Build Your Own Clone Message Board

Hi,
Just finished a Digital Delay and it sounds pretty good. Does everything it's supposed to. I was testing it and really diggin it when I decided to switch it off and my amp was now REALLY LOUD. It seems the delay cuts the volume of the signal pretty drastically when engaged. Enough that I won't be able to gig with it.

I know it wasn't designed this way so what should I look at first? I'm running it on a battery that tests at 8.95v.

Cheers,
- JJ

Thats not right. Probably a wrong value resistor somewhere.
Check all the resistor values on the board.

Or a short in your wiring. Some pics would help.

I'm pretty sure it's the blend pot that's my problem. I asked for advice on this and other boards about altering the value of a pot and was told by several people (many who pointed me to The Secret Life of Pots page) that is was possible to parallel resistors across the outputs of a pot to change the value. I knew it would alter the curve of the response, but I didn't think it would throw it this far out of whack, but it looks like it did. The reason I even wanted to alter the pot's value was to get the build done without waiting for parts to come in the mail. Looks like I'm waiting anyway. I was really hoping to get this done in time to use it on stage this week. Not going to happen - oh well.

On a hunch I measured the resistance of my altered blend pot (100k with a pair of 100k resistors paralleled across the outs) and in one direction, it works pretty smoothly with very even resistance of 0 to 50k across the sweep. The other side however is nearly no conductance until it's cranked, and even then it's only passing half of what it should.

The reason I thought to measure this pot was because I'd modded it anyway (always a good reason to check it out) and that on further investigation of the actual sound coming out of the delay when engaged, I found that the dry signal was weak no matter how it was set, but the dry & wet signals were very unbalanced in volume. When set completely dry, the volume was very low as I originally posted. When I cranked the blend pot all the way to full wet, the delay part of the output signal is about twice as loud as the dry signal. That made me think real hard about that blend pot.

Here are some guts shots of the circuit removed from the enclosure:









Here's a question:
How would this work if I put a 25k pot in for the blend? Would both the dry and wet signals be louder? If so, would a resistor on the output of the pot bring the volumes back to where they should be? I have a couple 25k pots here.

Cheers,
- JJ

try it with unmodded pots and see if that gives you something closer to unity gain.

OK, I'm all backwards here.
It's the DELAY pot that I modded and it works fine. The delay pot is the 50k, the blend is a 100k and that's what's in there. I replaced the 50K delay pot with a Ginourmous Rat Shack 50k pot just to test it and it works the same. Weak dry signal, strong wet signal.

I gotta get ready to play tonight so this will have to wait until tomorrow. Maybe I have a bad 100k pot in there? I have a couple of those here, but they're audio taper instead of linear. I doubt that would result in a more balanced sound.

So, how's it going with that delay then JJ? Need any help?

K

I haven't been able to find the problem yet.
I replaced all the pots with Alpha pots, just so I couldn't blame it on cheap pots.
Reflowed every solder joint on the board 3 times again.
Replaced every offboard wire on it.
I'm using a TL082 opamp in it instead of the TL072 called for because Rat Shak only carries TL082 around here. The data sheet for them looks identical to me. I also have a couple NJM072D JFET opamps here. Haven't stuck them in yet, I want to read the data sheet for it first to make sure it will even survive the experience and not take everything with them if they don't.

I also replaced the PT2399 itself. This gets interesting:
Now the initial signal is the same volume and tone as the repeats.
Still very treble heavy and about a 40%~50% drop in volume, but better than before.
At least the first hit and the repeats are in line volume wise. Still sounds thin & weak though.

Cheers,
- JJ

Don't bother with the other opamp, the TL082 is fine. It's just a more hifi version of the TL072.

If I read you correctly, the wet pot is now working as it should, it's just weak overall, is this correct? Since you've sourced your own parts, it's a bit harder to troubleshoot this one than if we'd known all the components, but I assume you've checked that all parts are correct values etc.

