Neovibe LFO won't oscillate ... help, R.G.!

george · September 10, 2006, 03:28:33 AM

Hi I built a neovibe using what looks like a very early version of the RG Keen layout. It uses discrete diodes in the power supply instead of a DIP diode bridge package.

I bought the PCB from Frank Clarke a few years ago - have only recently gotten around to building it.

I eventually got audio through it after a bit of effort with an audio probe.

But I canNOT get the LFO to work. It pulses on initial powerup but it gradually fades away.

Voltages:

junction of 2M2 and 3K3: 12.5V (a bit higher than the "correct" voltage of 11.7V but I've checked all the components and they are of correct value/orientation/pinout)

Q11:
C 21.6
B 10.1
E 11.8 (higher than the base, this should be a problem but it seems that other people get this and their LFO works OK)

Q12:
C 21.6
B 11.8 (should be same as Q11 E)
E 11.2 (looks like "correct" biasing ....)

Q13:
C 17.3 (6.3 V is being dropped thru the lamp)
B 3.4
E 2.7 (looks Ok in terms of BE voltage)

I have been over every nanometre of the LFO, I even traced it out and compared it to the schematic and it is correct. The only thing that I found wrong was that the 10uF cap connecting to terminal F (connects to 50K pot) was the wrong orientation. I fixed this but still no joy.

I used 2N3904's, in desparation I changed Q11 to MPSA18 (hFe > 500) and Q12 to 2n5088 but it still doesn't work ....

Your assistance with this very depressing problem would be hugely appreciated ....

I want to fix this so I can be Jimi Hendrix when I grow up to be a man.

R.G. · September 10, 2006, 08:51:36 AM

Have you read the most recent update on the wiring for the speed pot? There was a bug introduced into the last minus one version of the wiring diagram. That is corrected at GEO.
Check your speed pot wiring against that, then write back.

RedHouse · September 10, 2006, 11:32:11 AM

Quote from: george on September 10, 2006, 03:28:33 AM...I want to fix this so I can be Jimi Hendrix when I grow up to be a man...

Jimi said...

Quote: "Find yourself first, and then your tool, find yourself first, don'cha be no fool"
(Jimi Hendrix, "Message Of Love" from the Band Of Gypsys album New Years Eve 1969)

Yep, I'm a smart arse.

george · September 10, 2006, 07:40:57 PM

Quote from: RedHouse on September 10, 2006, 11:32:11 AM

Quote: "Find yourself first, and then your tool, find yourself first, don'cha be no fool"
(Jimi Hendrix, "Message Of Love" from the Band Of Gypsys album New Years Eve 1969)

<ADULT THEMES>yes from what I heard lots of people had no problem finding Jimi's tool .....</ADULT THEMES>

george · September 10, 2006, 09:09:31 PM

Quote from: R.G. on September 10, 2006, 08:51:36 AM
Have you read the most recent update on the wiring for the speed pot? There was a bug introduced into the last minus one version of the wiring diagram. That is corrected at GEO.
Check your speed pot wiring against that, then write back.

Thanks RG. I think it was originally wired up the right way but I changed it after I looked at the GGG pdf. I'll try it again as per your update and get back to you.

Thanks again

george · September 11, 2006, 07:15:43 AM

OK I rewired the speed pot as per the latest update but still no joy.

I notice that if I touch the terminals of the LFO caps the lamp starts blinking but then just fades away.

I'm using "low leakage" electros for the 1uF - that wouldn't be the issue would it?

Thanks
George

alderbody · September 11, 2006, 07:45:27 AM

I had exactly the same problem with my neo-vibe, and what made my LFO work,
was the replacement of the 1uF electros with film caps.
You'll have to find 1uF film caps with appropriate size, though. (i found mine in the local market ...in Greece...)

Just replace the ones in the LFO part of the circuit.

I remember i saw great improvement when i initially replaced those electros with NP's,
but what really solved my problem was the film caps.

Try them.

I guess they'll work for you, too.

good luck.

alderbody · September 11, 2006, 08:23:06 AM

there are two 10uF caps in there (C22, C23).

I had to replace them with NP's, too.

R.G. · September 11, 2006, 09:31:23 AM

I find this a little unusual, because I've used 1uF 50V polarized in all of the vibes I've built. But that doesn't mean that all polarized caps will work, I guess.

The 1uFs in the LFO are critical, because the LFO has just about enough gain as it is, and any leakage in these caps will cause a loss of gain. That would give exactly your symptoms, so I do see where leaky caps would cause problems, and film parts would fix it if that was the problem.

george · September 11, 2006, 08:00:08 PM

Thanks R.G., alderbody

I'll try those square MKT caps for the 1uF, they're NP film but still a reasonable size ....

don't know if I can get 10uF film caps, if I can they'll be huge .... I might have to ask Alderbody to send me some from Greece! Do you take paypal?

cheers

alderbody · September 12, 2006, 02:27:43 AM

Quote from: george on September 11, 2006, 08:00:08 PM
Thanks R.G., alderbody

I'll try those square MKT caps for the 1uF, they're NP film but still a reasonable size ....

don't know if I can get 10uF film caps, if I can they'll be huge .... I might have to ask Alderbody to send me some from Greece! Do you take paypal?

cheers

You don't have to use film 10uF caps. I used NP electrolytic 10uF caps and they worked fine.

