N]

Count me in with the failures.

I was dubious about that oscillator so I tried wiring it so it looked more like the relaxation oscillators found on the web:

http://users.rio.com/senorris/junk/vrptry.gif

(crossing the legs of the uni and moving the 300K). Nada. I do not have a genuine 2n2646. I'm trying an old Radio Shack MU4849 (276-2029).

It would be fun to knock this sucker down. Till then, there's always the Walco Sound-Go-Round for choppy fun.

[soundguy]

I obsessed over this thing many many years ago.  I have two original units and following that schematic could never get my protos to work.  I never traced the circuit but now that Im a much smarter guy then I was then I really should, I dont think that schematic is correct.  For those wondering what the unit sounds like put on a spacemen 3 record, there you go...  There are some similar sounding circuits, the kay trem (which will work) is sorta close, farfisa had that orange repeater pedal which was similar also.  The vox has a really unique "poing" to it, would be lovely to figure this one out.

dave

[Dan N]

Photos of the top and bottom of the board would be great!

[somasix]

I have a the plug-in model Repeat Percussion that I can't get to until I visit my family home in mid-August.  I was planning on picking it up anyway.  If ya'll haven't nailed this one yet, I'll be happy to post images of the original board. 

Best wishes on this circuit guys.  This is my favorite super slow tremolo circuit.

[somasix]

I decided to get a little more proactive on this thread because the Repeater is a sound I love.  Here's were it sounds kinda goofy.  I emailed a guy I know who worked with a guy who built a working copy of the Repeat Percussion.  He wrote back and said he'd contact this guy for a copy of the schematic that he used to build it.  Should I be able to contact this guy directly, I will ask him what transistors he used in Q1, Q2, and Q3.

RG mentioned the UJT.  Because I am new to electronics and pedal building I do not recognize the UJT on the schematic.  Can anyone isolate provide me more details on exactly what the UJT is so that I can get into particulars should the situation allow?

Again, provide direct contact is available, is there anything else I should pose from you who are currently building this circuit?

Thanks.  I look forward to this effect getting built.  The original Vox Repeaters are so small, I never imagined they would be so hard to clone.

[RDV]

Would this work @ Q1?

Mouser has it.

RDV

[darryl]

Hello again, the UJT is at Q1 the original was a 2N2646, I am using a NTE cross a NTE6401. Did not work per the schematic posted, I try the relaxation Oscillator that was posted, it oscillated but did not work in the circuit. I have also try another saw tooth wave nothing. I have try a NPN and PNP for Q2. Change parts around nothing yet. Hey and thanks for repling to my email, yes this would be great to figure out. Why is it the simple circuit seem like more work.

Thanks to you and everybody so far who is trying figure this one out.

Thanks Darryl

[somasix]

So far no word back on these transistors.   This might be a dead avenue.  I hate to say, it might be best just to dig for my old one in August.  Other than that, nothing new to report.  How about you guys?

[darryl]

No nothing I even bought the factory schematic, which is the one posted. I have mess around and around nothing, I think RGs comment on the Unijunct transistor is what it is coming down to. I going to buy some different transistors and go from there. If I get it going I will let everybody know.

Thanks for the update
Darryl

[theehman]

Saw this post while poking around and had to help everyone out.  The original Vox schematic is WRONG!!!  I, too, tried to build one from the schematic only to have it not work.  I ended up buying an original Repeat Percussion, doing some R&D, and now I've got the actual schematic for it.  The transistors are: 2N2646, BC108A, BC109B.  In the original schematic Q2 is shown at the wrong polarity, several resistors are incorrectly labeled, and the whole oscillator section is screwed up.  Also, it seems that only one of my 7 pieces of 2N2646 would work in this circuit so hopefully I've just got a funky bunch of them.  They all worked in a test oscillator circuit, though.  I've got some more on the way for my new pedal, the Trem-O-Matic.  Anyone wanting a free copy of the corrected Vox schematic should email me at my profile email.

[Dan N]

Bump in case anyone missed this.

Thanks Ron!!!

[R.G.]

Ron was kind enough to supplyme with the schemo. I'll compare it to the internal repeat percussion effect from the Thomas Vox amplifiers. These were quite similar, perhaps there are more clues to the details in the comparison.

[9 volts]

 I've been working on a layout for this..... which is causing a little grief. I get a straight sound coming through, no pulsing. When i take the first transistor out it doesn't seem to affect it. Anyone have any suggestions, like should I send in some transistor voltages etc.
Thanks

[Paul Perry (Frostwave)]

The only reason people used those UJTs was, the 555 timer hadn't been invented yet. So I wouldn't hesitate to ditch the UJT. (incidentally, UJTs are like fets, in that there is a bit of a variation between units. Maybe ones that don't oscillate could do with a tad more volts on the power rails?)

[9 volts]

I'll try it out. I'll put in a few trim pots and see what happens. Thanks

[stobiepole]

Okay, I just built one of these using Ron's revised schematic (thanks!)...

I initially got just the straight sound going through with no only the slightest hint of tremolo. I turned around the Q1 unijunction transistor (a 2n2646) and got a muffled sound. Then I twisted the legs so the emitter was in the middle and bingo: it worked. And I have to say, it's a really great effect - more musical than your usual tremolo. Great for anything choppy/funky or electronic sounding. At times you half believe you're playing through a delay...

Any one suggest any mods? I don't know that a depth switch would be that useful, but a double rate switch (like on the tremulus lune) would work well, I think.

Chris