Repeat Percussion troubleshooting

[Gavanti]

Hi all,

I hope it's ok that my first post will be a request for help. I'm on my third pedal build (I did an amp switcher and a SHO clone earlier), and I upped the ante a bit for this one with a pedal version of the three Vox Starstream guitar effects. The effect that I'm having problems with is the Repeat Percussion. Any help folks can offer would be much appreciated.

Sound: I'm getting none from the RP circuit. I have working Bass/Treble Boost and Fuzz circuits hooked up before it and when I turn this circuit on it kills everything (which makes sense given the wiring between the switches). I have verified that my build matches the layout and checked for accidental bridges. I have also verified that signal continuity goes to and from the switches and the battery and output jack.  My suspicion is that the problem is one or more of the transistors.

A small additional question: I used the specified 5k lin pot for the Fuzz (V8162 Distortion/Booster) , but I'm not getting much usable range (mostly little fuzz, then suddenly all out fuzz war). Would another pot work better?


Source for layout:
http://tagboardeffects.blogspot.com/2012/08/vox-repeat-percussion.html
I used the Modified layout

Battery: 8.72v
+lead 8.56v
-lead 0.00v

Trans Voltage

Q1 2n2646
•   E: 0.595v
•   B1: 0.163v
•   B2: 1.343v

Q2 MPSA18
•   E 0.000v
•   B 0.118v
•   C 0.000v

Q3 MPSA18
•   E 0.232v
•   B 0.797v
•   C  3.36v

Subs: None

[Gavanti]

Bumping request for help.

If I am right that the 2n2646, isn't working (and I suspect my spare is bad too), does anyone know of a source with a relatively consistent record of working ones?

Many thanks.

[PRR]

Is there a Schematic?

It is very hard to suss-out circuit function (and happy voltages) from a very compact LayOut.

Not to mention how long it has been since I thought about Unijunction devices. Which have changed a lot over the decades.

Wild-stab: shorted or reversed capacitor on UniJ's Emitter pin to ground.

[psychedelicfish]

Whereabouts are you? If you happen to be in NZ or Aussie, Jaycar sells them at ~$5 each

[R.G.]

Let me bump the request for a schematic. Working from just a layout, especially a veroboard layout, means that before any serious thought can be done, it must be reverse-engineered/traced.

That being said, if the transistors are what I think they are, the UJT is latched up and providing current to the signal-quieting transistor's base all the time.

This could be from a marginal UJT, a wrong-connections UJT, an open UJT capacitor/connection, a solder bridge, an open solder joint, a whole lot of things.

The 2N2646 is obsolete, but can still be found. They vary. When I have built this circuit, I subbed in a 2N6027 Programmable Unijunction Transistor (PUT), which is what modern semiconductor practice uses to minimize the variations for similar applications. It needs another resistor and a modded circuit.

As another comment, the issue may not be the RP at all. Do you have links to all three of the schematics you're using? They may interact. As far as I've found, the schemos from the Starstream/Ultrasonic haven't been available on the web, only the schemos for the separate Vox pedals, which may be different.

[Gavanti]

Thanks to all of you for your help. R.G., your tagline about magic made me laugh, partially because that is what I'm constantly telling my students about the world they live in, and partially because I don't understand much of it either. My builds are still a bit more like puzzle solving than actual electronics, but I'm learning more about how everything works as I go, especially from the mistakes. 

You're also correct that I'm hooking these up as three separate circuits, each linked to a footswitch in a true bypass configuration. I am running their power in parallel from the DC jack/battery and have verified that each of the circuits is getting power. I'm including links to what I believe are accurate schematics for the three circuits in the box and also the vero layouts I followed. There are some discrepancies there. 

I used MPSA18s as per the Repeat Percussion layout instead of the BC108/BC109 pair specified schematic, and my rate pot is 50k anti-log rather than lin. In the other circuits, there are a few small-value resistor substitutions from the ones specified in the vero layout in the treble/bass booster and distortion booster circuits. The T/B Booster is using a BC108, and the Distortion is using a pair of 2n3392s. Both of those circuits seem to work as described.