I only recommended 1uF film caps for the LFO part.
I tried them once in other parts too, but i felt like i was deviating from the original sound, so i replaced them with electros.

So films only for the LFO. They really worked!

Now, if you can't find conveniently sized film caps, i think i could send you some.

But i believe that Small Bear must have small size 1uF film caps.

Ask Steve and if not, we can work something out.

alderbody · September 12, 2006, 02:30:42 AM

btw, here's how my clone looks like...

george · September 12, 2006, 06:13:07 PM

Nice box! Expect a "cease and desist" from Dunlop! (only kidding ....;-))

Can you hook up a pedal for the speed control?

That's OK for the caps - I found some nice compact greencaps (8mm x 10mm) rated at 63V yesterday. Thanks very much for the offer.

I'm going to use tants for the 10uF caps though - just to be sure

Thanks again

alderbody · September 13, 2006, 02:15:13 AM

QuoteNice box!

Thanks George!

QuoteCan you hook up a pedal for the speed control?

No, it's only controlled by the pot. I thought about "expanding" it, but it would get things more complicated.
That big socket that can be seen next to the volume control is for the power.
It's a 24V dedicated supply, and i had to make it kinda unique so there won't be any mistakes powering another pedal with 24V!!!

QuoteI found some nice compact greencaps (8mm x 10mm) rated at 63V yesterday.

That's great news!

QuoteI'm going to use tants for the 10uF caps though - just to be sure

Hmmm, i tried tants in there too, before the NP's, but i didn't get a good result. (at least as good as with the NP's)
You got nothing to lose trying them, though. Maybe i ran into not-so-good parts.

Good Luck with those caps

george · September 13, 2006, 07:57:16 PM

tried the film caps - LFO still won't work.

I checked all the component values and offboard wiring they all look OK.

The voltage at 3K3/2M2 is still a bit high - about 12.5V

alderbody · September 14, 2006, 03:19:46 AM

Quote from: george on September 13, 2006, 07:57:16 PM
The voltage at 3K3/2M2 is still a bit high - about 12.5V

hmmm...

did you try metal film resistors in the LFO?

how adout the transistors; did you try any higher gain stuff?

darlingtons maybe? or a darlington and a normal one. (i remember, one recommended FETs, but they didn't work in mine)

You could socket these transistors for easier replacements.

btw, i have put power transistors in there and it works... i think they were BD139.

but i had to put them off board.

Also the lamp driver is a BD137.

heavy stuff, but they worked fine.

alderbody · September 14, 2006, 03:28:24 AM

another trick would be to power the LFO directly from the 24V supply and not through the regulator.

but this requires some more tricks that i can't transfer to you safely...

maybe an expert (like R.G. or JCmaillet) could help you.

george · September 14, 2006, 08:35:42 AM

Hi Alderbody

thanks for the suggestions, but I already have a MPSA18 for Q11 and a 2N5088 for Q12.

I'm pretty sure greencaps are metal film ... and the LFO is powered from the unregulated side of the power supply which gives more than 20V DC ...

I was hoping that someone might pick up on the voltages I posted and give me some guidance.

what is this trick that you mention? I'm not sure I understand what you mean by 24v supply ... I am using 16V AC which gets rectified to 20 - 21V DC

RedHouse · September 14, 2006, 08:55:24 AM

Have you tried turning your depth/intensity pot back-n-forth? I hate to ask this seemingly stupid question, but I remember on my very 1st Univibe build I had the depth/intensity pot turned all the way down (to set the bulb-bias trimmer) and then forgot about it when I tried to get the circuit running, it seemed to "not work" but it came to life as I turned the depth/intensity pot up.

alderbody · September 14, 2006, 09:01:43 AM

QuoteI already have a MPSA18 for Q11 and a 2N5088 for Q12.

How about a 5089 for Q12, then?

QuoteI'm pretty sure greencaps are metal film ...

i meant that you should try metal film resistors, not caps. They have much narrower tolerance than carbon films.
Just to make sure that you get as close as possible to the "theory"...

Quoteand the LFO is powered from the unregulated side of the power supply which gives more than 20V DC

cool.

Quotewhat is this trick that you mention?

Not quite a trick actually... i remeber that you have to do something extra than just powering the LFO straight from the P.S.

I

Quote'm not sure I understand what you mean by 24v supply ... I am using 16V AC which gets rectified to 20 - 21V DC

Sorry, my bad. I power my pedal with a 24V DC external dedicated "wallwart" and i assumed that you might have done the same...

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Neovibe LFO won't oscillate ... help, R.G.!