Mistress pases just clean signal

[yeeshkul]

Hi guys i have a Mistress to repair, it is the 1976 model. The owner switched polarity by mistake and after that it stopped working.
I've managed to repair the voltage regulator part, so that it gives 13V, however the circuit pases just clean signal,
no response to pots and trimpots (just the bias trimpot turns it off at low voltages).

Would you have any idea?

[GibsonGM]

Got a schematic?   Look here: http://www.metzgerralf.de/elekt/stomp/mistress/index.shtml

You get clean with it switched ON, not just in bypass??

If he switched polarity, you basically need to look at all the polarity-sensitive components.  Caps, diodes (think there's a Zener in there)...possibly the opamps, too    Hope not!

I'd go thru quickly (o'scope with an octopus, maybe) and check those....then audio probe the first opamp (a 4558, I think), see if you get signal there.   Whichever is easiest.     I hope your ICs didn't go!   See where the signal path takes you....   

[Govmnt_Lacky]

Everything in that version is easily replaceable EXCEPT FOR the SAD1024 chip!! 

Cross your fingers and pray to the electron gods that it is NOT that BBD!!!  They are SPENDY!!!

[GibsonGM]

Everything in that version is easily replaceable EXCEPT FOR the SAD1024 chip!! 

Cross your fingers and pray to the electron gods that it is NOT that BBD!!!  They are SPENDY!!!

Right.  The 4558's are 4 for a dollar, but the SAD is NOT a pretty purchase...

[yeeshkul]

Guys i don't believe the sound would go through if the 4558 was dead. I am afraid the Bucket brigade kicked a bucket. Maybe LM339, or the CMOS, let's see.

[Govmnt_Lacky]

OR the Clock chip.....

Point is.... you should ABSOLUTELY verify every possibility before pronouncing the SAD chip DOA. 

[R.G.]

The only definitive way to check that it's being clocked right is with an oscilloscope.

To test the SAD, you fire the thing up, use the oscope to verify that the clocks are good, solid logic signals very close to the power supply limits on the SAD, then you look for dry signal going into the input. Good clocks + good input signal + no delayed signal at the output of the SAD = many dollars spent to get a new chip. If you get good clocks, good signal at the SAD input, and a stepped approximation of the input signal at the output, the problem is elsewhere.

[yeeshkul]

Guys i did some measurements on my oscilloscope, and here are the results:

1. I used 200mV, 500Hz input signal
2. Filter matrix switch, so the clock is steady
3. Color all the way down, so there is no feedback
4. Range and Rate in the middle position
5. Fed by 18V, V+ adjusted to 13V during the voltage regulator repair.

Clocks (both working)


4558 input


4558 output / SAD1024 input


SAD1024 output


effect output


The output signal is just clean, no effect .

Turning VR1 bias trimpot doesn't influence the sound. When i move the trimpot, the signal "jumps around" but then it sits where it had been before.

[GibsonGM]

I have some thoughts on this, but would rather see what the big dogs come up with, having had their hands on clocked circuits a lot more than I have!   I could embarrass myself, ha ha. 

I wouldn't call the output "clean", Yeeshkul - and I'd say you're a lot closer to having an answer, based on what I see!   Hang in there.

R.G.?

[yeeshkul]

When i put an audio probe on input of SAD, i can hear the clean signal with phasing noise in the background - same thing can be heard on the output of SAD, just a lot quieter than the input one. This was an audio sound test before i switched the switch to Filter matrix to stop the clock from sweeping.

[Govmnt_Lacky]

Time to head on over to the debugging thread and repost with the information....

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

[yeeshkul]

The weird thing is that the output of the SAD chip was really quiet, however from the picture it seems pretty loud. I'll re-check it, just to be sure.
I guess i am picturing a 47kHz signal that was modulated on 500Hz signal.

[R.G.]

Your SAD1024 is working OK. The problem is likely in the filtering and stuff after the 1024.

[Govmnt_Lacky]

A couple of "so we dont get caught with our pants around our ankles" questions:

- You are CERTAIN that you are not getting any kind of filtering while in Filter Matrix mode correct? Did you try turning the Range pot while playing?

- Does the LFO work while in Flange mode? Can you hear the sound fluxuating OR can you see the voltages on the LM339 fluxuate with the Speed pot?

