Dirtbag, very very distorted (NOW SOLVED, or at least I'm happy with it)

[lumpylipton]

Hi, just got my Dirtbag finished up.  I included all of Brian's suggested mods for the v3205 version (ie not the R21 and C14 listed in the docs, but instead the whole list of them in the errata in the forum post).  In attempting to bias the unit, I can get it to pass delayed signal, but the repeats are super crazy distorted.  Also, when biasing, I had to turn the bias trims almost completely CCW, I have a tiny amount of trim to work with before the delays don't pass anymore.  I have verified the trims, R23 and R22, everything is correct.

Any tips?  Many thanks.
Warren

[lumpylipton]

Some additional info, after checking my voltages versus the post Brian references from a working unit  http://www.madbeanpedals.com/forum/index.php?topic=3380.0

Everything looks pretty close except pin 7 on each of the BBD are reading the same as pin 8, around 8V.  MadebyMikes are closer to 6 on pin 7 and 8V on pin 8.  As I noted, I've already verified the bias trimmer and the 6.8k and 82k resistors and they are right...

Thx
Warren

[lumpylipton]

Still no progress on this, but more to report.  Essentially, the signal is getting to the first BBD IC4 pin 7 just fine, no distortion, however, when it comes out of pin 3 and 4 of the BBD, it is fizzy and brutally distorted.  The only way I can get anything out of it is with the bias trim turned nearly completely CCW.  There is a tiny amount of trim where I get sound out, otherwise it cuts out completely.  Where can I start looking for problems?
Thx,
Warren

[madbean]

Well, you have the source of the problem more or less pinpointed so that's a good start.

What about pin5 on IC4? What you should be reading is
pin5 - 8.2v
pin8 - 7.7v (or around that---something less than pin5)
pin7 - depends on where BIAS1 is set, but between 6-8v usually.

Your voltages don't seem too far off, and it's normal to have a narrow range on the BIAS trim where the delay passes. Usually this range is not at the extreme, in my experience.

Try this: turn BIAS1 fully counter clockwise and do a voltage reading on pin7 of IC4. Now turn it fully clockwise and take the reading again. What do you get for these two?

Also, if you are getting a lot of distortion on IC4, try swapping in the BBD from IC5 there temporarily and audio probe pins 3 and 4. Still get the same thing? (just wondering if it could be a bum v3205).

[lumpylipton]

Hi Brian,

Pin 5: 8.4V
Pin8: 8.2V
Pin 7: 8.3V

with bias CCW: 8.3V
with bias CW: 0.6V
I've actually already swapped the BBD with some others I had, and it does exactly the same thing.
Thx
Warren

[djaaz]

Take a look at these videos posted here:

http://www.madbeanpedals.com/forum/index.php?topic=3701.msg32963#msg32963

Solved it by swapping the ne570.

[madbean]

Hi Brian,

Pin 5: 8.4V
Pin8: 8.2V
Pin 7: 8.3V

with bias CCW: 8.3V
with bias CW: 0.6V
I've actually already swapped the BBD with some others I had, and it does exactly the same thing.
Thx
Warren

At 8.4v supply, pin8 needs to be around 7.8v. Looks like the voltage drop across D2 is not high enough. You should use two 1n914s in series there to try and get that voltage down. Or, socket D2 and plug in some resistors.

I'm not sure changing the NE570 will fix the problem, since the distortion seems to be generated at the BBD. What if you remove IC4 altogether and audio probe pin7 on the socket? Any distortion? The signal should be loud there but not like a fuzz.

Maybe the problem area is actually in the clock, not the BBD. I do think you need to get pin8 voltage down as mentioned above, but perhaps the BBD is not the source of the distortion.

Do you know the actual value of C41 (i.e. does your multimeter have a capacitance reader)? Do you have a way to measure the frequency of the clock signals on pins 10 or 11 of the CD4047? Lastly, what brand and suffix is your CD4047?

[lumpylipton]

Some progress: 
Adding another diode in series brought the voltage down to about 7.7.  That helped a bit, in that now the bias setting is not on the extreme end of the trim, but closer to the centre.  There is also less distortion, but it's still unusably bad.  Another observation is that I have to have the feedback pot almost completely CCW, otherwise it self oscillates terribly, that's with both gain trims fully CCW.  I can't bias it with the feedback half way, it's oscillating way too much and is a distorted mess.

I unfortunately don't have a scope to see the clock signal, my 4047 is a BE suffix, from TI.  I do have a capacitance meter, and C41 is very close to 240pF, I made it with 2 caps in parallel to get it close.  I'm going to try to rig up a signal to my DAW to get a reading of the clock signal.

Incidently, I happened to have another NE570, so for kicks I swapped it, it acts exactly the same.

[lumpylipton]

Measured the clock with my DAW.  9.75kHz

[madbean]

That actually sounds about right. I'm a bit stumped.

Couple more questions: you are using 9v to supply the circuit? Have you used 47k on both R11 and R12? Positive you set all three jumpers correctly for the v3205 version?

I'm kind of baffled at the signal being okay at pin7 but coming out distorted on pin3 and 4.

[Scruffie]

I think you've got a wrong value some where, it sounds like there's too much gain which would cause too much feedback and distorted repeats from too hot a signal feeding back.

As long as Pins 2 & 6 on the BBDs are both the same voltage and at around half supply, then well assume the clock is fine (if you have a scope you'd want to see a clean squarewave there).

[lumpylipton]

Brian:
Yes, I'm using a 9V supply.  I reverified the jumpers for the 3205 (correct) and R11 and R12 are definitely 47K.  Perhaps the signal is too hot at pin 7 and it's just not as noticeable until the repeats and feedback enter the picture.  It is pretty loud on pin 7.  What sets the gain out of the compander?

Scruffie, the pin 2 & 6 of the BBD are almost exactly half supply so that seems fine, you're probably right about a bad value somewhere that's affecting the gain.  Where should I start looking?

Thanks guys, I appreciate the help.
Warren

[madbean]

The voltage gain is determined out of the compander is determined by the two resistors which are 47k in your case. You can lower those resistors to see if helps. You should be able to half them down to 24k or so.

The signal should be hot at pin7 on the BBD. After the signal is processed through the BBDs and filtering it is then "normalized" by the expander portion to a lossy but cleaner singal. I take it you are getting distorted signals out of pins 14 and 15 of the NE570 given your description.

If you want to post a hi-resolution close up of the top side of the board, I can help you spot check resistor values in a few key areas. Maybe another set of eyes will pick up something you might have missed.

Also, from your earlier post: using 10k for the Feedback definitely results in much sooner oscillation than a higher value. I have suggested 100k in place of that in the mods section of the forum, I think. I've found this to be true on every DMM I've built and on the one unit I own, as well.

[lumpylipton]

Alright.  I went through every single resistor, measured it and compared it's color code where it couldn't be measured accurately (due to a cap charging or a parallel resistance) and found it 100% correct.   Now, being thoroughly perplexed,  I decided to think out of the box a bit, and even though I'd done all the recommended mods and replaced R21 and C14 back to 1K, I put them both back to 100k.  Dirtortion is now gone.  Excellent, however, my signal is too weak now.  So, I decided to put a 50k trim (all I had on hand) for R21, and now....voila, I can dial in just the right amount of signal until I get distortion.

I rebiased everything, and also replaced the feedback pot as Brian directed.

Long story short, it now sounds fantastic and works perfectly!  I'm very happy with it.
So, thanks to Brian and Scruffie for the assistance.  Whew, I now deserve a beer.  Then after the beer, I can throw it into an enclosure!

Thanks a lot!
Warren