Long time listener, first time caller.
I just finished a PT80 homebrew. (I etched my own board, supplied my own case and parts, etc.)
This one is a bit different from the original PT80. I decided not to do a "stomp box," instead opting for a case that would sit atop my amp. The bypass switch is a toggle switch rather than a foot switch, but it's electrically the same. Also, instead of a battery, it's powered by a 12VDC wall wart (which has been tested). I also subbed the TL072 with a 5532, because it was handy.
When I first powered it up, it obviously passed signal in the "bypass" position, but when switching in the effect, all I get is a hum. This hum was consistent in level and frequency regardless of what I did with the controls or whether or not there was a guitar plugged in. After looking for stray solder or misplaced wiring, I checked the voltages on the list. This is where it got interesting. The voltages for the 5532 (TL072) and the PT2399 were all within normal range, but most of the pins on the SA571 were reading 10VDC! Closer inspection revealed the problem. My supplier (Mouser) had accidentally sent me an SA572. Same basic circuit, but completely different pinout. Fortunately, they were happy to send me the right one free of charge. I plopped it in today.
All the voltages were checked again, and this time everything looked good except for a few of them....
PT2399 Pin 7 -- 1.2V -- supposed to be 0.9V
PT2399 Pin 8 -- 1.3V -- supposed to be 0.9V
SA571 Pin 16 -- 1.4V -- supposed to be 1V
Not sure if these are within tolerance or not.
Anyway, the thing sounds exactly the same. When switched in, I just get a consistent hum with no other audio.
I figured I post my tale of woe here in case someone else encountered the same trouble. Didn't want to "reinvent the wheel" so to speak. (Although I seem to have done just that!)
Thanks in advance.
Update:
Having spent a bit of time with an O-scope and a tone generator, tracing the signal through the unit. I did find a less-than-hair-sized solder bridge. Removing the offending item, the unit now passes signal.
However, the signal is accompanied by a significant hum. Tried additional grounding to no avail. Rechecked the voltages. There are still two or three that stand out as unusual to me:
PT2399 pins 7 & 8 = 1.5VDC
SA571 pin 16 = 1.5VDC
Checked the board again for bad traces or bridges. None.
Incidentally, the hum is only present when the effect is switched in, and is not affected by any pot position.
Updated update:
Got rid of the hum by placing a .01uF cap across the input. The only problem is that this fix only works if I run the unit off a battery. If I run it off a wall wart, like I want to, the hum comes back. Foo.
Are you using the charge pump?
Could very well be the wall wart. I have a 12V supply too (from an old laptop I believe) and that hums on every project I tried it on. Supply filter caps did not work.
Quote from: geertjacobs on July 28, 2010, 03:28:16 AM
Are you using the charge pump?
Yes, although it's probably unnecessary.
Just my 2 pennies but, you mention the unit working if you use battery power. Could you be plugging a center-positive wall wart into your box? That circuit is center negative. Could have damaged something if this is the case.
I assume you are referring to the small barrel connectors that are often found on the other end of the cable from the actual wall wart. This particular wall wart does not have such a connector. It it simply hardwired right to the PCB. I used the white trace on one of the wires to determine which is + or "hot." (Tested it with a voltmeter to verify.)
Is this correct?
Who else is running theirs off a wall wart, and what kind of luck are you having?
Would bypassing the charge pump help or make no difference?
I run mine straight off an 18V wall wart so I didn't use the charge pump. It gets regulated down to 12V on the board.
When you traced through with the scope, did you find the point where the hum first appears?
That, I suppose, is the next step. When I used the scope before, that was just to find where the signal went dead. Now that I have it actually passing signal, I'll hook up the scope again and trace the hum.
And I might as well bypass the charge pump.
Well, I went ahead and bypassed the charge pump, using the wiring diagram from the GGG website. I used a 15VDC wall wart. The unit is pretty much dead until you plug something into the input, at which point the only sound from the unit is a horrific screeching sound. Yikes!
I guess the next step is to put the charge pump back in and try to trace the hum with a scope.
Well, I put it all back together and fired up the scope. Hooked up a 9VDC wall wart. Beaucoup hum! Started trying to trace it, and it seemed to be everywhere (along with some other hash and trash). Finally, I put the scope probes across the wall wart itself and saw a pretty significant sawtooth waveform. Tried the same test with the battery and got a clean flat line, so obviously the wall wart is noisy as hell (read: unregulated!).
So now what?? I'd rather just plug this thing into the little AC outlet on the back of my amp than feed it batteries. What about a regulated 9V or 12V supply?
O.K. forget what I just said. The sawtooth waveform I saw when reading just the wall wart was due to a defective probe. After consigning said probe to the trashcan, I checked again with a good one. Result? The output of the wall wart is pool table flat. Here's where it get's interesting....
Connecting the wall wart to the circuit and then checking it with the scope reveals a significant waveform. It's almost as if the circuit is generating the noise when connected with a wall wart! Again, running with a 9V battery works fine.
Help!
Did you ever read out your wall wart connections to your PCB to see if you have the Positive Wall Wart wire going into the Positive input of the board? It still sounds like you may have the polarities of the wall wart reversed on the PCB and probably even fried some components.
You also mention that you "hard-wired" the wall wart to your PCB. If you did this, then How do you connect your battery power to the PCB? Usually, a 2.1mm DC input jack is used with an internal switch to switch between wall wart and battery power.
How are you switching between the two ???
Sorry, Gov. Poor choice of words. By "hard wired" I meant I wasn't using a panel mount connector. I have two pigtails coming from the PCB, one for the "+" and one for the ground. This allows me to try either the wall wart or the battery by just changing what's connected to the pigtails. Once I solve the issue, the wall wart will be connected permanently. (Or the battery, if that's the only thing that works.)
I've verified all the polarities and have double-checked all voltages on all chips. Everything checks out.
If it works fine with a battery but not a power supply, it's your power supply.
So I tried it with a regulated bench supply. Noise be WAY GONE!!
So it would seem that the wall wart is not sufficiently regulated. I've tried a couple other wall warts with the same result. Hum city.
At this point, would it be worth a try to run the output of a wall wart through a couple filter caps and a voltage regulator (like a 7815 or similar)?
Because running it from a power supply the size of a toaster would be cumbersome.
Success!!
I built a tiny regulator board with a 7815 and couple filter caps, then attached a better wall wart. No more hum!
The regulator might be overkill, but the new wall wart puts out 24VDC and I was afraid that would be too much, so I decided to keep the regulator in place. Anyway, the thing is quiet.