YAND - Yet Another Neovibe Debug

[pyroxitis]

Hey there good people of the diystompboxes forum,

I am trying to debug my neovibe.
The audio path I believe is working - using my hand to change light on the LDRs the vibe works.
The problem is in the LFO section I believe - more specifically Q13 (bulb driver):

My measured DC voltages are as follows:

Q11: C   18.4
        B   3.35
        E   9.67 to 10.2

Q12: C   18.4
        B   9.65 to 10.2
        E   9.15 to 9.65

Q13: C   -1.8 to -2.15
        B   -1.8 to -2.2
        E   0.0 to 0.01

Q11-Q13 are 2N3904s.

("to" means it's oscillating depending on the speed pot)

Also, on the R39-R40 junction I have 10.8v.

I'm getting no light at all. I replaced C22 and C23 with new caps (the old ones were new as well) to no avail. I checked everything for continuity and for resistance values.

Please help! thanks in advance.

-Lee

[Brossman]

Not to sound silly, but did you possibly wire the LED backwards?

[PRR]

There's several different re-drawings of the neovibe schematic and they don't all use the same part numbers.... point to the plan you are working from so we's all on the same page.

How the heck can you have -negative- voltages on your Q13? There's no negative supply. I can think of a way that can happen.....

You say there is NO light? Maybe blown lamp? Try another lamp. Try a 2K resistor where the lamp goes... it won't blow, and the voltmeter waggle will tell if Q13 is doing a right thing.

[pyroxitis]

Thanks for your replies - I'm using the GGG board: http://www.generalguitargadgets.com/pdf/neovibeupdate.pdf

I seriously have no idea how I can have a negatives voltage there but that's what my multimeter check was - one probe touches circuit ground and another the collector of Q13. I'm using all stock parts except for the diode bridge which I subtituted a DB104 instead of DB102.

How can I check whether the lamp works? I'm using this one: http://www.smallbearelec.com/Detail.bok?no=646

thanks!

[R.G.]

Did the lamp ever light, even once?

It's rated at 1.5V, 25ma. You can test it by connecting it temporarily across an AA, a C, or a D cell and seeing if it lights. Those batteries are rated 1.5V.  You can test it with a 9V battery by hooking a 330 ohm or 470 ohm resistor in series and connecting bulb and series resistor across a 9V battery.

Another thing to do is to use your ohmmeter setting to see if the bulb is open (in which case it will never light again) or if it appears nearly a short, which means the filament is OK.

[PRR]

Yeah, if you have not seen the lamp light, do what RG says. Put it on a 1.5V battery, or in series with 470 ohms on a 9V to 18V supply. Or ohm it. Under 10 ohms is probably good. (Under 1 ohm may be shorted across the fine-wire legs.) 

(Britt is thinking of something else. This incandescent lamp has no polarity. I dunno why it is sold with 2-color leads.)

Smallbear doesn't sell dead lamps, but lamps are fragile. It could have cracked in shipping or been blown in-circuit. It would be good to have another.

FedEx/USPS will take $5 to ship a $0.69 part. Are you near a Radio Shack? They sell an equivalent lamp for a buck more (bubblepack).

1.5V/25mA Miniature Lamp
Catalog #: 272-1139
$1.69
Availability: In stock {all 7 stores within 50 miles of me}

For rough-testing, you can replace the lamp with 100 ohms (sorry, 2K was a high guess) and meter for voltage wiggle. Or 100 ohms plus an LED and look for blink. With LED the polarity matters. Conceptually the 'vibe can work with an LED. In practice you can't cover four photo-resistors with one LED, and the fast LED will sound different than the slow incandescent. Still you can confirm everything works except the lamp.

[pyroxitis]

Did the lamp ever light, even once?

It's rated at 1.5V, 25ma. You can test it by connecting it temporarily across an AA, a C, or a D cell and seeing if it lights. Those batteries are rated 1.5V.  You can test it with a 9V battery by hooking a 330 ohm or 470 ohm resistor in series and connecting bulb and series resistor across a 9V battery.

Another thing to do is to use your ohmmeter setting to see if the bulb is open (in which case it will never light again) or if it appears nearly a short, which means the filament is OK.

it did light when shorting the base and collector of Q13 in the past (however now not even that works). the voltages on Q13 were different then also - collector being at least 16v if I recall correctly... during that time I had a mistake in wiring the speed pot, now fixed.

to the naked eye I can barely see the filament and it seems fine, however with an ohmeter and continuity checker I don't get any readings... not even the smallest one - I assume it's open.

If it is open I will order another one - but I wouldn't want to burn another lamp - is my Q13 voltage reading reasonable? could it be the culprit? I will subtitue a 100 ohm resistor tomorrow and get new voltage readings.

thanks everyone for taking the time and trying to help!

[R.G.]

it did light when shorting the base and collector of Q13 in the past (however now not even that works). the voltages on Q13 were different then also - collector being at least 16v if I recall correctly... during that time I had a mistake in wiring the speed pot, now fixed.

