KR MEGAVIBE issue

[harrisxr650]

Reopening the thread cause somehow its lost and i really need some help as i have a gigi next week and i need the megavibe.

Hey guys, hope you're all fine . I'm having weird issue with this pedal... "sometimes" (lets say about 8 out of 10 which makes the pedal completely unusefull) when i turn on the pedal the lamp stops blinking, it doesn turn off completely but it stops blinking and it lights but very weak..when i turn off the pedal after 4-5 seconds it starts blinking normaly again, when i turn it on it does exactly the same...same thing happens when i max out the speed pot, lamp stops blinking and it fades a lot but not turn off completely, when i backup speed pot a bit ( around 12'oclock) after 3-4 seconds the lamp starts blinking normaly again. Sometimes it doesnt start blinking again unless dc jack is been removed for a couple of secs but when power it again same thing happens....Any idea where to start looking? I've made an optical check for blown caps, resistors, bad solder joints and everything seems fine. I've read that issues like this one have to do with the transistor driving the lamp (c828) after the internal trimpot and changing it would fix the problem? I havent any experience with lfo/ldr's but i have very good soldering skills and i'm a quick learner so if anyone could help a bit would be nice.

R.G. posted a layout and suggest the the problem should be in the transistor (Q11?) connecting to the 2.2M res in the upper right corner? what type is the transistor? should i look for 2sc828 or a bc549 as suggested in some other post will do the work? 
http://img844.imageshack.us/img844/8632/megavibelayout.jpg

thanks again
Harris

[R.G.]

You want high gain and then to stuff the proper lead (i.e. collector, base, emitter) into the proper hole.  2N5088 is my fave, but other transistors with high gain work too. 2N5089, MPSA18, probably the BC549, and others with hfe>400 typically.

For reference, the problem is that the LFO on the univibe needs a minimum current gain to keep operating over variations in the speed pot. If the gain is a bit too low, it oscillates a cycle or two when turned on, then quits at some settings of the speed pot. Cure is to get more gain in there.

You can (generally) replace both Q11 and Q12 with one MPSA13 or MPSA14, putting the base lead in the Q11 base hole, the emitter in the Q12 emitter hole, and the collector in either collector hole.

[harrisxr650]

forgive my silliness here but any change the the psu cant supply the required ma for the unit to work? I'm running it off a 12v 500ma psu and i thought maybe somehow that thing cant supply the megavibe with the correct ma anymore.
To be honest i'd rather keep the pedal 100% original in case i'm going to sell it someday and the buyer is a mojo lover that doesnt like alterations and mods to the circuit ( just a thougt) in that case replacing Q11 and Q12 with the same 2SC828 will it fix the issue or there isn't no other way that the one you suggest?
Thanks again for the precious info.

[R.G.]

forgive my silliness here but any change the the psu cant supply the required ma for the unit to work? I'm running it off a 12v 500ma psu and i thought maybe somehow that thing cant supply the megavibe with the correct ma anymore.

It's possible that the power supply is defective,  I guess,  but 500ma is way more than it needs. I suggest you measure the DC power on the board and settle the issue.

To be honest i'd rather keep the pedal 100% original in case i'm going to sell it someday and the buyer is a mojo lover that doesnt like alterations and mods to the circuit ( just a thougt)

I'm probably not a good person to ask about issues like that. From everything I've heard, the KR Megavibe is a not-particularly-well-done copy of the Univibe, with a lot of advertising hype. Maybe it's great, but I've not seen that. So it doesn't strike me as a possible target of mojo-mania,  but then neither do the zillions of "boutique" pedal builders that each claim their pedals are better than anything else, ever. Many of them pestered me for info during their effects-building infancy. Some of them grew up and did good things, some didn't.

In any case, I feel about this much like I feel about hyper-vintage-original amps. If it's too valuable to fix so it runs, then it's only useful as a piece of sculpture. It's always at least possible that some art-fancier will pay a bigger price to own this or that piece of sculpture. But I can't afford to own sculpture, things that are pretty or have a great story behind them, but don't do anything useful.

But if you'd prefer to keep it original, it's yours, and it's your perfect right to do so. To do that, you kind of have to accept that it may not fully work.

in that case replacing Q11 and Q12 with the same 2SC828 will it fix the issue or there isn't no other way that the one you suggest?

Transistors vary; so you could order a bunch of 2SC828's and select for high gain, then sub one of them in there, or two of them in for both Q11 and Q12. The datasheet I just looked up says that 2SC828s have hfe from 130 to 520. If you have two 130's in there, it could cause the problem you're seeing. If you had two 520s, it might work fine.

So yes, you could find, buy, and hand test 2SC828's and select high gain ones, if you can find them. In my mind, I don't distinguish between this and putting another transistor type in there with a high gain. It's still not the transistor that came in it originally (which doesn't work right, as you know) and the only difference with using a transistor labeled "C828" is that the buyer maybe won't question it if they're not sharp enough to detect the different soldering.

But the part numbers will still be original, I guess, if the devices are not.

[harrisxr650]

I know what you mean, i'm not a mojo maniac and owning a pedal that it isnt usefull or it might die in live situtation its not what i'm after .
I was asked in the past to sell it for around 500$ (well...having the pedal and knowing its issues, i wouldnt give more than 200$,btw i'm not a great fun of the mega vibe for the reasons you mention i prefer Stuart's Castledine build more) but only if there is any mods or alterations to the circuit.
Now.. having a bunch of the transistors you recomended (2n5088,mpsa13,14) i'll go the way you suggest and put one in place of both Q11 and Q12 to eliminate the problem once and for all.
Thanks again R.G.
Harris

[R.G.]

Shoot, I'd call up that guy that wanted to give me $500 for it and make the trade.

