Please help debugging PT80: Clips and voltages inside

[finetuned]

Hi there,

I've finished my PT80 (perfboarded  ), but there's a problem.

What it does do:
» Straight signal and delayed signal both work (although low volume),
» If I play hard, there seems to be a point at which everything goes ten times louder and distorts. (www.finetuned.nl/PT80faulty.mp3)

As you can hear in the clip, there's some real filtering going on in there, almost like closing a wah pedal on from the 3rd repeat onwards. I'm aware of the analog character of this delay, but can anyone confirm that the sound in the clip is the way it's supposed to sound?

Also, although I wired the pots the way they are wired in the layout from GGG (the 3PDT version), both the Feedback and Repeats Level pots seem to work in the wrong direction: turning the Repeats volume pot clockwise decreases the volume of the repeats, and turning the Feedback pot clockwise results in less feedback signal (lesser repeats).

The Delay Time pot works as expected (turning counter-clockwise reduces delay time).

Also, as you can hear in the clip, there's quite some 50-cycle (Europe) hum, which may or may not be caused by the fact that the effect is not yet boxed (unshielded cables from the jacks to the perfboard picking up interference).

I have all parts stock, except:
» the 50k pot for delay time replaced by 100k pot for longer delay times
» the TL072 opamp replaced by TL082CP


I used an SA571N compander.
I used WIMA non-polarized capacitors. I used electros for the values larger than 1uF.


Voltages as follows:
» PSU output: 12,16V
» Output 5V regulator: 5,01V
» Output 12V regulator: 10,21V (So I connected the outer lugs of the transistor so the output voltage of the 12V regulator now becomes 12,16V as well.)

» Vgnd = 0,00V

PT2399 (Voltages as opposed to ground)
Pin 1: +5,01 V
Pin 2: +2,49 V
Pin 3: 0,00 V
Pin 4: 0,00 V
Pin 5: +2,60V
Pin 6: +2,49 V
Pin 7: +0,83 V
Pin 8: +0,86 V

Pin 9~16: +2,49 V


SA571N:
Pin 1: +1,10 V
Pin 2: +1,80 V
Pin 3: +1,79 V
Pin 4: 0,00 V
Pin 5: +1,81 V
Pin 6: +6,47 V
Pin 7: +6,49 V
Pin 8: +1,80 V
Pin 9: +1,80 V
Pin 10: +6,57 V
Pin 11: +1,77 V
Pin 12: +1,80 V
Pin 13: +12,16 V
Pin 14: 1,80 V
Pin 15: 1,80 V
Pin 16: 1,12 V


TL082CP:
Pin 1: +1,45 V
Pin 2: +1,44 V
Pin 3: +1,19 V
Pin 4: 0,00 V
Pin 5: +1,01 V
Pin 6: +1,44 V
Pin 7: +1,45 V
Pin 8: +12,16 V

I've spent quite some time looking at everything and double-checking all the leads, but I can't really trace the problem. I would say everything around the PT2399 is fine, maybe there's a problem around the 082 or the 571, but I can't find it.

I've replaced all three ICs but to no avail.

I hope someone can help me with this.

Thanks

Stan.

[geertjacobs]

Voltages as follows:
» PSU output: 12,16V
» Output 5V regulator: 5,01V
» Output 12V regulator: 10,21V (So I connected the outer lugs of the transistor so the output voltage of the 12V regulator now becomes 12,16V as well.)

power supply:
- Be careful with the power supply:
the datasheet of the 7812 mentions a required input voltage of minimum 15 volts, so your 12.16V PSU wouldn't be enough.
Distorted sound:
- is the transistor oriented correctly?
- Have you checked for bad solder joints?
- You could try to use an audio probe to find out where the signal is distorted.
Filtering:
- double check the values of resistors/caps.

A working PT-80 is a great pedal 

[Andre]

The voltages of the PT2399 and the SA571 are OK. I checked with my PT-80.
However,all the voltages of the TL082 except pin 8 (Vcc) and pin 4 (GND) are wrong.
They should all be around 6V,
So, the problem, I think, must be the 1/2 Vcc ref. marked "R" created by the voltage divider (2 x 1k) at the output of the 7812.
You should measure about 6 V at the junction of these two 1K resistors.
If not, one of them has the wrong value or maybe there's a solderbridge which shorts the 1/2 Vcc.

André

[finetuned]

Thank you thank you thank you!

I found 1 bad resistor value and I actually forgot to make a connection between the junction of the two 1k resistors and the TL082 

All pins now have 6V except for GND (0 V), Vcc+ (12,16V) and pin 5, which is around 3 volts now, is that correct?

Anyway, it works now, although it still has a loud hum.... could it be because the circuit is not yet boxed?


Anyway, big thanks, I'm now sure that all the resistor and cap values are right...
I believe all connections are made.

Can anyone confirm my question regarding the pots' directions?

Thanks a lot and have a nice Christmas Eve (and a merry Christmas, too)

Stan.

[Andre]

I am glad you got it to work.
I think the hum is coming from your power supply.
Since this delay also works with 9 Volts and you have bypassed the 7812, you could easily try a 9V battery.
I think the hum will be gone.

Most cheap power adapters only use a transformer, bridge rectifier and a big electrolytic capacitor, so there's always some hum left.
Since you bypassed the 7812 , the unstabilized power is fed into the circuit and causes the hum.

I guess you need to find a 15V DC powersupply.

As far as the pot's directions concern I connected the pot's in a way that the delay volume and number of repeats increase when turning clockwise,
no matter what the diagram says.