3904 & 5088 - Strange Problem

KHStudio · December 27, 2006, 05:20:58 PM

I built a Neo-vibe & it sounds VERY good but I'm having a strange problem.

With 3904's in the LFO (Darlignton) there's a hum & the speed will not reach it's MAXIMUM speed unless I touch the speed pot, intensity pot or (the PCB side) right on the Darlinton

With 5088's in the same spot - ALL the problems go away

Speed is fine, noise is gone, etc... BUT, the bulb intensity & SWING is not as good as it is with the 3904!

I tried a 1000ma 24v DC supply to see if the extra current would help & I think for the most part the noise level did go down BUT I'm still having the speed issue.

What could be causing the 5088's to be fine & the 3904's NOT???

This is frustrating... the pedal is ready to go. PLEASE HELP ME OUT.

Kevin

StephenGiles · December 27, 2006, 05:48:10 PM

Are you sure that the 3904s are connected properly?

KHStudio · December 27, 2006, 09:27:39 PM

Quote from: StephenGiles on December 27, 2006, 05:48:10 PM
Are you sure that the 3904s are connected properly?

If they're the same pinout as the 5088's then YES.

#1 The noise problem with the 3904's installed in the LFO is GONE with the beefier Power supply

I was able to get the fast speed to stabalize:

With the 3904's installed (LFO) & all the components mounted in the case (WITH THE BACK-COVER OFF!) & a ground wire run directly to the out jack from the extra ground next to the Power Supply.

When I put the cover on the speed INSTANTLY SLOWS DOWN

NO it is NOT shorting ANYTHING out

Also, when I dissconect the added ground to the out jack the fast speed falls back down again.

Kevin

markm · December 27, 2006, 10:20:25 PM

Are ya sure the cover isn't contacting a jack possibly?

KHStudio · December 27, 2006, 10:52:16 PM

No. I think there's a ghost in there

I've checked everything in the LFO & most other area's including PS & GND.

How in the hell could the 5088's work & the 3904's NOT? That's got to be the BIGGEST Questions... man, I'm suppossed to be finishing my third LA-2A & my NEVE mic pre right now but this damn pedal sounds so good I want to complete the job. I'm not beginner but I'm also no expert but I've built about 15 guitar pedals (Never a Phaser Type), 2x LA-2A's, 2x 1176's, 8 channel API, 3x SSL Buss Comp & almost every famous guitar amp out there. I'm about to run over this thing with my car

Could a bad transistor or an electrolytic-cap in another area (if it's bad) effect the LFO somehow? ... thru the ground maybe?

I may have shocked the board when I first fired it up... my desoldering braid was under it & I couldn't see it. I can see a tiny burn/spark mark on the ground trace next to the large 1000uf cap (which I changed).

What are the most sebsitive components this could have affected, maybe thru the ground & could this be my problem?

Thanks for helping,
Kevin

KHStudio · December 28, 2006, 12:34:55 AM

I got it to stabalize by putting a Ceramic CAP across the base & collector of Q11

I don't even know what I've done but I'd like to know

The voltage swing of the bulb is much lower now though... maybe even closer to the 5088 which didn't have enough but it still isn't bad.
It's also pretty quiet for the most part.

Weird stuff here.

Kevin

Paul Perry (Frostwave) · December 28, 2006, 06:32:09 AM

Modern transistors - even the lowly 3904 - can have so much gain that they oscillate all by themselves (due to stray capacitances). usually around 80 MHz or so, don't ask me why!!
A fix: tiny ferrite bead on one leg. I'd go for the base.

KHStudio · December 28, 2006, 01:01:48 PM

Quote from: Paul Perry (Frostwave) on December 28, 2006, 06:32:09 AM
Modern transistors - even the lowly 3904 - can have so much gain that they oscillate all by themselves (due to stray capacitances). usually around 80 MHz or so, don't ask me why!!
A fix: tiny ferrite bead on one leg. I'd go for the base.

Thanks for responding... so basically my circuiy is more sensitive to the 3904's over the 5088's

or should I say the 3904's are sensitive to the circuit? It's a little confusing but I guess the oscillation can cause the speed to go slower or faster... strange - an oscillating device in an oscillating circuit

I played it for hours last night & it sounds & works GREAT (thank the Lord

I forgot to mention that the ceramic cap I used on Q11's base to collector = 100pf
Is it OK to leave it like this or is there a downside to this method?

If so I will try to find a bead... I think I have some.

Thanks again,
Kevin

Paul Perry (Frostwave) · December 29, 2006, 08:09:32 AM

It's still only a guess that the problem was ultrasonic or RF oscillation, but putting a cap there would sure cut the RF gain down

No problem with leaving that ceramic there, it won't damage anything.
And when a circuit is unstable & changes behavior wiht different power supplies.. that often means that it needs a couple of caps across the power supply, right at the effect PCB. Say an electrolytic (anything from 25mfd up) and one of those little .1mfd ceramic bypass caps.

KHStudio · December 30, 2006, 12:53:56 AM

Thanks man... this Neo-Vibe is KILLER! I Love it.
Sold my Dunlopoo-poo pedal after doing a comparison... there wasn't any

The Neo-vibe has 1000uf caps filtering the PS so I'm not too sure what you mean?

Could it be I have a bad cap somewhere?

Kevin

Paul Perry (Frostwave) · December 30, 2006, 08:07:09 AM

Well, put one of those little bypass caps in as well. Those big electros have low impedance across the audio range but at radio frequencies, there is series inductance there that can cause resonance peaks.
The other thing is, when stuff works but not when the base is screwed on, is feedback between input and output wires (or pot wiring) that is squashd closer together when the base is put on. Yeahe I have been there.....

petemoore · December 30, 2006, 08:27:15 AM

] is alot easier than figuring out what it was or is.

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3904 & 5088 - Strange Problem