Here are the schematics:

http://analogguru.an.ohost.de/001/schematics/Vox_TrebleBassBoost.gif
http://analogguru.an.ohost.de/193/schematics/Vox_V8162_DistortionBooster.gif
http://analogguru.an.ohost.de/193/schematics/Vox_RepeatPercussion.gif

And the vero for all three:
http://www.turretboard.org/wp-content/uploads/2012/03/voxtreblebassboost.pdf
http://mirosol.kapsi.fi/varasto/layouts/VOX-Distortion-Booster-V8162.png
http://tagboardeffects.blogspot.com/2012/08/vox-repeat-percussion.html (the third version with the tone and rate switches)


I think I have a basic understanding of how the UJT functions. I had read about the PUT on another post and had considered trying to set up the circuit with one, but I had as much trouble sourcing that as I did the UJT. I'll check for solder bridges again, but I think I'm ok there. I also think I read the pinout correctly with the tab between E and B₂ unless something was wrong with the can. I could probably run in a socket and see if a different pinout or my spare work any better. If I'm reading the layout right, the UJT emitter goes to ground via the rate switch and the rate pot. Unless I have some sort of current arc, those connections should be clean. 

Thanks again.

[PRR]

> UJT emitter goes to ground via the rate switch and the rate pot

I don't see a rate switch. (Unless you mean Q2?)

Ignore R6-R7-Q2 for now. Rate pot pulls-up C2 toward +9V. UJT does nothing. At some point around 4V, UJT E starts conducting heavily, pulls C2 nearly to zero V, then shuts-off. Cycle repeats.

Slowest repeat is around a half second. This may be too fast to see on DMM. Add 100uFd to C2. Now it bops at 5 second intervals. You should be able to watch C2 voltage rise, then drop and rise again. Rate pot makes it faster (until the DMM can't follow).

Over where R6 R7 come together we should find a fraction of that ramp voltage. Without Q2 it should rise to about 0.8V then drop. Except Q2 (which is used "inverted") will clamp it at about 0.6V. When clamping the base, it will also clamp the input signal.

To verify that, jumper the input jack to +9V. Watch the R8 R9 junction. Unclamped we expect 3V; clamped should be super-close to zero (~~50mV).

BUT....

> Sound: I'm getting none from the RP circuit.

NONE?

Snip the leg of Q2 that clamps the audio. It should be gain-of-3 circuit. Simple no-doubt booster. If so, follow R.G.'s thoughts on a stuck-on UJT circuit.

Except: your voltages around Q3 don't add-up. If there's 0.232V across the 3K3 emitter resistor, there should be 2.32V dropped across the 10X larger 33K collector resistor (plus a wee bit for R10). But you have 5.36V dropped across the 33K. If you are using color-stripe resistors, all org-org-??, maybe red org yel 3rd-stripe got mixed up?

[R.G.]

Yes, that helps.

Paul has the right idea - poke the circuit a bit and see how it reacts.

I would pull out Q2, and then see if I got some signal through Q3. If not, then no matter what Q1 and Q2 do, you can't hear it. And the voltages on Q3 do look funny.

Once you get signal through Q3, reinstall Q2. Open the C2/R5 end of R6. You should now have what you heard before - that is, signal on the output, but no effect.
Touch the open end of R6 to +9V. Sound should instantly disappear from the output, because Q2 is turned on and clamping it to zero. If it doesn't, there's something wrong with the Q2 modulator section.

Once that's working as expected, look at the UJT. You can reconnect R6 or not at this stage. The UJT works as a voltage-sensitive trap door. The voltage on C2 rises til it hits a critical voltage for the UJT. When that happens, the UJT suddenly starts conducting HARD and dumps the cap through the UJT's E and B1. When the current from this dump drizzles down to some tiny value, the UJT resets and no longer conducts. The voltage on C2 then starts rising again.

