3.3volts for digital question re. Guyatone MD-3 rehouse

[BDuguay]

Hey All
I was asked recently to rehouse one of these micro digital delay pedals and provide for all the controls to be expression pedal operated. A big undertaking sure but I got it to the point where everything was still working with half of the guts in the new box. Then something went south.
The pedal still passes signal in bypass and when engaged but no signal echo, just a thumping noise that changes accordingly when the delay, mix, and repeat controls are changed. This tells me it still works...sort of.
The pedal circuit is made up of 2 boards, the main board has board mounted in and out jacks and dc power plug and connects by 3 ribbon cables to the second board that has board mounted pots, a slide switch for selecting short, medium, and long delays, and a proprietary digital delay chip, an EPROM, and a few other digital doo dads.
There is also a JRC 7200L33 that google tells me is a 3.3 volt regulator only now, it's putting out slightly over 5 volts.
There is some chassis grounding done through the mounting of the pots which I missed when I rehoused it and I'm thinking that may have caused the problem.
My question is how important is it that the digital components see exactly 3.3volts and can replacing the regulator possibly fix the problem?
Any input on this would be helpful.
Thanks in advance.
B.

[Mark Hammer]

One wonders how widespread a standard 3.3V is.  The Source Audio Hot Hand 3 outputs 0-3.3v control voltages.

[BDuguay]

No matter how little or how I often I post, I can always count on Magic Mark to reply! I was fortunate that I had another MD-3 that is not functioning and I was using the working one to take voltage measurements in an attempt to sort out the other. I do remember getting 3.26 voltage readings on most of the EPROM legs so I'm very convinced there's a voltage regulator.
I've spent way too much time on this project to give up now so keep those cards and letters coming.
B.

[Mark Hammer]

I'm currently awaiting word on a job I put in for that, should I be successful, will likely reduce my post output for a year (although it would dramatically increase my national travel).  Wish me luck, ....but maybe don't.

[BDuguay]

Well then , I wish you nothing but the most basic level of luck!
I was thinking about this dilemma I'm in and since I don't have any 3.3volt regulators handy to swap out, I suppose I could use the old tried and true LM317 configured to deliver 3.3volts and see if that solves the problem right?
I'm starting to lose sleep on this one....
B.

[Mark Hammer]

...or stick a suitable 1N4xxx type (one or more than one) in series after the regulator output to drop it in 500mv increments.

[jblack547]

3.3V is a standard digital logic level. So is 5V. 5V is often referred as TTL levels. The 3.3V is often referred to "Low Voltage" or LV. You see the LV designator in part numbers like 74LVCxxx.

[BDuguay]

Thanks for that info J.B.
One sure way to check the regulator is to pop it out and test it. That's my project for tonight. If that doesn't help, it's inching that much closer to the curb at the end of my driveway (tomorrow is garbage day btw) 
B.

[BDuguay]

Another thing occurred to me yesterday. I was once told by a co-worker for whom I was building a test box, that one should not mix digital and analog grounds. I may have done this when I transferred the in and outputs form the original circuit (board mounted isolated plastic jacks) to chassis mounted non-isolated jacks in a new aluminum enclosure.
Thoughts?
B.

[jblack547]

You generally want to bring the digital and analog grounds together at one point---A single point ground. Unless they are optically isolated or another form of isolation. You definitely don't want digital signals running through analog sections. For instance, don't run a digital signal close to an op amp input. This assumes that the digital circuits are running at digital speeds like in the MHZ or have fast edges on the transitions. There are so many things written about "mixed signal systems," where do we start?

If we are talking about the signal switching in a stomp box then there is not much danger of coupling noise except during the digital transitions. Try to run the digital ground return as close to the digital signal as possible. Preferably right under it. Hence, ground planes on the circuit board. If the digital signal is in a wire, then put the digital ground in a pair with the signal.

Check out "Noise Reduction Techniques in Electronic Systems" by Henry W. Ott. This is an engineering text and has a ton of info and may be more than you want to know. It was my go to book when I was designing mixed signal Video cards in the 90s.

[BDuguay]

Well, I killed the pedal and I think I know why. I pulled the voltage regulator and it was pooched for sure so devices that were only supposed to see 3.3 volts were getting 9 and change.
I'm going to check the data sheets on the digital bits and see if anything is salvageable.
B.

[Mark Hammer]

What sort of digital chips are in there?  An on-line pic of the back of the main board suggests there might be a PT2399, but the specs (2600ms delay) suggest it couldn't be.  Is there some sort of proprietary chip, or perhaps a 2nd board we can't see?

[BDuguay]

There's a proprietary chip and an eprom and a 2 other surface mount doo dads. There's 2 boards to that pedal and it's the second board that holds the pots, a switch for Short, Medium, and Long delay settings, and all the digital stuff that doesn't like 9volts.
B.

[BDuguay]

Further investigating reveals a Hex inverter and a Flip Flop device. Also, the main chip is a T14L1024N which is a High Speed C-Mos Static Ram chip.
Still, after replacing the voltage regulator with a functioning one, the pedal still demonstrated the thumping noise that reacts to the Time, Level, and Repeats pots.
It just won't echo the guitar signal.
B.

[Mark Hammer]

The hex invertor and flip-flop is going to be for bypass switching.  The RAM chip will be an adjunct to whatever is serving as the A/D/A chip producing the delay.

Safe to assume - especially given the size - that the static Ram chip is SMD?

[BDuguay]

Yessir, SMD indeed.

[BDuguay]

I think there's one last attempt left to revive this bad boy. I have another MD-3 that doesn't work, or at least it doesn't behave the way the other one does. It has the proper voltage regulator providing 3.28volts so I can somewhat assume there's no damage to it's sensitive bits. What do we say if I was to swap the static ram chips and see what that does? I.E. take the one from the board that has the thumping thing going on and put it on the second non-functioning board that likely hasn't been damaged by high voltage.
I'm guessing it's the ram chip responsible for the delay?
B.

[Mark Hammer]

The RAM chip isn't "responsible", as such, but there needs to be somewhere to stick the samples, right?  So the RAM could be fine but still no delay, or the converter chip could be fine but the RAM shot.

[BDuguay]

Okay, now I'm getting it. Jeez, if I hang out here long enough, all this voodoo might start to make sense.
So, any idea how to check/test these chips to see which are working and which aren't?
There may be a happy ending to this story yet...
B.

[BDuguay]

Shameless but desperate bump...
B.