- Did you check the Filter Matrix/Flange switch to make certain that it WAS switching?

Looking at the schematic, and going by RG's comment above, there isn't much after the SAD to check. Basically the Color trimmer and some resistors/caps for broken parts or cold solder joints.

[yeeshkul]

Thank you for your concern guys, i will be back to it tomorrow, i am off for some beer now

[yeeshkul]

So i am back to this. I realized that when i switch it to Flange mode, and turn the rate knob way up, there IS an effect - it sounds like a weak tremolo. No flanging though, and the Color knob doesn't have any effect. The trimpot does have some influence on the sound but not the pot itself.

Here is the SAD output in 1ms range just for comparsion with the input signal pictured above.

[yeeshkul]

You are CERTAIN that you are not getting any kind of filtering while in Filter Matrix mode correct? Did you try turning the Range pot while playing?

Yes, i am. No flanging at any mode. I'm getting a clean sound in matrix mode + none of the pots seem to work.
I am getting tremolo in the Flange mode - Rate pot changes the tremolo rate and Range pot also seems to change the sound a bit. Color does absolutely nothing at all.

Does the LFO work while in Flange mode? Can you hear the sound fluxuating OR can you see the voltages on the LM339 fluxuate with the Speed pot?

Measured on the Range pot viper, Flange mode, Range all the way to the left.

Rate at max


Rate at min


Turning Range pot clockwise adds on amplitude, and keeps the same frequency (almost).

Did you check the Filter Matrix/Flange switch to make certain that it WAS switching?

Yes it does switch. Measured.

[Govmnt_Lacky]

I think it is time for debug voltages.

GO to the link I posted above and repost here... with that format... and the requested info! 

[Fender3D]

I think it is time for debug voltages.

^^

Check what's the bias voltage @BBD's pins 2 and 15 (it should be slightly lower than half power supply)

[yeeshkul]

Model:Electric Mistress 1976.

Brief description:
The early model with 2N3563 tranzistors in place of Zener diode and between LM741 output and common base tranny input.
The pedal was apparently damaged by wrong polarity feeding. I repaired the voltage regulator, inserting Zener in
and adjusting 18k resistor (in the output voltage divider) to 22k so now it gives 12.96V (V+).  I also replaced the common base transistor for
BC327-25, however the original one didn't seem to be burned. I replaced LM741, as my previous experience with burned our Mistress
pedals leads me to belief that this one kicks the bucket first.

- the 12.96V (negative ground) voltage regulator is working fine and it is not an issue.
- the pedal had the color pot replaced, i measured it and it seems to be all right.

The issue:
The pedal doesn't provide any flanging, just tremolo sound and color pot doesn't have any impact on the sound.
All LFO, VCO, and both clock working fine (the two clocks seem to be in phase, hope that is all right), SAD have an input and provide an output signal - see above.

Here are the voltages:

4558 chip
1. 6.56V
2. 6.56V
3. 6.5V
4. 0
5. 6.5V
6. 6.57V
7. 6.57V
8. 12.96V

LM339 chip
1. signal swinging around 6V ish
2. signal swinging
3. 12.96V
4. 6.57V
5. 6.58V
6. signal swinging around 2V ish
7. signal swinging around 3V ish
8. 0.08V
9. signal swinging around 4V ish
10.  signal swinging
11. 0.43V
12. 0V
13. 0.02V
14. signal swinging around 2V ish

CD4013 chip
1. signal swinging
2. signal swinging
3. 0V
4. signal swinging around 3V ish
5. 0V
6. 0.02V
7. 0V
8. 0V
9. 6.43-6.46V
10. 0V
11. signal swinging around 6V ish
12. 6.43-6.46V
13.  6.43-6.46V
14. 12.96V

SAD1024 BBD chip
1. 0V
2. 6.56V
3. 6.43-6.45V
4. 0V
5. 12.95V
6. 1.16-1.23V
7. 12.96V
8. 6.43-6.46V
9. 12.96V
10. 6.43-6.46V
11. 12.96V
12. 1.16-1.23V
13. 0V
14. 6.43-6.45V
15. 6.56V
16. 0V

BC177 tranny
b. 11.82V
e. signal swinging around 2V ish
c. 12.41V


There don't seem to be a DC voltage leak to the output filtering stage.