Some pointers please: FSH1 debug (voltages included)

[Auke Haarsma]

I've built the tonepad FSH1, but it is not working as expected. Some pointers are greatly appreciated to spot my mistake.

I do get sound. But only in a very weird range. There is no 'normal' filtering going on. Hard to describe. When I strum hard I do get a very high pitched filter. When I strum normally, I do not get any sound. At some notes I do get an arpeggiated sound. Some notes in the arpeggio are very very loud (much louder than the others). The freq range of the arpeggio is low (say below the 6th (e) string of the guitar).

It does not sound as I've heard on the clips. Therefor I assume I made a mistake somewhere.

Link to schematic/layout: http://tonepad.com/project.asp?id=50

'special' parts or subs:
LM13700 instead of LM13600 (LM13700 is verified to work in this layout)
2n5457 for the FETs (Q2, Q5, Q6)
Q1 2n3904 (noise trannie)
Q3 2n3906
MAX1044 = LTC1044

All pots are the correct value, but sometimes they are LOG instead of LIN. I think this should not matter for functionality.

LTC1044
1: 8.8V
2: 4.4V
3: 0.03V
4: -4.0V
5: -8.2V
6: 0.05V
7: 4.3V
8: 8.8V
This one seems ok to me. I get a +8V/-8V supply.


LM13700
1: -6.3V
2: -1.5V
3: 0.03V
4: 0.03V
5: -0.9V
6: -7.5V
7: 0V
8: 0.3V
9: -1.1V
10:  0.03V
11: 8.1V
12: 0.03V
13: 0.03V
14: 0.03V
15: -1.3V
16: -6.3V

IC2 (TL072)
1: -0.3V
2: 0.03V
3: 0.03V
4: -8.3V
5: 3.5V
6: 3.5V
7: 5.1V
8: 8.8V

IC3 (TL072)
1: 0.03V
2: 0.03V
3: 0.03V
4: -8.3V
5: -4.7V
6: LFO (-0.9v with min speed, 0.9V <-> -4V with max speed)
7: -6.6V - -6.8V
8: 8.8V

Q1 2n3904
C: -8.2V
B: -7.6V (nc)
E: 0.5V

Q2 2n5457
G: -0.9v
S: 8V
D: 0v

Q3 2n3906
C: -2.3V
B: 0.03V
E: 0.7V

Q4 (does not exist)

Q5 2n5457
G: -6V
S: 0.03V
D: 0.03V

Q6 2n5457
G: 0.03V
S: 0.9V
D: 8.8V

Both trimpots are set in the middle. I've been playing around with them, it doesn't seem to matter much where I set them.

Q5 has Source and Drain at 0.03V (which is the 'ground'value in my build). I wonder if this is correct. If I twist the trimpot (the 10k one) I can get the Q5 Gate up to about 0.4V. As said, this does not seem to affect the sound.

Any help is greatly appreaciated!

[Ry]

An initial thought comes to mind without looking at your voltages, did you use multi-turn trim pots?  I've built a couple of these and found that there is a very narrow range of the pot travel that allow them to work.  I would suggest slowly turning the trim pots.  From your description, it sounds like my builds before I got the trim pots set correctly.

[Auke Haarsma]

Thanks for your reply.

No I do not have multi-turn trimmers. Is there something I can measure to see if the trim-pot is set correctly, or is trim-by-ear the only option?

[Auke Haarsma]

can't really hear much difference with the trimpots.

With an audio-probe I lose the signal (or most of it) pretty early in the schematic. It comes out nice from Pin 9 of IC1 (LM13700) but loose it right after the 100k resistor (pin 9 -> 0.15uF -> 100k resistir -> pin 4). At pin 4 the audio is barely audible.

Resistor values around that area have been checked and confirmed. Any clues?

[Ry]

I will try to dig out my tonepad version and see what I can find for you.

[Auke Haarsma]

much appreciated. Looking forward to it!

[Ry]

I haven't had a chance to look at mine yet, but this thread may help you get started (voltages included for the Tonepad version):

http://www.diystompboxes.com/smfforum/index.php?topic=69381.0

[gigimarga]

Look very carefully at BFs (the post above)...my FSH didn't work because on the Tonepad's layout they are misoriented!

[oskar]

I think you have a flaky Q2.

-You have sound on IC1 pin4. Not alot but the 100k/3k3 divider lowers the signal for the OTA which hates big signal.
-Q3 collector is having -2.3V. This should provide enough current on IC1 pin 1 and 16 (which are seeing correct voltages) for the filter to open.

I would try a new OTA chip (it's in sockets right?) and/ or a new Q2.

ps. I don't know how the OTA would behave if the feedback is incorrect such as the 30n cap being damaged, shorted or unconnected.

[Auke Haarsma]

Thanks for all the suggestions.

I swapped in some other LM13700's (I happen to have a bunch of them, don't know why...)
Tried some other Fets.
Swapped the pinouts.

All the same result: no difference in sound.

Either I get a low cartoon-like sound which goes bohwohowhwowo, or I get chirping birds.

I get a big volume boost for specifically the low A-string. Seems that frequency is boosted the most.

Am I right to think that the trimmers only affect the S/H-setting? So in Filter-mode the trimmers play no part?
(all debugging thus far has been done in Filter-mode)

Any more pointers?

[fogwolf]

Play around with the settings of the pots. I've found that it's touchy. Some settings, particularly if you are in "down" mode, don't necessarily produce much at the output, depending on the range, attack and decay settings.

[Auke Haarsma]

Yeah, in up-mode I get a little more sound. I'll play around with the pots. As of now I have not found anything musically useful.


And in addition to my previous post: when swapping the pinouts it ofcourse made a difference: no sound at all , so my pinouts are and were correct.

[gigimarga]

Use multi-turn trimpots...i lost a lot of time to tweak both of them 

[Auke Haarsma]

But the trimpots only affect the S/H-setting, right?

So in the filter setting the sound should be 'ok' regardless of the trimpotsetting. Or am I missing the point?

[gigimarga]

But the trimpots only affect the S/H-setting, right?

So in the filter setting the sound should be 'ok' regardless of the trimpotsetting. Or am I missing the point?

You're right, sorry...i forgot how i set them ...the trimpots are for the S/H part only.

[Auke Haarsma]

No problem!

Does anyone know where I can read about the functionality of this circuit? What does what and why? More knowledge on my side about what each block does can help me narrow the search to get this baby to work as it should.

[Auke Haarsma]

More specific:
-Which parts set/control the frequency (Q)?
-Which parts set/control the amount of boost/cut?

[Auke Haarsma]

anyone?