Vox Repeat Percussion trouble

[daydreamdelay]

I built this a while back and it wasn't working correctly so I ended up putting it on the back burner for a while in favor of other projects. I just got back to it, tried a few things but I still cannot figure out what the problem is or how to fix it

basically it works but I have to turn my amp up half way just to hear the effect.. I can keep going, basically cranking the amp (a loud JC-120) all the way up and the effect doesn't get any louder.. thankfully I turned the amp down before switching the pedal off

I poked around looking for iffy connections and noticed I get a VERY loud pop and noise when touching the 1 meg resistor (R10 on the fixed Vox schematic) and Q3. That is until I tried turning Q3 around, the pedal worked the same but I only got the loud pop and white noise when touching the resistor. I also get a loud pop when touching the right side (output) of the 3PDT

I don't know if any of this makes sense.. it's just the only thing that I could find that is sticking out as a problem

any help would be very much appreciated

[drewl]

This a pedal, or an old amp w/percussion?

[daydreamdelay]

it's a pedal

[DougH]

There was a bum schematic for this being circulated a few years back. Make sure you have the right one first. Check this thread:

http://www.diystompboxes.com/smfforum/index.php?topic=38473.0

I haven't built mine yet  but I have the UJT's.

[daydreamdelay]

I got gut shots and the schematic from the same person and his works so I'm pretty confident that it's the right one.. I've just messed it up somewhere

[DougH]

Start by reflowing all your solder joints and check your wiring.

[jdanner722]

Hey all,

I started this project a year ago but ended up abandoning it for a bit. I ordered the transistors recently as I'd finally regained an interest.

Everything is soldered together and works fine however I did run into a couple of issuesand being as I'm relatively new to all of this I'm not exactly sure how to troubleshoot them.

1. Things are wired together so that everything sounds great as a tremolo pedal. The issue here is that there are no "percussive" qualities at all. I found a video online some time back for comparison and it doesn't  exactly sound the same. I did notice that if I touch transistor 1 or 3 the signal changes. When I touch Q1 there is some clicking going on. If I leave my finger attached the clicking continues even when I haven't played a single note.

2. I went with a depth mod by replacing R9 with a 10K linear pot which works fine when it's turned about halfway or more.  Anything less than that it sounds like the pedal hasn't been activated at all.

Any suggestions?

Thanks for your help, it's much appreciated!

J.

[R.G.]

Read and follow "Debugging: what to do when it doesn't work". That batch of advice gives us the info we need to help you.

[jdanner722]

Sorry about that...

Like I mentioned I guess there is a tremolo effect going on with the pedal but no real percussion like I'd heard n the sound samples posted online.

Here's the schematic I had found a while ago :
http://analogguru.an.ohost.de/193/schematics/Vox_RepeatPercussion.gif

I only did the depth mod as was outlined in the schematic and I substituted the 68n (C3) cap with a 100n cap and Q2 (BC 108A) with a BC 108B.

The red battery lead came back as 8.53 with my black battery lead reading 0.00.

I measured the leads of all three transistors

Q1 (2N 2646)
E=3.11 and fluctuated between this and 3.49 (it started low and steadily increased, decreased, then repeated)
B1=0.03
B2=6.15

Q2 (BC 108B)
E=0.00
B=0.49
C=0.00

Q3 (BC 109B)
E=0.35
B=0.94
C=4.49

Thanks again,
J.

[R.G.]

Like I mentioned I guess there is a tremolo effect going on with the pedal but no real percussion like I'd heard n the sound samples posted online.

I only did the depth mod as was outlined in the schematic and I substituted the 68n (C3) cap with a 100n cap and Q2 (BC 108A) with a BC 108B.

Thanks. That helps.

Q1 (2N 2646)
E=3.11 and fluctuated between this and 3.49 (it started low and steadily increased, decreased, then repeated)
B1=0.03
B2=6.15

What should happen here is that the voltage on E should suddenly drop to near 0, then increase steadily, then drop to near zero quickly. If it's just wobbling between 3.11 and 3.49, then it's not dropping fully, and that could account for the "no percussion".

What is the range of speeds you get with the rate control?

Q2 (BC 108B)
E=0.00
B=0.49
C=0.00

Q3 (BC 109B)
E=0.35
B=0.94
C=4.49

These look to be OK so far.

I think the problem is in the UJT circuit or some of the parts between Q1 Emitter and Q2 base.

[jdanner722]

Thanks R.G.!

After some investigating, it looks like I may have used a 1.5K resistor as R2 rather than the 1K resistor from the schematic. Do you think this could be an issue?

I'm also wondering if it would be advisable (as a replacement for the 68n cap C3) rather than the 100n cap I initially used?

[R.G.]

After some investigating, it looks like I may have used a 1.5K resistor as R2 rather than the 1K resistor from the schematic. Do you think this could be an issue?

It could be. UJTs are quite sensitive to their resistances. Make it 1K and see if it gets better.

I'm also wondering if it would be advisable (as a replacement for the 68n cap C3) rather than the 100n cap I initially used?

That's less of a problem (probably!). It would smooth the volume variation out more. Maybe, but probably not the entire problem.

[jdanner722]

So I tried replacing the 1k resistor without much luck. I even required the off-board components (as I was receiving a lot of hum with a wall wart plugged in). I looked at theat schematic again and noticed that c2 has a positive charge on both sides. Either I hadn't noticed it before or thought it might have been a mistake. Is it possible now that I can put two 5uF caps in line with each other to resolve the issue?