Deep Blue Delay clone voltage drop problem

[balkanizeyou]

hi guys,
it's my first post here so i guess i'll introduce myself a bit - i'm pretty new to electronics, i've built a few pedals this past year or two but i'm still a newbie so don't get too angry about my stupid problems and questions! also, sorry for my english, but i'm from poland, so i'm sorry in advance for all the mistakes.

anyways, after building few distortions & fuzzes & tremolos & stuff, i wanted to build some pt2399 based delay (because they are the simplest delays available i guess). I've read good opinions about the Deep Blue Delay, so i breadboarded it. It worked well and sounded great to me, so i decided to make one.

I've made the PCB using the layout from this website: http://revolutiondeux.blogspot.com/2012/01/mad-professor-deep-blue-delay.html

The problem is, that when i put it all together it just wouldn't work. I could hear only the clean signal but no delays. I assumed a broken pt2399, but changing it wouldn't help. I started checking the voltages and then i realised that the output of the 7805 (i'm using 7805s instead of 78l05, but i think that shouldn't be a problem as it has 1A current rating as opposed to 100mA of 78l05) wasn't exactly 5v, but more like 4,60. I assumed it was broken, but changing it three times wouldn't help either.

The problem seems to be in some other place, because the power supply voltage measured 9,03volts, but the input pin of the 7805 was at 6,32v, so there was a voltage drop of 2,70volts at the 33R resistor, which is absurd, considering the fact that it forms a voltage divider with two 10k resistors if i'm not mistaken.

The funny thing is that when i socket out the 2399, the voltages are fine - input of 7805 shows 9volts and the output is exactly 5volts, but everything breaks down when i socket the pt2399.

So i thought that there was a mistake somewhere, but after few hours of checking all connections i couldn't find it, so i decided to make another one from the scratch.

But the same problem occured! I made a total of four copies of this circuit and in all of them there was the same problem.

I got really desperate, so in one of them i removed the 33R resistor (R7 on the schematic) and, weirdly enough, it started working. But it sounded a bit like crap - a lot of digital noise in the background, not enough feedback for me even with the feedback knob turned all the way up, and with the delay set to maximum it really degraded the repeats. I know that it is the issue with all pt2399 based delays, but the one i breadboarded really sounded much, much better.

Does anyone have any idea why doesn't it work with the 33R resistor? Yes, i checked a few times - it measures 33R, and the 10k resistors are really 10k resistors. The schematic seems to be verified, as a lot of people have made those using this PCB design i guess.

I'm really annoyed because i've been trying to get them to work for a few days now and i cannot spot any mistakes. I'm sure i did something wrong, but i don't know where to look for them anymore.

[PRR]

Welcome!

> voltage drop of 2,70volts at the 33R resistor,

2.7V across 33 Ohms is 82mA. IIRC the current demand of the PT is something under 100mA. The regulator has to pass all this.

> forms a voltage divider with two 10k resistors if i'm not mistaken.

If it is 33r, reg, PT+10K, then the PT's demand dominates and (without actually looking-up PT specs) it looks reasonable to me.

EDIT- http://www.princeton.com.tw/Portals/0/Product/PT2399_1.pdf

They do not actually say the PT's normal consumption. But (page 5) it must not exceed 100mA.

If it runs close to 100mA, 33r is dubious. 9V-3.3V= 5.7V, but the 7805 needs 2V of headroom to make good 5V, so no lower than 7V into the 7805.

Change 33r to 10r. Or since you have a lot of 33r, tack three in parallel (11r). That should give 7.9V+ into the 7805, almost 3V headroom over 5V, good regulation. However if your PT is totally sick (shorted), the 10r-11r will smoke before gross damage happens.

Are you *certain* of the chip pinout? Which way is pin 1? Which way is up? On the chip? On the PCB?

I can tell you (it has been a while) that a 8088 CPU can be plugged in backward, and acts like a very good short. (Oddly enough, putting it right made it work fine. The backward chip was tougher than the power supply.)

The schematic at the link posted shows 7805 (not 78L05).

[balkanizeyou]

thank you very much! I changed the 33R resistor to 10R and now everything seems to work fine - the voltage at 7805 input is more than 8V, the 2399 gets exactly 5V and the delay works as it should. Thank you for your help and insight!

However, there seems to be a little more noise in the background compared to the one i breadboarded a long time ago, and with longer delay times the repeats tend to get a little choppy - is my memory failing me and the breadboarded one was probably noisy as well because of the pt2399 imperfection, or is it possible to get a noise-free DBD and could it be a mistake in the circuit somewhere?

[PRR]

> is my memory failing me

Mine is.

Sonic comparisons over long time periods are often dubious.

> is it possible to get a noise-free DBD

Hiss-FREE? No.

> could it be a mistake in the circuit somewhere?

Always possible.

[balkanizeyou]

ok, so i guess i'll have to breadboard one and compare the performances, thanks a lot