Electric Mistress not working

[saxtim]

Just finished building my Electric Mistress and it's not working.  

The unit passes audio but it doesn't do anything else - the pots and the switch have no effect on the sound.  

I'm using a NOS SAD1024a from small bear, and I socketed all my IC's so there should be no damage from soldering.

I've traced the circuit through for continuity with the schematic and everything links to what it should without a problem.

I subbed a couple of values in the circuit, from memory I used and 11k resistors a couple of times instead of a 10k and 39nf instead of a 33nf (becuase I ran out of parts).  I wouldn't think this would be the problem

I haven't managed to get many voltage readings - most points I tend to get numbers that swirl within a .5 volt range - but I read the same range of voltages at all the V+ points.  I think this is because of the LFO??  I'm getting around 15 votls power to the board.

I've checked for shorts.

I have no idea where to go next on this one!  Help!?

thanks
tim

[saxtim]

I did some more testing, but I don't think I'm any closer to figuring it out.

Looking at the schematic:

http://www.generalguitargadgets.com/v2/diagrams/dmistsc.gif

From what I can tell I'm getting dry signal through the system, but nothing from the wet signal (off the SAD1024a)

I used an audio probe to trace the signal, but to be honest I'm not really sure with what I'm doing with this.  I got signal at pin 6 of IC1, pin 1 IC2A and pin 7 IC2B.  This makes sense as I'm getting dry signal out.  

I don't know if this is supposed to be, but the signal at pin 1 of IC2A is much softer than the signal at pin 6 of IC1.

Anyway, I've got signal at pins 2 and 15 of the SAD1024A but no signal at any other pin.  There obviously should be signal at pins 12 and 5 - as thats where it's mixed back in no?

As for IC4 (LM324) I've got no signal on any pins.  I should be getting signal at 1,8,7, and 14 no (the outputs)?

Also no signal at LM311 at pin 7.  

I've got no idea if I should get anything out of the 4013, i imagine not?

Any comments? I'm not sure where to follow the signal after it hits pins 2 and 15 of the SAD1024a - I assume that these have to be outputed somewhere in order for there to be signal in the bottom half of the schematic?

thanks
tim

[EdJ]

have you tried adjusting the trimmers?
I spent an evening adjusting the wrong ones,cursing at the unit.
The follwing day i discovered my fault and with a little twisting of the correct one the unit came to live.
HTH,Ed

[saxtim]

Ok,

I have some more evidence to consider:

I had all the trimpots centred.  I started changing each one whilst leaving every over one centred.  Non of them had any effect except the balance trim.

When centred there is a high pitch frequency that is present, but still fairly soft when compared to the rest of the clean, dry signal I'm getting through.  I hadn't really noticed it before because it's reasonable soft and I had been testing the unit at soft volumes.

This frequencys volume increases when I adjust the balance trim either way of centre.  I remember reading in another thread some time ago something Mark Hammer said about this circuit and the clock signal arriving at the balance trim out of phase - could this be the clock signal I'm hearing?

The actual freqeuncy itself can be modulated via the range pot.  At one extreme the frequency is at it's lowest, but when you rotate the pot the frequency increase (pitch increases) up till around 3/4 rotation or so and then it disappears, so at the other extreme the signal is gone.  This only happens when the filter matrix switch is in one position.  The other position of the filter switch the frequency disappears altogether (I'm not sure which position is which to be honest and I haven't had a chance to check it yet.) I should also say that the range pot doesn't effect the dry signal I'm getting out, only this other high frequency that is present.

Any ideas? I have no idea where to proceed with this.  

tim

[Mark Hammer]

Yes, the balance trim will appear to increase and decrease the level of the whining sound, as you produce greater and lesser amount of cancellation of the two opposite phase clock signals coming from the two output pins.

The Range control will also make the whine change in pitch because the whine IS the clock signal setting the amount of delay.  At a certain point the clock signal will be too high to hear, and will also be more effectively filtered by the onboard circuitry.

I sort of stumped as to why adjusting the bias control does not yield a delay signal at any point in its rotation.

[saxtim]

So the bias trim is the one I should concentrate on to get a delay?  I have fiddled with all of the pots, but I'll concentrate on this one and do some more fiddling and see if I can get it going.


thanks
tim

[saxtim]

ok a did a little bit of fiddling with the bias trim pot and that made no difference in any position.  Still no delay.  Any other suggestions?  I hate to think it, but could the SAD1024a be dead?  I mean I get dry signal to it, but nothing out of it.  Considering the clock appears to be working, should I look for trouble at the LM324 or LM311?