Paul is correct about trying to slow it down with a bigger cap. The only caveat I'd add to that is the issue with UJTs being funny and variable. I've seen several problems with this specific circuit. It happens to be a favorite of mine. That's why I was questioning the schematic - I've never been able to find the exact schematic for the schemos that are inside the Starstream/Ultrasonic, only the pedal versions. There are issues with some 2N2646 devices hanging up in the "on" position, or conversely never tripping and dumping the cap. If this last is what yours does, you can tell by the trick Paul mentions of slowing it down. Some UJTs can be damaged by very high dump currents, but in this one I think the 33 ohm in series with B1 will prevent this.

As a side note, PUTs are like SCRs, but triggered by a voltage on the gate lead which is one diode-drop below the anode. The connection for a PUT in this circuit is to tie the cathode where B1 is, the anode to the timing cap, and the gate where B2 is, but with a new resistor to ground from the B2 position. This fixes the gate at a diode below your desired trip voltage and then when the cap rises above it, the PUT is triggered, working the same as the UJT did. The resistor values on the B1/B2 position need messed with a bit, but I have built and tested a PUT version of this circuit.

[Gavanti]

Excellent. Thanks, Paul and R.G. It will probably take me a day or two to get back to the pedal, but these ideas should be helpful.

[kaycee]

A few people have got this going from the IVIark layout, pretty sure that I've built that varient with success. The UJT's are varible, but I've found that most of them work, some better than others though. So, initially I'd run through your build again for a misplaced/wrong value part, cap round the wrong way, track bridge etc. sometimes with little builds like this its quicker just to build another than scratch yer head at it.

Another thing you can try is replacing the 47k resistor to the emitter with a pot, sometimes you can 'tune' one in using a trim here, or get one that already works going a bit better, I usually put a trim on the board for this. The PUT works just as well, and is more readily available, needs a few mods so you can't change like for like, check out Moosapotumus' 'Skippy' build for more info.

On thing if you do get it going is be prepared for it to tick, I spent weeks trying to stop my builds from ticking but nothing would shut it up completely.

[R.G.]

On thing if you do get it going is be prepared for it to tick, I spent weeks trying to stop my builds from ticking but nothing would shut it up completely.

It is *possible* to do a non-ticking version. The secret is to realize that all wires are resistors, and that current through a resistor makes a voltage.

The tick comes from the sudden discharge of the timing cap through the 33 ohm b1 resistor. If any part of the analog circuit, including the modulator transistor, shares any part of the copper trace between the capacitor negative and the 33 ohm resistor, it will tick, and nothing can be done about it. The fix is to make that current path have one and only one wire back to the power supply negative.  If the tick changes the + power, that will tick as well, but there is already good decoupling on that.

[Gavanti]

OK. So I've had a chance to work through a lot of these suggestions. Thanks to your help, I managed to get the signal through Q2 and Q3--the main problem there seems to have been a bad joint--and I found a few other offboard wiring errors along the way.

I've got a new set of MPSA18s in, and I have installed some sockets to make transistor swaps easier for now, but neither of the UJTs I've tried seems to be producing the repeater effect. I get sound, but the circuit mainly seems to boost it and modify the tone.  I tried adding a 100uf cap to the rate switch (which is where C2 ends up in the modified layout if I'm right), but that doesn't seem to have any effect.  The Rate switch and Rate and Depth pots have no effect on the sound. The Tone switch on the other hand works very well.  Also, I'm still getting the odd voltage readings on Q3. I did run through all the resistor color codes to make certain those matched the layout, and things check out.

I may order some UJTs from a different batch to see if that's the problem. If I get a free evening, maybe I'll rebuild the circuit, but I'm still hoping to sort this one out.

DC: 8.54v

Q1
E  0.752v
B1: cycling .000 to .003v
B2: 0.185v


Q2
E 0.000v
B 0.148v
C  0.000v

Q3
E  0.233v
B  0.793v
C  3.3v

[moosapotamus]

Sometimes adjusting the value of R5 (4k7) can get the oscillator going. You might try replacing it with a pot (20k or 50k) and adjusting until you hear the effect. If that works, make sure you hear the effect at both extremes of the Rate pot before settling on a new value for R5. In the layout that I recently did, I actually replaced R5 with an onboard trim pot... http://moosapotamus.net/ideas/skippy-tremolo/

~ Charlie

[R.G.]

