Univibe question: What happens when....

[Mark Hammer]

What potentially happen when someone accidentally plugs their guitar cable into the speed-control jack for the external cntroller pedal?

I'm asking on behalf of someone else who has commited this accident, and now finds that the pedal is stuck on slow speed.  If there is any sort of modulation at all, I'm assuming all is well with the light source.  The person who commited the act says there is nothing wrong with the speed pots.

My guess is that the manner in which a mono plug would likely short the pots may have damaged one or both of the transistor pair that form the Darlington.  But would any of the electros or diodes in the LFO possibly be compromised?

Thanks for any help.

[PRR]

Schematic?

Stock original, modified, cloned, easy-ed?

Looking at R.G.'s page, the only "external controller" is an LFO signal kill. I don't see how a guitar here would do any harm.

Maybe the incident was mis-reported? (Plugged the 400 Watt bass-amp output??!)

If it wobbles at all, anywhere like a normal rate, the speed pots should change the wobble rate. At best it just-barely wobbles, but that wobble *will* be at the pot-cap rate.

Insufficient information. I gather you have not inspected the culprit personally, and are just groping for easy answers? None from me.

[mth5044]

The originals had a 6 or so DIN connector for the speed control that also had the bypass switch on it. It would take some serious doing to get a guitar into there. +1 on schematic.

[Burstbucker]

Hi guys, I'm the poor bugger that originally posted for help with this pedal and Mark Hammer graciously posted this here in hopes of solving the problem.

The pedal in question is a small-box KR Mega Vibe which I bought used probably 10 years ago.  The speed pot is a 100k, linear I believe and is connected to the PCB via a switched, stereo 1/4" jack located on the righthand side of the pedal, this is where you can plug in an external speed controller pedal if desired.  The speed pot in the pedal and the pot inside the speed controller pedal both have dual-gang 100K pots that have their centre lugs jumpered together.  There are three wires going between the speed pot and the PCB but they go through that switched stereo jack which is wired between the speed pot and the PCB.

One wire goes to the jumpered center lugs of the dual-gang pot.  It's basically two pots wired in parallel with the wipers jumpered together.  So when the pot is completely counter-clockwise, the resistance between the common jumpered wipers and the other two wires is appx, 68k and when at extreme clockwise position, it's zero ohms.

I had a few pedals plugged in before the KR Mega Vibe and accidently plugged the output of the last one into the ext.speed control jack mounted on the side instead of the Mega Vibe's input jack which is top-mounted.  I found that I had no sound and upon investigation, I noticed my error.  This wasn't the first time that I had done that and never had a problem in the past.

After plugging it all together correctly, the Mega Vibe's speed pot is unresponsive.  It's just stuck on low speed.  This could be a coincidence and that plugging into the ext.jack had nothing to do with my current problem.

I noticed one time that after using my external speed pedal and unplugging it, the pedal's speed knob wasn't working but when I checked it with the ohmmeter, the switched stereo jack was glitchy and was staying open on one or two of the wires hooked to it.  But with a blast of De-Oxit it seemed to come back.

But this time, whatever readings that I have on the speed pot are present right on the PCB, there's continuity between the speed pot and the PCB, the switched stereo jack doesn't seem to be giving me problems this time.

I'm stumped!

I'm not an electronics technician but I can read a schematic and get around with a meter ok but that's about it.  I was just worried that I might have damaged something on the PCB by plugging the output of a fuzz pedal into this ext.speed jack.

It appears that whatever the problem might be, it's on the PCB somewhere.  Dang!

Anyways, I'm planning on bringing it to a tech here in my town next week.  He's pretty knowledgeable about music gear, I'm hoping that he'll be able to bring her back to life without breaking the bank.

But I sure would appreciate any input from you guys if you'd like to chime in!

[R.G.]

I'm away from my stack of schematics. Got a link?

If the  KR is like the original univibe, chances are that it's a dead cap in the LFO. If the darlingtons were dead, it would not phase at all. Ditto the lamp driver. But I'll need the schemo to look at to tell more than that.

Got links?

[Burstbucker]

Sorry R.G. but I don't have a schematic for this thing but from what I've read in the past, it was basically a direct copy of the original Univibe pedal.  It sounds great but the actual construction is surprisingly sloppy inside.  But at least it sounded awesome before it crapped out.

If accidently plugging a mono cable into this switched stereo jack for ext.speed control would potential fry something on the PCB, it sure wasn't a good design choice on KR's part........  It should be something different like a 1/8" mini jack, or whatever they're called.

I hope it's not going to be an expensive fix.  But the pedal did sound really nice, a big fat chewy sounding Univibe tone but wow, but the back of that PCB is nasty looking, lots of messy solder everywhere, it's pretty scary looking.

[aion]

Edit: (basically I just suggested that you clean the contacts of the switched jack without reading the rest of the thread)

[Mark Hammer]

I'm away from my stack of schematics. Got a link?

