Mobius Trip question

[Ry]

I really like my Mobius Trip but it has an audible hiss when not engaged.  By not engaged, I mean that it is neither recording nor playing, just sitting patiently in my effects chain.  As soon as I engage it, the hiss goes away.  

I'm kind of assuming that what I'm hearing is a high frequency oscillation bleeding through the recording chip when idle.

Has anyone else noticed this?  It's possible that I may have damaged my chip by not properly grounding it when I first had it powered up...

Ry

[idiot savant]

i dont have any problems with mine. there is some small amount of hiss when engaged, but not in standby. make sure you add the extra jumper detailed in this thread, though honestly i couldn't tell you if that would make a huge difference.

http://www.diystompboxes.com/sboxforum/viewtopic.php?t=32651&highlight=mobius+trip+looper

hows your bypass wired up?

[Ry]

make sure you add the extra jumper detailed in this thread

Yeah, I have that wired up.  It's the ground connection that I mentioned not being hooked up when I initially powered mine up.

I have my bypass wired per Dean's schematic at GGG.  I may have to switch to a true bypass version.

Thanks for the reply!

[makkimo]

hi!
i've my looper not working and i'm so sad i can't find what is gone wrong...
please is there someone who can post the wright wiring and can cure the link (it doesn't work) for the additional jumper
thank u all

[makkimo]

no-one???

[Dean Hazelwanter]

Hi,

Ry, I'm not sure what is causing the hiss. I haven't had that problem, (or the clicking at the end of each playback cycle that Andrew reported a number of times) in the 2 that I built. I ran my first unit for quite awhile before someone pointed out the missing jumper, so I don't think your chip is damaged. You can try tacking a low value cap (50pF to 500pF) to ground at various points in the audio signal path, to bleed the hiss to ground. Definitely try the true bypass too, to see if it makes a difference.

Makkimo, take a read through this thread: http://www.diystompboxes.com/smfforum/index.php?topic=32651.0 It explains the missing jumper, as well as off-board wiring.

Hope that helps, and get back to us with your results.

[Ry]

Wow, this is an old question of mine!  I will try adding the cap to ground and see if it fixes the hiss.  I haven't gotten back to playing with the circuit, but I remember really liking it even with the hiss. 

Thanks for contributing the design, Dean.

[makkimo]

thank u very much for the help
still got it in non-working condition....
today i was reading voltages and find all the op amps pins have 4.98v (except for the 9v and the ground ones)is this correct i think it is a little strange ..
can someone post voltages on the isd?
i'd really like to find the error fearing of a burned ISD chip

[Dean Hazelwanter]

What do you mean by 'it's not working'? Do you get the input signal appearing at the output jack when the 'dry' pot is adjusted?

What voltages do you see at the following points (all with reference to ground)?

- Pin 4 of both opamps
- Pin 8 of both opamps
- Pin 5 of both opamps

- Pin 16 and 28 of the looper chip
- Pin 12 and 13 of the looper chip

[makkimo]

i mean i can't get it to record
only the dry signal

voltages are on opamps 0v on pin4
                                   9v on 8
                                   5v on 5

on IDS    4.77v on pin 16
              0v on pin 28
              0c on 12/13

leds working properly

thank u for your time
                 

[Dean Hazelwanter]

If there isn't +5 volts at the ISD chip's pin 28, that's a big problem. Is the jumper above pins 1 and 14 of the 4001 IC in place? If not, put it in. If it is in, check the voltage on both sides of it.

[makkimo]

  :icon_redface:sorry it's 3.30 a.m. here got some problems with sleep....
the voltage on pin 28 is 5v it 's correct and the jumper is in place..

[Dean Hazelwanter]

Ok, good.

when you apply a signal to the input jack, do you see that signal at pin 20 of the ISD chip, and is it's level controlled by the 'REC' pot?

BTW, which looper chip are you using?

[makkimo]

hi!
i'm testing.........
hoooops ... i can't get any voltage on pin 20 -> ov  trying to change the two 0,1uf going to the rec pot
on pin 7 of the op amp before these the voltage is 5v but after, on the rec pot, there is  0v
i'm using an isd1020ap from small bear
thank u

[Dean Hazelwanter]

i'm using an isd1020ap from small bear

Good, just checking.

hoooops ... i can't get any voltage on pin 20 -> ov

Instead of using a meter, try using an audio probe, since you are looking for an audio signal not a DC voltage level. If you don't already have one, do a search for it, and build it. I'm sure that it (like most other gems of wisdom) can be found at GEO. It's basically just a cap connected to the hot connection of a cable pugged into an amp. Then you can follow the signal through the circuit.

[makkimo]

  going crazy
ok i've probed it !!! find that the signal arrive at pin 20 and the rec pot work properly .....sobh
the idea of a fried chip is calling???

[Dean Hazelwanter]

Hi,

the idea of a fried chip is calling?

It may be toast, but hopefully not. Is it socketed? You don't happen to have another to swap in, do you?

Do you hear the un-affected input at the output, and are you able to vary the volume of this un-affected signal with the 'Dry' pot?
Are the LEDs (Enable and Record) behaving correctly? If so, make sure it is reset by disconnecting then reconnecting the power, place it in record mode, enable it, play something into it (just 5 seconds or so), disable it, switch it to play mode, and enable it. If you don't hear the looped signal at the output, even after varying the 'Play' pot, use your audio probe to see if there is a signal at pins 14 and 15 of the looper chip.

[makkimo]

hi!
dry pot works good
leds are ok
play pot does nothing
probed at 15° pin but  nothing only the  pop when you put the probe on it ...a normal reaction...
probe on 14° pin is TOTALLY MUTE nothing at all -> death->silence -> sigh