AC-2 WORKS! (Vb 4.5 volts is 1.54 ... Huh?)

[MartyMart]

I may have found my "DIY" Boss AC-2  problem.
I have the usual 9v to a pair of 10k resistors, one to ground paralleled
with a 22uf cap...... should be 4.5 v ( approx ) off 10k number 1 right ?
9Volts has a reverse 1N4001 and 100uf to ground just before the Vb setup .....

What's happening here, I cant figure it, should I change them for some
1% metal film 10k's  ??
They are 10% carbon films at present.
It means that all my "board 4.5v" points are reading 1.54v

Thanks,
Marty.  :shock:

[BJF]

Hi,

You have exceeded the current capability of your voltagedivider or in other words current is not drawn symmetrically.

You can buffer the mid supply - asimple transistor would likely due
and you just hook up a diode
like this:
10K from B+ through diode, trough 10K to ground and connect base of transistorat the top of the diode seen from B+. Use emitter with suitable emitter load , say 4,7 K
and connect all common points to this new output.
For this you can also use an OP but may perhaps not room
You can use a Zener stabilised mid point if you have a set B+ voltage.

If you do not worry about current draw you can lower the total resistance of the divider and use about 2K2 and 2K2.

You might wonder why this happens and want to search the circuit and you can most likely find what is drawing, could be LED's et.c

Good luck and no Metalfilms won't help

Best
BJ

[R.G.]

You might want to try reading about bias voltage sources at GEO.

You have connected something to the resistor string that sucks too much current.

Try this: disconnect everything from the 10K/10K resistor string. I suspect that you will now read 4.5V - or half your actual battery voltage, whatever that really is.

As pointed out in the article at GEO, a typical resistor string divider for Vb cannot really maintain a half-voltage unless the two resistors conduct about ten times as much current through them as goes out (or comes in) of the junction in the middle.

[MartyMart]

Thanks RG, there's around thirty connections to Vb ! so a total disconnect
is a no no !
I'll have to find out what is causing the "drain"   :shock:
It's a big circuit so I knew it wouldn't be an easy one !

Marty.

[Doug_H]

Look for a short, Marty. If the 10k/10k is part of the original design, that should provide enough current if everything else is functioning properly. This could be a symptom of something touching something it is not supposed to, or a mis-read resistor color code, etc, etc...


Doug

[Eb7+9]

... paralleled with a 22uf cap......

try disconnecting that cap - see what happens ...

[mat]

Hi,

Best
BJ

Hi There,

Sorry (Marty) for the OT but are You (BJ) the man behind the BJF pedals ?

I owned the Pale Green Compressor and it was superb  :!:

mat

[BJF]

Hi,

Thank you Mat.

Yes I am BJ of BJF Electronics, known on some boards as BJF.

Oh, yes I completetly forgot there would nowadays be links to anything you'd like to know- education is good!

I remember bumping into a similar situation about 20 years ago and could rectify it from moving some parts that went to pseudoground.
In those days internet was only for the military and while you could ask technicians  they would either give you a scientific explination above your head or completely have forgotten about analouge electronics and amplifiers in particurlar and would hardly understand anything meant for use with electric guitar.
I think this exact thing with pseudo ground is mentioned in Craig Anderton's book but not clearly why  and when you should use this- because some insight is assumed- once you know the electronflow you can do the stroll of Kirshof and find out what is going on
It is however explained in Walter G. Jungs IC OP amp Cookbook, and in some articles in Elector.
Having said that I got much help from various technicians and many books at the library, but I wish somebody had told me what to do with the pseudoground when it first occoured to me- it would have saved me a lot of work or maybe not

Also if this thing is a ready circuit that is known to work with 10K divider, you should check first the cap and then if that doesn't do it you should be able to see from the tree that runs fro pseudoground where the current is going and if it seems complicated try disconnecting
suspected parts until you find the culprit.

Best regards
BJ

[puretube]

...
It is however explained in Walter G. Jungs IC OP amp Cookbook...
BJ

this one:
http://diystompboxes.com/sboxforum/viewtopic.php?t=34907 ?

[BJF]

Hi,

Yes, looks like it.

Actually I got the spinoff 'Audio IC OP amp Applications' first and then
bought IC- OP amp Cookbook the 1983 edition, might have been updated a few times since then but through the years it has remained a good read for anything OP amp based and I highly recommend it for anyone interested in OP amps.

Regards
BJ

[MartyMart]

To :
Doug H
Ton
BJF
RG
------- THANK YOU - THANK YOU - THANK YOU ------

The 4.5v line was being "drained" by the third ( of four ) dual O/A's
I had a connection to pin 5 which should then have been "cut" as I was using that trace for caps/resistors to pin 6.

Done  
Thanks to cbiere for his work on this Boss AC-2 clone, I'm sure it would
have been much easier to use his PCB rather than this HUGE vero
layout ...... but now it works and sound marvelous !!  

Regards,
Marty.