If the  KR is like the original univibe, chances are that it's a dead cap in the LFO. If the darlingtons were dead, it would not phase at all. Ditto the lamp driver. But I'll need the schemo to look at to tell more than that.

Got links?

I wasn't able to find any, but there are several gutshots available, like the one below, which suggest it likely follows the original very closely.

[Burstbucker]

Yes that's it.

The three wires in question are the yellow, red and blue wires in the top right hand corner or that PCB.

[MrStab]

The wires have continuity to the PCB, but do they have continuity between each other ( when they shouldn't)?

[analogguru]

Hi guys, I'm the poor bugger that originally posted for help with this pedal and Mark Hammer graciously posted this here in hopes of solving the problem.

The pedal in question is a small-box KR Mega Vibe which I bought used probably 10 years ago.
..... 

So we are not talking then about a professional made product like the "Univibe", instead we are talking probably about a tunnel-of-horror-clone of a not-so-able-to-solder-jockey.  Is yours looking like this too ?:





Then you are a poor soul.  Everything can brake only by looking at it.

What does the manufacturer say to this problem ?
No support for his junk ?  Like: "take the money and run" ?

Sorry R.G. but I don't have a schematic for this thing but from what I've read in the past, it was basically a direct copy of the original Univibe pedal.
.... 

Did you already ask him for a schematic ?

Hows about taking some sharp pictures of the guts of your unit ?

[Burstbucker]

MrStab,

I know that the readings across the pot lugs are correct as you change the position of the speed knob but I will check to see if this is the case right on the PCB itself.  I'm pretty sure that I did check that already but will double check to make sure.  Thanks!

[Burstbucker]

Analogguru,

Unfortunately, yes this is exactly what my PCB looks like.  Scary!  I'm astonished that it's worked this long and sounded so good, I mean really good!

I didn't even consider trying to contact Kevin at KR because of all the negative posts that I've read over the years about his lack of communications.  I never bought this direct from him, just somebody online probably ten or eleven years ago.

[Burstbucker]

Ok, 

I just sent an email to Kevin asking for a schematic.  I'm crossing my fingers but not holding my breath!   

[PRR]

> external cntroller pedal?
> basically a direct copy of the original Univibe pedal.

I don't see an "external controller pedal" on the original original.

But by the story, this is what failed?

Then if it is 95% original and 5% novel, we need that 5% with context.

Looking at R.G.'s trace-out:


We could replace the onboard pots with offboard pots with 3 wires, 2 switched. This could be done with a stereo switched jack (and I assume it has to be 1/4" or else the mix-up would be unlikely/impossible?).

A dead-short could blow the transistor, but then it would not wobble at all, and you say it does.

A high voltage (speaker line) could do much more damage. But all those parts are needed to wobble, which you say it does.

Ah? If (both?) pot-wires are broken, it will wobble, but at a very slow speed. Maybe 1/3rd of the lowest stock speed?

I'd think if just one pot got disconnected, this barely-unstable oscillator would just up-and-quit, at least for some setting of the still-connected pot.

Sadly it works much more subtly than trailer-lights or even a Fuzz-face. There isn't any simple diagnostic path that I see. Staring at a 'scope may give clues, but it works on very slight shifts of phase and that can be hard to prove on a 'scope.

I'm actually thinking '8038 or similar fun-generator heart-replacement. Someone may have a digital LFO plan or product. A particular advantage is that these schemes can use a single pot, and are pretty SURE to oscillate as desired. The UniVibe LFO is a very clever good-sine with more range than a 1-pot phase-shift LFO, but it needs a gain over unity and that transistor is resolutely under-unity. (It does work, breaks no natural law, but to real-world techs it is a miracle every time.)

As the 1uFd caps are past their due dates, it may be wise to replace them on general suspicion. 2 of them have to be very-near the same value, and film caps may do better (if they can be fit).

[Burstbucker]

Man, now I've just noticed that the speed indicator LED isn't even blinking anymore, it's just stuck on all the time and not flashing.  This thing is possessed!

I hope that my local tech can figure this out for me........................

[Burstbucker]

BTW guys,

A little update on my Mega Vibe.  I brought it in to my local tech and explained the problem and what you guys thought it might be and within an hour and a half of leaving his shop, he called me and said that it was fixed!

He found that one of the two transistors in the LFO circuit was blown, he had a similar replacement on hand and put it in.  Works like a charm again!

I have had intermittent hiss/crackling noises when adjusting the volume knob, so I gave it a couple of shots of Craig's DeOxit 5 and the pedal sounds like new.

I want to thank everybody here for helping me with this and a special thank you goes out to Mark Hammer for posting this on here for me.  Thanks a bunch guys, my pedal is ready to rock again!!!

[Mark Hammer]

Thanks for the followup, and confirmation of our suspicions.  Pleased to hear that you are back to swirling again.  I knew you'd arrive at a successful outcome if you came here.  Some VERY knowledgable and generous people.

See you back in Canada!