Vintage Mistress ticking

[yeeshkul]

I own two vintage Electric Mistress pedals - the big boxes from 1976 and 1977.
They differ just a bit - the 1977 pedal has a few extra caps.
What bothers me about the 1977 pedal is loud ticking that follows the Rate (LFO ticking). The 1976 pedal is way quieter.

Is there any way get rid of the ticking?

[Plexi]

I own the RI..and have the clock noise too.
I follow your thread...

[Scruffie]

Basically just how that design rolls, but there's a large value cap on the 4013 or 339, like 100uF or so, you can try replacing that or adding another in parallel with it.

[robthequiet]

2 links,

http://www.metzgerralf.de/elekt/stomp/mistress/index.shtml -- to help identify version and possibly get schematic


http://fuzzcentral.ssguitar.com/tremulus.php -- how bump was eliminated, different circuit but an idea of where to look for a solution

HTH,

Rob

[yeeshkul]

In the later designs they added a trim-pot on the VCO buffer collector that would null the clock. I wonder if that would work on the early design. My is V2.

Yes, there was 33uF filtering cap between V+ and ground, it replaced it with a brand new 100uF.

[Fender3D]

In the later designs they added a trim-pot on the VCO buffer collector that would null the clock...

You're confusing VCO and LFO....
the ticking you described above, comes from LFO since it follows rate control.
Look for the correct schematic, amidst the zillions issued by EH, then search for Stompboxology @ Mark Hammer's web site.
If you're lucky, you might apply the same advices used for tremolos there (many Mistress don't use op-amp for LFO...)

[yeeshkul]

It is Mark Hammer, who describes the trimpot on VCO as "null clock" trimpot that tames the ticking.
http://archive.ampage.org/threads/1/fxgd/019145/Electric_mistress_mods-1.html

"... just below the mix trimpot is the clock null trimpot for cancelling the clock signal noise,"

Yes, the ticking is most definitely produced by LFO. The trimpot MH is talking about is a part of VCO buffer though.
I have no idea how would that trimpot cancel the clock noise, that's why i am asking first.

My Mistress:


Here is DeLuxe with the trimpot installed:


All schematics are here
http://www.metzgerralf.de/elekt/stomp/mistress/index.shtml

[Fender3D]

The clock null trimmer, is connected @ BBD's outputs.
You will find it on Deluxe models, according to the schematics @ Metzgerralf.

The trimpot you posted, sets the minimum VCO frequency (clock).

If the null trimmer is out of setting, you will hear a whistle, sometimes a white noise "swoosh", definitely not a "tick"

[yeeshkul]

I can see "balance/mix" and "gain" trimmer at the BBD output.

[Fender3D]

the balance/mix is also called null trim

[yeeshkul]

Here is whar Mark Hammer says about the trimmers in DeLuxe Mistress:

"If you were to magically change the metal case for an invisible one, and looked down at the unit from the standard operating position, you would see the following trimpots: 
- between the RANGE and COLOR controls is the regeneration trim (EH used the term COLOR instead of feedback or regen), 
- just to the left of the RANGE control you would see the bias trimpot (used to set the bias voltage going into the SAD-1024 BBD chip), 
- on the left hand side of the board, you'll see three trimpots; in the upper left corner near the line cord is a trimpot that sets the dry/wet mix, 
- just below the mix trimpot is the clock null trimpot for cancelling the clock signal noise,
- closest to front of the pedal on the left is the initial delay trimpot. "

[Fender3D]

Well english is not my mother tongue, but to me - just below the mix trimpot is the clock null trimpot for cancelling the clock signal noise describes the trimmer @ BBD's out and - closest to front of the pedal on the left is the initial delay trimpot describes

The trimpot you posted, sets the minimum VCO frequency (clock).

Anyway you're just a screwdriver away from discovering it 

[yeeshkul]

You are right, thank you for deciphering it for me

[Scruffie]

I've got a V2 too and i've repaired a couple, none have not ticked, true some are better than others but that's neither here nor there.

If I remember from my experiments the problem is actually in the VCO portion (IC4C/D) but there's not a whole lot that can be done about it, it's the design.

[yeeshkul]

I have two suspects.
1. The funny voltage regulator may have gone bad - i will replace it with 7812 i guess, why would i keep things weird when they can be done simply
2. The input chip may be baked and thus more prone to catch noise. There actually is some frying eggs sound drowned in the background hiss.

Is there any mojo recommendation for the V+ voltage? Or is 12V just what the mighty mojo pedal and thus my musical career needs? 
They sell 13.7 regulators here, so i can basically do whatever i am meant to do.

[Scruffie]

If you get a steady supply voltage, the voltage regulator is working, why tear up a vintage pedal? Those traces are very unforgiving too.

The 'frying egg noise' can be solved, put an 820pF cap in parallel with the 6k8 resistor that connects between pins 1 & 2 of the 4558 in the above schematic.

[yeeshkul]

If you get a steady supply voltage, the voltage regulator is working, why tear up a vintage pedal? Those traces are very unforgiving too.

It is just a power supply, no magic in that i suppose.

The 'frying egg noise' can be solved, put an 820pF cap in parallel with the 6k8 resistor that connects between pins 1 & 2 of the 4558 in the above schematic.

That will cut trebles. I have no idea how would that sort the noise produced by a semi-fried chip, but thanks. I guess i will replace the chip.

[Scruffie]

It will cut highs but not so you'd really notice, above about 15khZ if I remember correctly.

The noise is because there's no low pass filtering to the input of the BBD (if you see the 9V mistress schematic, what i'm suggesting was added to that model) so you're hearing aliasing noise.

[yeeshkul]

Thank you for your advice. I will try that

[DrAlx]

The layout on those boards is horrible.  The tracks that run to the rate pot snake through the whole circuit.  Nice big square waves coupling to all the other tracks.  Doesn't explain why one version ticks less than the other though as the layouts are pretty much the same according the pics on the EM Mystery Page.  I would guess the voltage regulation was the problem and audio probe the supply pins on the chips in the audio section.