Q1
E  0.752v
B1: cycling .000 to .003v
B2: 0.185v

Yep, your UJT isn't doing its job.  I believe from the voltages that it's staying latched up, with the emitter turned on all the time.

As Moose says, increase the resistance into the timing cap. If this resistor provides enough holding current or your UJT is very sensitive, it will never turn off and let the cap start ramping back up. In fact, go back to a smaller timing cap and 10 to 100 times the timing resistance and see if it will start.

There is always the possibility of more wiring/soldering issues, at least with my handiwork, but the symptoms read like too much holding current.

Issues like this are why PUTs replaced UJTs - even though PUTs have the same issue with too much hold current stopping the oscillation.

[PRR]

I don't know what the box is supposed to do.

But it looks like the UJT is a free-running pulse generator? Or ramp since we take the cap voltage. Why not a 555 timer and a small increase in R6 (the divider down to the chop-transistor base)? Yeah it is pennies more cost and 5 more pins, but at this point J.J.'s time and hair-loss exceeds part cost. And 555 is likely to be around a while.

[R.G.]

I don't know what the box is supposed to do.

It imparts a tapering-off, suddenly-on tremolo. The envelope is a miniature, shorter version of a plucked string, and can sound like very short, percussive string plucks. The waveform is important.

But it looks like the UJT is a free-running pulse generator? Or ramp since we take the cap voltage. Why not a 555 timer and a small increase in R6 (the divider down to the chop-transistor base)? Yeah it is pennies more cost and 5 more pins, but at this point J.J.'s time and hair-loss exceeds part cost. And 555 is likely to be around a while.

... which is why a 555 or other square wave oscillator doesn't work well.

I experimented with the circuit a bit when I was playing with it. The raw circuit from the vintage pedal has some real rough edges, a product of doing it simply and cheaply with the parts that were available when it was designed. The variation in the UJTs is a problem, as is the current flows in filling and dumping that big timing cap. Subbing in a PUT fixes the issues with the variability of the UJT, and careful ground layout fixes the ticking.

But you can provide the waveform to the modulator transistor other ways. It needs only a very small current, so any way to generate the LFO voltage works OK. A slow-rise, fast-drop sawtooth works fine too. A couple of opamps can give you a good-enough version, as can some other approaches. But complexity goes up, and that scares off the beginners who like the idea of three transistors and a handful of resistors giving you a vintage effect.

[nocentelli]

But you can provide the waveform to the modulator transistor other ways. It needs only a very small current, so any way to generate the LFO voltage works OK. A slow-rise, fast-drop sawtooth works fine too. A couple of opamps can give you a good-enough version, as can some other approaches. But complexity goes up, and that scares off the beginners who like the idea of three transistors and a handful of resistors giving you a vintage effect.

Could you give me a few leads to chase up on this? I'd like to cop some of those utterly stuttery Repeat Percussion sounds without paying ~ $3 or more per UJT that might not even work.

[induction]

These work great with just one extra resistor.

MU4892's also work without modifying the schematic, but they're harder to find, so I assume they're expensive.

[nocentelli]

Great! Thanks.

[R.G.]

Could you give me a few leads to chase up on this? I'd like to cop some of those utterly stuttery Repeat Percussion sounds without paying ~ $3 or more per UJT that might not even work.

These cost $0.16 each:
http://www.mouser.com/ProductDetail/ON-Semiconductor/2N6027RL1G/?qs=sGAEpiMZZMuAO0%252bGuNbnQuTGsEEz0QMQHzlVS1cgwJQ%3d

You can sub them into the existing circuit with the following mods:
1. cathode goes where B1 was on the UJT
2. anode goes where the B2 was on the UJT
3. 33R resistor may need to be shorted (that is, made 0 ohms)
4. 1K to B2 changed to 2.7K
5. add new resistor 4.7K from B2 to ground
6. gate goes where E was on the UJT

I've tried several 2N6027s and they all worked.