[harrisxr650]

haha!!! Actually that guy is a mojo freak...I wouldnt dare to explain that it would be no difference with that transistor repair  .  For him i'm about to destroy a mojo vintage pedal .

[R.G.]

Ah. Well, call him first!  He'll understand that original is better than working, for sure!   

[harrisxr650]

after some time i managed to do the above repair sadly with no luck. still does the same. i used 2x 2n5088 ( hand selected using  http://www.geofex.com/Article_Folders/ffselect.htm   circuit) for q11 and q12 with around 480gain each. but still does the same. any ideas? any voltage i could take and post?
thanks again

[R.G.]

after some time i managed to do the above repair sadly with no luck. still does the same. i used 2x 2n5088 ( hand selected using  http://www.geofex.com/Article_Folders/ffselect.htm   circuit) for q11 and q12 with around 480gain each. but still does the same. any ideas? any voltage i could take and post?

Hmmm. Have to think a bit. Usually more gain in Q11-12 fixes this. Could be another problem that's rarer.

[harrisxr650]

well.. i know the i info i  provide are  not what it should be but as i said i dont have any experience with lfo's. If you could guide and tell me anything i could check with my dmm (voltages etc) i might make it easier for you . thanks again

[harrisxr650]

btw i've notice something else. the bulb flashes one time when i plug the power then immediately goes off then lights on very weak, no matter where the intensity knob is set

[Digital Larry]

This is a total guess but it's probably a 3-capacitor phase shift LFO.   

Did it ever work or was it always like this?  Stop working suddenly or got worse over time?

If your transistors are good I'd suspect something else in the LFO circuit such as a bad solder joint or a leaky capacitor.  But if it never worked I'd be going back to fundamental principles and replace the LFO with something I knew worked well.  That would wipe out the vintage mojo for sure and might be a little hard if you're not all that comfortable with electronic design.

I was unable to locate a schematic for this piece in casual searching.

[harrisxr650]

i've been working with that pedal since yesterday afternoon and found out today that one of the  transistors i swaped was defective or i burn it while soldering... i put a new pair but before i plug it i did check once more for solder joints or anything else that doesnt look good and i noticed that between the contacts of the exp. jack was a bit of something like grease (?) i removed it pressed the contacts agains each other and it worked!!!!! So i decide to let it run for some time to make sure its ok.. I left on with a loop on tha input about 2hours mabe more at full speed full intensity and it worked all just fine!!! But then it stopped again suddenly...same issue :S dont know why though..i've noticed that the LFO trimpot was turned full clockwise so i'm gonna put it in the middle swap the 2 transistors again and set it up from the beginning to see how will go..maybe leaving it 2+ hours on with internal trimpot full clockwise made one or two transistors defective. will see.
and yes the pedal was working very good before that. maybe the previous owner messed up with the trimpot and turned full clockwise.
thanks anyways will post again soon i guess

[harrisxr650]

here is a layout. a bit different regarding components placement but other than that its the same.
http://img844.imageshack.us/img844/8632/megavibelayout.jpg

[R.G.]

i've been working with that pedal since yesterday afternoon and found out today that one of the  transistors i swaped was defective or i burn it while soldering... i put a new pair but before i plug it i did check once more for solder joints or anything else that doesnt look good and i noticed that between the contacts of the exp. jack was a bit of something like grease (?) i removed it pressed the contacts agains each other and it worked!!!!! So i decide to let it run for some time to make sure its ok.. I left on with a loop on tha input about 2hours mabe more at full speed full intensity and it worked all just fine!!! But then it stopped again suddenly...same issue :S dont know why though..i've noticed that the LFO trimpot was turned full clockwise so i'm gonna put it in the middle swap the 2 transistors again and set it up from the beginning to see how will go..maybe leaving it 2+ hours on with internal trimpot full clockwise made one or two transistors defective.

You're hot on the trail of the problem. Looks like it might be a build issue.

[harrisxr650]

I hope so. Its to difficult to change components cause of the build design and the (full of solder) paths ( jesus...). If this is the issue then maybe its a good idea to set the trimpot correct and then swap it with a resistor and forget it. anyway i hope i will find a solution and fix it. i call it a day for now and try later and post again.

[harrisxr650]

AAAAND!!! problem solved thanks to you and your help guys!!! For reference original  lfo transistors (2SC828) needed to be replaced anyways (one was defected the other one was low gain around 200 hfe) either with the same or a substitute but higher gain as R.G. mentioned . Put two 2N5088 around 450-480 in place and worked like a charm for 2+ hours then went dead again, noticed some dirt between the exp jack contacts which i've cleaned  push the contacts against to make sure contact achieved and checked with DMM replaced the two 2n5088 as both were gone to meet the Maker. Power on, played for some time again then went dead again... checked everything again  and found internal trim-pot fully clockwise
and i think that was the problem...when the trim-pot is fully clockwise somehow both lfo transistors gets defected...haven't take voltages yet but i guess it will be good to know. so what i did was : set the speed pot at 5 o'clock, set the intensity pot around 8 o'clock and start to turn the trim-pot slowly clockwise until i've heard a faint vibe, it went a tad less than 3/4 of its full range and then turn the intensity pot all the way up to check that is working perfectly, and it does after that i did another test, turned the trim-pot fully clockwise to see if anything goes wrong and after 20-30 mins it went dead again...so i did the previous again and so far its working perfectly, trim-pot around 3/4 and it fired up nicely and ive been playing for more than 2 hours with no problem.

R.G once more thanks for the help
Harris

[harrisxr650]

btw i'm goin to test it the next days in rehearsal and live situation to make sure its fine and then post again. btw what exactly the trimpot does? its for intensity bias?