I went through it again last night and check all component values - all are correct as far as I can see

tim

[axis]

HI, It could be the sad1024,because I just got some bad ICs from small bear myself.This is unfortunate  because I like and trust Steve.

[uncle boko]

What voltage do you have at pins 1 & 5 on the 4013? They should be roughly the same around 7-8volts I think - if not suspect the 311 or the 4013. Do you get any hiss at the output of the SAD1024?

[saxtim]

What voltage do you have at pins 1 & 5 on the 4013? They should be roughly the same around 7-8volts I think - if not suspect the 311 or the 4013.

7.3 at both pins.  Looks good

Do you get any hiss at the output of the SAD1024?

Nothing

HI, It could be the sad1024,because I just got some bad ICs from small bear myself.This is unfortunate because I like and trust Steve.

I hope this isn't the case, as I bought a MN3007 from him too which cost a packet.  The clock chip was from steve though and that appears to be working, so I haven't given up on the SAD1024A yet.

I have another idea - I was fiddling with the bias trim pot whilst probing the input to the SAD1024A - the level there is quit low compared to after the first opamp (LM741).  I was moving the bias pot around and it had no effect on the signal level at the SAD1024a - this should not be the case right??  I'm going to go and check over that part of the circuit carefully.  Hopefully that might yield something.  

Any confirmation of this or other suggestions are welcome - I'm still really just stabbing in the dark at the moment.

thanks
tim

[saxtim]

Well, I had no luck there.  I can't find any shorts in that area and everything has continuity.  As another hunch I tried taking out the 4558 and putting in a Tl072, no difference.

But I still think this is a problem.  Can anyone tell me that if,

a.  the signal should be softer after IC2a than it is after IC1?

and

b.  If the bias trim should increase/decrease the output from IC2a (into the SAD1024A?

thanks

tim

[saxtim]

Ok I think I'm now getting somewhere.

I'd missed a connection ground on the bias trim pot.  I now have the same level signal out of IC2a on to the SAD1024a as I was getting out of the IC1.

I've also got output on pins 6 and 11 of the SAD1024a.  The output of these is very very soft though.  I've tried adjusting the trim pots etc, but no difference.  adjust the other controls (rate,colour etc) made no difference either.  I'm still only getting a dry signal out of the unit at the output jack (or at least the delay signal is so soft that it's not in the mix)

I seem to have lost the whining clock noise too - don't know if that's good or bad, or it's just not as present as it was before.

What now?  I think I'm getting there, but I still don't know where to look next

thanks

tim

[StephenGiles]

What happens to the 1024 output when you adjust the bias trim? Do you get nothing then a distorted sound, then clean sound, then distorted then nothing - even at low volume? If yes, then the chances are that the 1024 is working as you have the correct clock voltages. I would suspect the audio trail between input and 1024.

[saxtim]

Ok, I'm quite stupid!  After I discovered the missing ground connection off the bias trim pot I didn't really sit down and slowly move it through the whole rotation to see if any setting would make the effect work, I just turned it quickly round and figured that hadn't solved the problem.

But, today I sat down and moved it really slowly and the effect came to life!  There is only a very small range on the trim pot where the effect will work.

Now I still have one problem - the range pot doesn't function correctly.  On one extreme, the effect works fine, but as I turn it I start to get loud, regular pops and the effect seems to switch from dry to wet signal back and forth every half second or do.  Sometimes it just pops once and switches back to dry and stays until you turn back far enough and it pops again and it goes into effect mode.

I doesn't do this with the filter matrix switch down - you can sweep the entire range pot with that on and it works correctly.  

At least I'm nearly there now! Thanks to everyones help so fay - but does anyone have an idea what's causing this last (hopefully) problem?

thanks

tim

[soundcollage]

I recently made one of these pedals and wired it for true bypass, and did not include the final resistor in the circuit (a volume buffer i believe as buth signals were originally passing through it). You are right to say that the trim knobs are sensitive it took me a while to dial in a good sound. now my only problem is a slight hum while the effect is engaged. I believe this is because my transformer is not shielded and possibly overkill at 600mA. I actually had a larger 1A one and replaced it with the smaller one which greatly reduced the hum. Mouser did not have the 300mA version in stock. I am hoping some copper tape shielding will fix the problem, as now it is not aparent at stage volumes but i would not use it in a studio.
   What transformer did you use?