ADA flanger slapback echo

[Ry]

I've been working on tuning in my ADA flanger with the trimmers and timing caps.  I currently have a 42pf C29 and 1M R69. The only problem is that no matter what I do, there is a noticeable and very annoying slapback echo present.  The timing of the echo changes based on changes in C29 and R69, but it never goes away.  I'm guessing that something else in the circuit could be to blame, maybe R68?

Any ideas what could be the cause of this? 

[StephenGiles]

Something is causing the clock frequency to be way too low.

[Ry]

It's strange because I get flanging and echo at the same time.  Maybe one side of the clock is slow and the other is okay.  I'll check the clock chips.

Thanks

[Ry]

Okay, I changed out the 4047 and 4049 with no change.  Is there something I should be looking at on the 1024?

[StephenGiles]

It has to be a clock/modulation circuitry problem

[oldschoolanalog]

Are you using the Moosapotamus A/DA board or one you made/designed yourself?
Do you have a meter w/a frequency counter? I know this is not necessary for setting it up by ear; but for troubleshooting it's a big help.
I have a few ideas/things for you to check, depending on what test equipment you have. Get back to me/us on this.
All the Best,
osa

[Ry]

I'm using a moose board, rev A.  I have a meter with a frequency counter, but it doesn't go very high (maybe 20k).  I also have an old 'stereo' o-scope that I need to figure out, but it should work for taking some readings.

Thanks for the help!

[oldschoolanalog]

First the obvious. Check all component values, especially the trimpots (I'm sure you did, but do it again anyway). Check your supply V at the output pin of the V reg. should be 15V. Check supply V of all the chips.
For the echo problem a frequency counter w/a 20k limit should be fine. When properly set up, you're looking for a low clock rate of ~17k. Since you're getting flange, it would appear that the LFO is functioning properly. Now, turn your Range pot full CCW. This takes the LFO out of the mix. Starting with the Manual pot full CCW take clock freq. readings at the SAD 1024 pins 8 & 14 (these should be the same) then 3 & 10 (again, these should be the same, and equal to the ones at 8 & 14). Do these readings with your probe at the pins of the SAD. From your description, I'd guess your clock is, as Stephen said "way low". Probably somewhere around 10k, if I were to give a guess. Play around with T5 and see how high this takes your reading. Then go to T4 and see how much higher this takes your reading. These 2 trimpots are very interactive. Be patient. Next, if that didn't help, check IC2a/ pin 1. Should be 5V here. Go to IC3d/pin 14. Make sure your Manual "sweep" spans ~3V to ~12V. Turn the Man pot from one extreme to the other for these readings. If all of the aforementioned is in order and there is still this problem, you might want to try a 68k resistor at R65 (Or tack on an appropriate parallel resistor to get ~ that value). Try some of this and see what gives. Sorry this is all I could think of right now. If you still have problems, I'll open mine up and we can compare voltages. Keep us posted. Don't go "Postal".
osa

[Ry]

Thanks for the detailed tips, I will try these out as soon as I can.  With company coming in from overseas, it might be this weekend.

I think the problem may be with the Manual pot.  I've noticed that it doesn't seem to do anything right now.  I didn't connect the two problems, though.

I will certainly keep you posted and post some voltages when I get them.  I've been working on this off and on for a few months now, no danger of going postal with it.  I wouldn't have this hobby if I were easily frustrated by it

[oldschoolanalog]

  Just a thought. Did you wire in the CV input jack? If not, 2 of those pads need to be connected. Sw & S. Without those connected you disable the manual pot. Have a look/see at the schematic & layout...
osa

[Ry]

Okay, I connected up the S and SW pads on my board.  It didn't really help that much.  Then I traced out the voltages.  All of them looked good.  The manual sweep goes from 4v to 14v.  Then I tested the frequency on the SAD.  Both sides were equal, at about 7k.  Adjusting Tr5 and Tr4 only brought it up to 8k.  With the Manual knob turned all the way up, I get about 9.5k.

For R65, I have an 82k resistor.  I will change it to 68k when I can get to my iron.

Thanks again!

[Ry]

I changed R65 down to about 68k.  It affected nothing regarding clock frequency.

What voltages can I post that will help out in the debugging?

[oldschoolanalog]

OK, I guess it's time to open mine up and take some V readings. No problem. I'll post them tomorrow (Sun.). I'm a bit crisp from work right now and am prone to errors when in this state of mind . FWIW, the static CV seems OK and the LFO seems OK; so we are narrowing things down a bit (always a good thing). Seems to be something "to the right" (on the schem) of the Range pot.
More on the way...
osa

[slacker]

What voltages can I post that will help out in the debugging?

The voltage on pin 10 of the 4007 sets the frequency of the clock. On mine using the standard component values ie: C29 39pf and R69 2M2, the voltages are.
Manual on minimum 7.32volts which gives a frequency of ~35Khz at the test point
Manual on maximum 4.81volts which gives a frequency of ~1.3Mhz at the test point

I don't know how much these would vary from one build to another but if yours are a lot different that might point to a problem. The voltage on pin 10 should be controllable using TR4 and TR5.

[oldschoolanalog]

Ian is spot on in his assessment. I used C29 39pf and R69 2M on mine.  At the 4007/pin 10 I get: Man on min (ccw) 9V. Clock f at test point=~31KHz. Man on max (cw) 5.7V. Clock f at test point=~1.4MHz. Re center TR 4&5, and do the min/max V test at pin 10/4007. If you don't get some kind of variation, the problem is around IC3c. If this is the case, I'll take more V readings later. Now, if you do get some kind of variation, look to C29. There are 2 places this can be installed. The correct place, when using only one cap, is "in between" ICs 5&6; parallel to R69. If you used the spot directly to the right of pad F, one end of C29 will be left "floating". This spot/pad is explained in Charlies build notes. Give it a look, and let us know what's up.
All the Best,
osa

[oldschoolanalog]

Note: When I said "C29, parallel to R69", I meant parallel physically on the layout, not electronically parallel. Just wanted to clear that up to avoid any confusion. 
osa

[Ry]

It's working!  All of this talk of values for R69 made me pull out the resistors for R68 and R69.  Turns out I swapped the values for the (1M for R68 and 47 for R69).     It still needs tweaking, but it sounds great already!

Just goes to show how I can never be sure of anything no matter how many times I check it...

Thanks a ton for your help, OSA and Slacker.  The light wouldn't have gone on (at least not so quickly) without the guidance.

Ry

[oldschoolanalog]

It's working! 

You're going to have fun with this one!
All the Best
osa