Doh! Another Zombie Chorus post!

[RJ]

Hi everybody, sorry for another thread about a nonfunctioning Zombie Chorus...I know there's been more than a few in the past and many people have written a considerable amount on this topic, without which I wouldn't have made it this far!

For the last couple days I've been fighting with a Zombie Chorus on my breadboard.  I'm using the schematics found at http://www.geofex.com/ and http://www.geocities.com/munkydiy/zombie.jpg.  I'm trying to make it work with an MN3207 but I'm only getting a quiet, uneffected signal at the output jack.  I'm using TL072s as the opamps (U1 and U3) and I've tried TL062s also...as well as 2 other MN3207 (U2) chips, just in case. U4 is a CD4046.

I've been digging through the archives like crazy for the last couple days...there's a ton of useful information regarding problems with the Zombie, which are almost always related to the biasing of the input pin of the MN3207 or MN3007.  I've followed the advice found in several threads but I'm afraid I'm not sure what my next step in debugging should be....or if I'm doing something totally wrong with the biasing...this circuit has had a big learning curve for me....yesterday the word 'bias' didn't really mean anything to me in regards to a circuit, now I understand it's pretty important!

As for the MN3207 chip, I have pins 1 and 8 going to ground.  Pin 5 (Vdd) is receiving V+, approximately 8.3 volts from a 9V battery.  I have a 100k trimpot set up for pin 4 (Vgg) which I understand is supposed to be 14/15 of V+, set for around 7.7 volts.  I have another 100k trimpot and a 15k resistor (in series) set up for the input pin, pin 3, set for around 4.3 volts.  (The trimpots are set up with one outside lug going to V+, the other outside lug to ground, and the wiper to the appropriate pin of the MN3207.)  For the Vb part of the circuit, I reversed the 10k and 15k resistors and Vb is around 3.3 volts.   I'm not sure if pin 7 is supposed to be connected to ground or to V+ (it's tied to ground on the schematics, but that's for an MN3007 chip), but neither connection has any effect on the output.  I'm also not sure exactly how U1 (the first TL072) is supposed to connect to pin 3 of the MN3207 now that I've changed how it receives its bias voltage.  The schematic shows pin 1 of U1 connected to pin 3 of U2 (the MN3207)...I didn't alter that connection.  The schematic also shows pin 2 of U1 connecting to pin 3 of U2 through a 47k resistor...I removed that connection.  

I've tried lots of different voltage values for Vb, the bias voltage for input pin 3, Vdd and Vgg (pins 4 and 5) but all I can get is a quiet, cleanish output...much quieter than the bypassed signal.  I'm not sure what else I can do to bring the MN3207 to life.  I'm not sure if maybe there's other modifications that need to be made to the circuit for a MN3207 chip to work...I've dug through many posts in the archives regarding the Zombie Chorus and MN3207s and biasing their input pin, but there isn't really a definitive list of changes that must be made to the schematic in order for a MN3207 to work properly.  Also, is it possible that all 3 of my MN3207 chips are damaged, or maybe lots of my chips?  They had to do a lot of travelling after all to get from New York to Alberta, Canada.

If anyone has any suggestions or observations it would be greatly appreciated, thanks in advance!

-Ryan

[puretube]

FIRST:
pin 8 of a MN3207 is one of the 2 outputs, which should not be grounded at all!

[puretube]

SECOND:
pin7 of a MN3207 is the other output: don`t ground!

b.t.w.: neither in GEO, nor Munky`s do I see it grounded or plussed!

[puretube]

THIRD:
the input-bias should go from the wiper of that pot through a 100k series-resistor to pin3 of the MN3207

[puretube]

FOURTH:
the output of U1 should connect through a capacitor (e.g. try 100nF) for decoupling, to the input-pin of the BBD, since you give that one a separate
bias, which differs from that of U1.

b.t.w.: the input of the BBD would be shorted without that 100k resistor mentioned  in the post before.

It would also be nice to put a cap (22uF) from the junction wiper & 100k resistor to ground

[puretube]

fifth: are we looking at the same schematix?...

[RJ]

Thanks for clearing those up, puretube!  Much appreciated!  Unfortunately after making those changes my output went from quiet and cleanish to quieter and very distorted...twiddling the bias supply to pin 3 of the MN3207 alters the degree of distortion and the output level by a small amount, but I guess I still have a major malfunction elsewhere.

The reason I wasn't sure what to do with pins 7 and 8 on the MN3207 is because on the schematics they're tied to V+ through a 47k resistor off of pin1...but on the MN3207 pin 1 goes to ground...so I assume I should still be supplying V+ to those pins?  I tried that and it had no effect on the output, but is that the right way to do it?

One final question regarding U1...should pin 8 be going to V+ and pin 4 to ground?  Or are those connections not supposed to be made?  Putting U1 to power and ground like that gives me a nice loud clean tone, but still no chorus I'm afraid.

