Is this how you test a Tl072 Op Amplifier

[rdhj]

This is the Landtone delay kit that I got off off amazon.  I think it is the same as the TTone kit.

[antonis]

So, what are DC voltage readings on all TL072 pins..??
(black probe on power supply ground and red probe on pins - DMM set on VDC..)

TL072 pin numbering..

[rdhj]

For some reason, when I put the black probe to the ground on the 9V jack, it wasnt iving me the right readings.  So, I stuck the probe on the fround of the output jack and got the following voltages.
1  1.468
2  4.16
3  4.09
4  0
5  3.95
6  4.06
7  6.97
8  8.09

[Rob Strand]

1  1.468
2  4.16
3  4.09
4  0
5  3.95
6  4.06
7  6.97
8  8.09

Your pin 2, 3, 5, 6 look reasonable for a working opamp.

The pin 1 and 7 voltages are close to the power rails and might explain the lack of headroom you are seeing.

It's not possible to judge beyond that if those voltages are normal for the circuit.   

It would be worth looking into why pin 1 and 7 are so close the supply rails.   
It's virtually impossible to debug further without a schematic.    At this point experienced debuggers
would draw out part of the circuit as it's the only clear way forward.

[rdhj]

can you exlain "lack of headroom" and "close to the supply rails"?

[11-90-an]

Lack of headroom: the signal can't get more gain without distorting (which may or may not be good, depends on what you're building.)

Close to the supply rails: the voltage of a certain area is nearly the same as the power supply voltage, which normally for guitar/bass pedals would be +9v and 0v (ground)... but since your power is at around 8v (as we can see from pin 8 of the op-amp), pin 7 (7v) is too close to the supply (8v) in which we would probably be expecting distortion... also the voltage on pin 1 (1.5v) is too close to ground (0v) this may give some volume problems... but again, without a schematic we can only guess...

[rdhj]

Thanks.  I emailed the chinese company to ask for a diagram.  Lets hope they can supply me with something.

[rdhj]

Based on the above about headroom and the rails, would you expect that the TL072 may be bad?  I can order a new one from ebay and try it.

[GibsonGM]

It's not possible to judge beyond that if those voltages are normal for the circuit.   

[amptramp]

When you are probing the non-inverting input, your meter may be dragging the voltage down that is set by the voltage dividers because it may have an input resistance on the order of megohms which may change the voltage at the voltage divider if the divider is high resistance.

When you are probing the output / inverting input, you have a low-impedance source and meter loading will be negligible.  If you have a second meter, leave one on the inverting input while you probe the inverting input.  They should read the same then.

If you want to test the full functionality of the op amp so you can measure all of the characteristics on the datasheet, that requires a number of setups and a lot of test equipment and most of the setups are shown on the datasheet.

[rdhj]

was messing around with it today.  Here is what I got now:

1. Bypass mode works totally fine and LED light is lit up
2.Hit switch and led gets brighter....with repeat knob all the way down yu can hear the guitar signal but it is weak
3.  as you turn the repeat up it gets loud and noisy like it is keeps repeated and getting louder as you turn the knob.

Does this happen to tell us anything....maybe issue with the delay IC?

[11-90-an]

Audio probe?

Pictures?

(I think tracing the board would be a *quicker* way to get a schematic...)

The delay IC seems good to me... again, inexperience speaks...

[rdhj]

Just found this which is the same exact kit and issue that I am having..


https://www.reddit.com/r/diypedals/comments/b2ozqn/delay_pedal_kit_problem/

[Rob Strand]

For some reason I have trouble with imgur,



That's the only image I see. (Sometimes imgur shows more than one.)

Weeding through some threads I got these,

[Rob Strand]

It's looking a lot like this one,  but there are some differences,

http://3.bp.blogspot.com/-bEMBMvJwA5Y/TwrbbnpmzoI/AAAAAAAAAbw/EQ70PKQOFes/s1600/seaUrchin_Schematic.gif

- The opamp has the A and B parts swapped:  Input opamp is pin 5, 6, 7,  output opamp is pins 1, 2, 3
- Input opamp has resistors 510k in and 510k feedback.   So gain of 1 not gain of 2 on this one.
  It also has 240k in series with pin 5;  which does nothing but add unnecessary noise.
- Output opamp has input resistor 24k and output resistor 24k; correctly matches the gain of 1.
  It also has 1k in series with pin 3.

Opamp is powered from 9V and the VB supply is made from two 12k resistors.


So getting back the opamp voltages.  The measurements on pins 7 and pins 1 look wrong.
I think the cause is around the input opamp, pins 5, 6, 7.    Pin 1 is wrong *because* pin 7 is wrong.

For the overlay pic I posted, the top left has a 1M, 82n and 510k, close to the 240k.
It seems if the 82n was faulty or there was a short across the 82n then we would probably see pin 7 having a high voltage.
Both sides of the 510k should be about 4.5V  (DMM loading can screw that up) if one side is at a lower voltage than the other it could be due to a fault in this area.

[Rob Strand]

It could well be this one,

https://aionfx.com/app/files/docs/vector_legacy_documentation.pdf

If you look at the Rebote 2.5 variant parts list - sort of close.   The values in the parts list are different to the schematic.  The values in the parts list match-up with the values in the kit (sort of).

The extra opamp resistor was on the Rebote 2.0,
http://www.matsumin.net/diy/jisaku2/vin_delay/Rebote2Delay.pdf

[rdhj]

when you say 82N are you referring to the capacitor labled 823?  If so, i took the voltage readings on each side.  One side was 3.9 and the other was giving me a voltage in millivolts that was different each time I checked it.

[Rob Strand]

when you say 82N are you referring to the capacitor labled 823?  If so, i took the voltage readings on each side.  One side was 3.9 and the other was giving me a voltage in millivolts that was different each time I checked it.

Yes, that 823 cap.  So your measurements look normal.

It might be worth measuring the voltages on the two 12k resistors located just below that group of components (510k, 510k, 240k, 82n).   Also measure the voltage on each side of the 240k .   For sanity you could re-measure voltages on the opamp, pins 5, 6, 7.

Based on your 8V supply, one of the 12k's should have 0V and 4V, the other 12k should have 4V and 8V.   The 240k should have about 4V on each side.

If those check out but the pin 7 voltage is still 7V, I'm struggling to see how the opamp (pins 5, 6, 7) can produce 7V.   It's like the pin 6 input is damaged by leaking current to ground.

[rdhj]

Here is what I got. Everything looks normal except for pin seven is 6.3 V

[Rob Strand]

The 3.78V measurement on the 510k resistor should match the 6.3V on pin 7.   If you look at the PCB tracks the two points are connected so they have to be at the same voltage.     When voltages don't match like this it's a sign of a connection issue.   A faulty IC cannot do this.

It's worth checking those again.    It's also worth check the pin 7 voltage from the bottom of the PCB and on top of the pin of the IC.   All the points should be at the same voltage.

Perhaps touch-up the soldering on pin 7 and the 510k resistor.

If the  pin 7 voltage is different on the top of the IC then it's likely pin 7 of the IC is not connecting to the socket.  That can be because of a dirty IC pin or dirty socket pin (these things can develop oxide coatings).  So try cleaning those up.

If you can get those two voltages to match and they settle on 4.0V then it should work and you can check pin 1, 2, 3 which should also match.    If they settle on 6.3V then there's still a problem,  perhaps pointing to the IC.