pt2399 datasheet aplication doesn't work...sort of

[spyrusthevirus]

i wanted a simple delay for my synth, so I bought a small board with the standard delay/echo application circuit soldered on it. I figured that by changing the trimpots and perhaps a cap or two I could pretty much use it as is.
It has arrived, but it's not working as I supposed it should.
When I try to use it at a normal volume (ie, synth master knob is at the usual level) I can hear no echo at all. When i crank it up (max setting), I can hear just one echo, that is overdriven to hell and distorted. My sound generator is a microbrute.
I tried using the line out and headphone output, same thing happens in both cases.
Am I doing something wrong, or is the board rubbish?
It's all made using SMDs and uses trwo trimpots with "104" printed on them (I was to lazy to google the trimpot code). It uses a 78L05 regulator and as far as I can see, it uses the exact "echo/delay" application schematic that can be found in the datasheet.

[Jdansti]

I've never tried the datasheet circuit, but since it only has one pot which controls the delay time, I don't see how you control the number of repeats. I'm sure that someone more knowledgeable than I can answer this.

If you decide to try again, consider the Ersatz Verben  here:
http://www.diystompboxes.com/smfforum/index.php?topic=107970.0
It's a very minimal circuit and will do the job

BTW - A 104 trimmer is 100k Ω. 10 plus 4 zeros.

Edit:Sorry-I see the feedback loop and 50k pot which should control the repeats.

[mth5044]

I will be less lazy for you and Google 104 trim pot. 10 seconds later, turns out they are 100k. I will also Google the pt2399 datasheet for you. The one I found says the delay pot is 20k and the feedback is 50k. Check that the total resistance coming off of pin 6 shouldn't be less than 1k with the delay time trim turned all the wAy down or your chip could lock up. You can do this by measuring between pin 6 and ground. You will need to pad the input of your synth, or turn it down, as the schematic in the datasheer doesn't have a lot of headroom.

[spyrusthevirus]

In a delay circuit, you control the number of repeats by varying the feedback amount-ie, how much of the output is fed back in the input. The higher that amount is, the longer they need to die out.
The resistance on pin 6 varies from 10k to about 100k, measured with a multimeter. This should be ok for the IC, even if not optimal. In fact, I can hear a satisfying and stable delay time in all settings of the trimmer.
While 100k is double the recommended amount for the feedback pot, i think it shouldn't be much of a problem. I'll eventually replace those trimmer with pots, so I might as well just do that today and see how it goes. Perhaps 100k messes up the feedback amount and that's why I don;t get any repetitions.
I tried every volume setting of the synth. low values (that would fall into the input range of a 5V powered IC) don't produce any echo at all (I can hear the original signal only). Higher settings (close to maximum)  are the only ones making any echo, but they are overly distorted (which does make sense) and don't, well, echo. Just one repetition and they stop.

[samhay]

>Perhaps 100k messes up the feedback amount and that's why I don;t get any repetitions.

I don't think so.
Feedback is usually taken from pin 12 or 14 of the PT2399 and fed back to pin 16. If you only have 1 repeat, then you don't have any feedback.

Do you have a schematic / photograph / link to where you got the circuit from?

[garcho]

For what it's worth, the PT2399 datasheet delay works just fine, sans suprise.

[spyrusthevirus]

samhay- it's the echo circuit on the pt2399 datasheed. the one with the feedback loop. At least that's what the seller claimed.
From what I can see on the board, it matches the schematic-ie, the signal from pin 12 is fed to the 100k potentiometer (50k on the schematic) and the pot. taper is connected, through a bypass cap, to the input.

page 3
http://sound.westhost.com/pt2399.pdf
The schematic he posted had a few slightly different component values (for example, the resistor after the feedback pot taper is 10k instead of 18k), I believe it comes from a different/older version of the datasheet. Also, the bypass caps he used are all 10uF, but that shouldn't be any trouble at all (if anything, it should cause less low freq. cutoff).

What troubles me is that even if the circuit was configured as a delay (which it isn't), the one repeat I'm hearing should be the same, or very damn close, to the original signal. What I'm getting now is an extremely distorted version of the input, and that only on very high amplitude inputs. On a lower input amplitude, I hear no repeats at all.

[slacker]

Is the dry signal ok? If you turn the feedback all the way down, you should get just the input signal on the output, same sort of volume level with maybe some high end rolled off.
If you can try tapping off the signal from the clockwise pin of the feedback pot, if you do this with the feedback all the way down this should give you just the delayed signal, is this clean?