Thanks for helping me out, it's appreciated!

-Ryan

[puretube]

ok, Ryan: I take back 1st and 2nd comments, after you said you took pins 7&8  via the 47k resistor to ground, where it should be hooked to.

b.t.w.: do you have a scope at hand?

[puretube]

SIXTH:
after you already decoupled the output of U1a with a capacitor from the input of the BBD, you also have to decouple (the newly biased BBD`s)
output, that is: pins 7/8 including the 47k, from the input of the lowpass-filter around U1b, which starts with R7 (I use the numerals of the GEO-schematic) with a 100nF capacitor.

oh, yes: now U1b needs to be re-biased in turn:
similar to R2 at U1a, hook up a resistor (10M or 1M) from the node of your new decoupling capacitor with R7, to Vb.

[puretube]

SEVENTH:
Mr. R.G. KEEN has a serious typo in the description of his schematic diagram
(while the schematic itself doesn`t depict the P.S. to the op-amps):

He writes:

" Pin 4 U2 and U3 connect to ground; Pin 8 of U2 and U3 connect to +9V. "

THIS IS WRONG!!! (although pin 4 of his U2 does connect to ground...)
he very probably intended to write correctly:

" Pin 4 U1 and U3 connect to ground; Pin 8 of U1 and U3 connect to +9V. "

[puretube]

EIGHTH:
now the BBD`s bias is completely independant from the op-amp`s,
you can re-do the original Vb voltage divider.
If neccessary, or if distorted (I assume by now you get some delayed/modulated sound), try equal values for R12 & R13 (10k).

Wish you success, and hope none of the BBDs is fried....

[gorohon]

Finally, some more info on using trimpots to bias and decoupling caps.
Thankyouthankyouthankyouthankyouthankyouthankyouthankyouthankyou, etc.

[puretube]

Biassing a MN3207:

10k trimpot.
left lug to ground.
right lug to +9V.
wiper to 100k resistor.
at this connexion (shorter expression: node),
the positive pole of a 47uF/16V electrolytic cap.
other pole (-) to ground.
other end of the 100k resistor to pin 3 (of the 3207).
set trimpot to middle,
then turn slowly in either direction, till ("wet" = delayed)
output signal offers lowest distortion/largest "headroom".

voila!

P.S.: now the BBD got it`s own bias (which gets transferred
to its output in a way...),
all that is going into it, and is taken out from it, needs to be
hooked to pin 3 (input) and pins 7/8 through series capacitors.
(I said 100nF, coz thats a value everybody should have at hand,
but you can take larger ones for better bass response, but don`t
ask me how they need to be polarized... that depends on the rest
of the individual circuits).

[gorohon]

I'm using the mn3007 replacement (NTE1641).  I tried a bastardized trimpot setup with no decoupling cap--I'm learning. A little info on this specific application would really help me out, if indeed there is a need to clarify.
Thanks for more info puretube!

[puretube]

Mark Hammer (thank you!) has (almost) ALL the BBD`s data sheets with manufacturer`s application schematics on his site (about 5MB download...).

[Marcos - Munky]

I'm using the mn3007 replacement (NTE1641).  I know pinout is different.  I tried a bastardized trimpot setup with no decoupling cap--I'm learning. A little info on this specific application would really help me out, if indeed there is a need to clarify.
Thanks for more info puretube!

Could you tell me the difference in the pinouts?

[gorohon]

Never mind what I said about different pinout.  My mind was somewhere else.   :oops:

[RJ]

Ok puretube, we're making some progress now!  I have things set up now just like you described, with a .1uF cap on the output of the MN3207, a 1M resistor off that node to Vb, U1 is now properly receiving V+,  I've redone the original Vb divider with 2 10k resistors, and I replaced the 100k trimpot I was using to bias the MN3207 with a 10k and a 47uF cap to ground.   Thanks a million for those suggestions, they definitely had a big impact.

Here's where my Zombie Chorus has progressed to...I still have a prominent clean tone but underneath that there's a distorted and modulating signal...about half as loud as the clean guitar sound is.  When I adjust the bias for the MN3207 I can make the distortion go away, but I also stop hearing the effect at all, it just sounds like clean guitar.  The Depth and Speed pots don't seem to have any effect on the distorted, modulating sound....but pulling out the MN3207 stops it, so I guess at least it's doing something now...which is a big improvement!  I tried the 2 other MN3207 chips I have to see if there's any difference, but it's the same with any of them.  

I'll keep going over the circuit, maybe I've laid something else out incorrectly.  Thanks again for the help, it's really appreciated!

-Ryan

[puretube]

NINETH:

where does your pin 4 of the BBD go now? (Vgg).
Ought to be 14/15th of +Ub.
In your 9V case, this would be +8.4V, which is roughly 1 diode-drop less than the 9V.
I haven`t tried it yet, but you could wire a 1N4148 form +9V to pin 4.
(the diode "pointing" to pin 4)....

[puretube]

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