Shorting base and collector of the lamp driver makes the transistor not limit current any more. It's only limited by the emitter resistor of the driver transistor. If that goes a lot over 25ma, the lamp gives its life for that one, bright moment of glory.

to the naked eye I can barely see the filament and it seems fine, however with an ohmeter and continuity checker I don't get any readings... not even the smallest one - I assume it's open.

Given the history, yep, it's probably open. If an ohmmeter shows over a meg, it's open.

If it is open I will order another one - but I wouldn't want to burn another lamp - is my Q13 voltage reading reasonable? could it be the culprit? I will subtitue a 100 ohm resistor tomorrow and get new voltage readings.

Sub in 100 ohms and look for wiggling on the Q13 collector.

You can't judge this by the voltage on the Q13 collector. Q13 lets current flow til it satisfies the voltage needs of the base and emitter circuits. The collector of Q13 may go to any voltage between the power supply and the voltage forced on the emitter as needed to get the current it's trying to pass.

[Jaicen_solo]

I had a similar problem with my build when I tried to use a grain of wheat bulb intended for model railway use. It was possible to light the bulb from DC, but the Neo couldn't provide enough current to get it to light up. I have a suspicion that the bulb has some sort of integrated voltage or current limiter that makes it not work in the Neo.

Either way, I tried all sorts of bulbs, most of which 'worked' in the sense that they lit up, but the only one that worked for the Neovibe was one I got from Steve at Small bear. It's an oldskool looking thing with a looped filament, that just seems to respond better than the more modern looking coiled filament for some reason.
I'm sure there are other factors at play, but that bulb really made the difference for me.

[pyroxitis]

So, I put in a 100 resistor instead of the lamp and the Q13 collector voltage looked fine - around 18.4v if I recall correctly. I then put a LED in instead of the resistor and it flashed, with the speed knob and depth knob seem to be doing what they're supposed to. Bottom line - it seems to be working! It even works reasonably with the LED in instead of a lamp (sound wise).

So what I believe happened is that: at first I didn't wire the speed knob correctly (forgot to make another connection there) so there was no oscillation. Then, I burnt out the lamp and fixed the speed knob - but still no visual feedback of it working (lamp was open) therefore I thought I still hadn't fixed the LFO.

So yeah, I'll have to wait for another Smallbear delivery but I think I can safely say I have a working Neovibe, and that I can be thankful it was solved quickly thanks to your help R.G., Jaicen_solo, and PRR! You guys rock!

[pyroxitis]

I'm back again!
This time both a lamp and a LED is lighting up - but no visible flashing what so ever. It just stays on. In the past I had successfully had the LED flashing only with Q13 emitter disconnected by accident - that's why it worked - and only with a high-brightness LED.

Now, with a lamp connected I can control the brightness of it using the lamp bias trimpot, but again no flashing.

Checked voltages and they are as follows:

Junction of R39+R40: 10.75v

Q11:   C: 18.27v
          B: 18.27v
          E: 18.17v

Q12:   C: 18.27v
          B: 18.19v
          E: 17.62v

Q13:   C: 11.45v
          B: 11.05v
          E: 9.18v

Transistors are: Q11,Q13 - 2N3904 and Q12 - 2N5088. I socketed these while trying to debug: If I put a 2N5088 in Q11 I get about 3.5v at the base.

Please... I'm going crazy over here....

[PRR]

>Q11:   C: 18.27v
>          B: 18.27v
>          E: 18.17v
>Q12:   C: 18.27v
>          B: 18.19v
>          E: 17.62v

"All" your voltages are essentially equal to supply voltage.

Best first guess is: missing ground under Q11 Q12, possibly at R41.

> If I put a 2N5088 in Q11 I get about 3.5v at the base.

_NO_ reason a change of transistor should make such large change of bias .... something else is going on. Bad joint?

[pyroxitis]

>Q11:   C: 18.27v
>          B: 18.27v
>          E: 18.17v
>Q12:   C: 18.27v
>          B: 18.19v
>          E: 17.62v

"All" your voltages are essentially equal to supply voltage.

Best first guess is: missing ground under Q11 Q12, possibly at R41.

> If I put a 2N5088 in Q11 I get about 3.5v at the base.

_NO_ reason a change of transistor should make such large change of bias .... something else is going on. Bad joint?

I believe the R41 area is connected correctly.
If it may help to diagnose - a LED in place of the lamp with the Q13 emitter disconnected is working pretty nicely - intensity and speed can be controlled... obviously the lamp bias trimpot does nothing then.

With Q13 connected normally (emitter connected) and a lamp connected I can see a slight (very very very small!) flash only with the intensity and speed turned up.

Also I rechecked the voltages:

Q11:
c 18.27
b 3.33
e 9.1-11

Q12:
c 18.27
b 9.1-11
e 8-12

Q13:
c 11.20-11.22
b 10.72-10.89
e 8.93-8.95

[R.G.]

Check for the actual resistor values by measurement, not what you think they are by the colors.