From your description, I would first check that the input buffer is working ok. It may be a wrong value resistor, a bad solder point, a short to ground here somewhere; you need to rule that out first. Ideally an audioprobe would help you here, to check what your clean signal sounds like at various places after the input stage, but we can try it without. (Be careful with the volume of your amp as you do this, or use another box inbetween that limits the signal). As a quick first test, make a jumper connecting pin 7 to pin 1 on the opamp; you should now hear your crisp clean signal, and it should be louder than in bypass. Confirmed?

If no, your input buffer isn't working correctly.

If yes lets try this; if you have a spare 10k/12k resistor, use it as a jumper between pin 7 and pin 2 now - this should sound the same as the prior test, loud and clear. Confirmed?

If no, your output buffer isn't working. Lets get these two out of the way first.

K

Yes I checked all the components for the correct value, and the wet pot does exactly what it's supposed to do. Actually all the pots function normally as they should.



Nope, very distorted sounding. - downright fuzztone distortion.



OK, here's what I did:

I tried both tests even though it failed the first one.

With a plain jumper (clipped off lead from a used resistor), it was not clean. Sounded like a low volume fuzz box. Not really a bad fuzz sound either, but nowhere near what this thing is supposed to do. There was also no delay produced this way.

I then tested it with nothing to confirm I hadn't killed anything. Back to the original problem, so no harm done.

I then tested it with a 10k resistor (measured 9.87k on my DMM) across pins 2 & 7 of the TL082. Now, the volume bypassed is only slightly more than engaged. The engaged tone loses a lot of bass yet though. Echos sound the same as the initial signal.

I appreciate the help - a lot!

What's next?

Cheers,
- JJ

OK, so the first test didn't work as I expected, I thought it should act like just a buffer circuit, but maybe I got that wrong. Still I think it indicates something wrong with the input buffer.

To get the delay part out of the way for now, just take the PT2399 out of the socket. We know the delay is working.

It's time for voltage readings. All 8 pins of the opamp. While you're at it, do the 3 pins on the voltage regulator too for reference.

K

OK, with the both the PT2399 & TL082 out of their sockets, here are the pin readings.

OpAmp socket:
pin 1 = 0.4vdc
pin 2 = 0.3vdc
pin 3 = 4.7vdc
pin 4 = 0vdc
pin 5 = 4.6vdc
pin 6 = 0.8vdc
pin 7 = 0.8vdc
pin 8 = 9.5vdc

regulator (soldered on board - flat side toward me, pins ordered right to left):
in = 9.5vdc
ground = 0vdc
out = 5.0vdc

Ground was taken with my DMM negative probe stuck in an alligator clamp on the input jack sleeve. A hot-shorted phone plug was inserted into the jack to complete the power circuit. Voltage readings were taken with the positive DMM probe in another alligator clamp with a short piece of extra lead/jumper wire to reach down into the small spaces without touching anything else. I used a DMM set for manual @ 99vdc range.
*** I'm describing this stuff mostly so anyone else looking in the archives will find all the details in case they have a similar problem.

I'm ready - What's next?

Cheers!

Put the TL082 back in!! Who told you to take that out?! Now do it again. Voltages in the 8 pins of the opamp, WITH the opamp in the socket.

(Numbers so for are crearly wrong and pointing one way though).

K

Ok, since I'm about to go to bed (GMT+1) let me just give you something to chew on:

From the numbers so far, the good news is, your power supply works perfectly, and so does the bias network and the voltage divider. The bad news is, you have a big short somewhere. Pin 1, 2 6 and 7 should all be around half voltage. Something is pulling your audio signal path all the way down to ground, no wonder it sounds like sh*t!

From the looks of it, lets first just desolder pin 2 on the wet pot to get that out of the way.

If that doesn't do it, remove the input and output jacks from the enclosure and make sure they don't touch anything. Go over the soldering on the jacks as you do this.

And then report back.

K

Hey guys - I moved this thread over to the Delay Lounge...