PT80 Oscillation problem and how i fixed it

[weeeugene]

Thought I'd share a problem I had with a PT80 going crazy and how I fixed it so that others with this problem can trying fixing it.

Everything on the PT80 i had worked fine except for 2 things

1) When i had the Delay Level maxed, it would get crazy feedback. instead of the repeats fading away, they actually got louder and louder and wouldn't stop repeating. and this happened even at minimum Delay repeat settings,

2) When I maxed the Delay Time, it would oscillate like crazy and make space ship sounds.

spent hours troubleshooting. tried replacing the 22k resistor at pin 12 of the compander with a 10k and 12k with a 0.01uf cap at the junction to ground, and it didnt work. all the while i thought it was my repeat pot that was causing problems. turned out it was the delay level. When the delay level is anything thats just shy of max, it works fine. finally hooked up a 1k resistor in line with the input to the delay level pot and now it doesnt go feedback crazy when maxed out.

as for problem number 2. i tried the same thing. added a 1k resistor in line with the delay time pot. but i dont think the 1k is enough cos it still goes into crazy oscillation. i think i can live with it since i dont dial it up so high. might be fun to be able to create those noises.

does anyone else have these problems? or any explanation why the delay level maxed out would send it into craxy feedback?

[alfafalfa]

Haven't built it yet but thanks for the info.

Another question , have you built the Rebote 2.5 ? I would like to know if the PT 80 is cleaner.
My rebote sounds very good but has this noise towards longer setting of the delaytime.


Alf

[soggybag]

I think the PT2399 chip is only good for about 680 ms of delay. After this it starts to get distorted. A 50K delay pot is recommended. Higher than this and the delay gets longer but you get distortion.

I changed the feedback resistor to 5k6 on my PT-80. This gives run way feed back at the max setting. And endless delay just below that.

[Bullet79]

i have trouble when raising the feedback to 12 o'clock, it would oscillates and get louder... where is the feedback resistor actually..?

[anchovie]

Repeat pot wiper -> Capacitor -> Feedback resistor (22K on schematic).

[Bullet79]

now do i have to raise or lower the value to fix the oscillation... i have done this with echobase wich had a same situation, raising the 24k to 100k solve the problem... is it the same story with this PT80..??

:icon_confused:why i always get the same kind of problem...

[anchovie]

Raise it - higher values decrease the maximum amount of feedback.