MXR Delay System 2

[Tav]

Hi, it's my first post in the forum, cheers to all
Here's my problem, I own an old MXR Digital Delay System 2 unit that sounds great. From time to time the unit messes the delay times (you can see the display that shows the time set changing the delay), then returns to the correct delay time, starting the problem all over again.
It's the second unit that does this (they seem to be not very reliable).
What could be the problem? Anyone owns the unit and esperienced this type of behaviour?

[Mark Hammer]

Erratic behaviour such as that often stems from old dried out, out-of-spec electrolytic capacitors. That unit is over 25 years old, if I'm not mistaken.

[Tav]

Yeah, it's a pretty old unit. I will change all the capacitors inside just to be sure that the problem resides in those.
Thanks for your help, I will let you know if I solve the problem.
Regards

[Tav]

Problem solved
I've changed all the electrolytics caps in the unit and it seems to work right now.
Thanks for your suggestion.
Merry XMas

[Mark Hammer]

Excellent!!

[Tav]

Nope, it started doing the same thing again...what the hell could it be???

[Tav]

And now a new problem arose, the unit turns off by itself.
What a pity, I've got the schematic of the unit BTW, it really disappoints me. Could it be something regarding the power section, non correct voltages? ICs that don't work anymore? Damn...
Like I said I have changed all the electrolytic caps and memory ICs.

[Mark Hammer]

Could you possibly post the schematic? That might help to pinpoint some strategc places to look.

[Tav]

http://www.pinkover.com/IMG/MXR_151_...Schematics.pdf
Here it is.

[Tav]

BTW Here's the story of this unit. This is the second MXR delay system 2 that I bought since the first one stopped working completely (incidentally it had the same problems of the first one). The first unit had a 220V power transformer (I live in Europe). When the second one arrived from the States it originally came with a 115V PT. I interchanged the trafos in order to use it in the second unit, and after a while it started with the problems I described earlier.
Suspecting that the issue involved the 220V Trafo, I replaced it with the original 115V one but the problems remained there. The first thing I did was to replace all the memory IC (TMS 6118 20nl), for a few hours it worked properly, then the problem arose again. I then changed all the electrolytic caps, the problem vanished for a few weeks, then it happened again. Now the unit turns off by itself, and it does not turn on. (still with the original defect). If I wait some time it powers on normally.
I will change the voltage regulators (but I don't think that it's the problem). What else?

[TheEHMan]

http://www.pinkover.com/IMG/MXR_151_...Schematics.pdf
Here it is.

If you just purchased that from MusicParts I'd have them redo page 6. It's got something weird right through the middle of the page.

[cjlectronics]

When you say turns off by itself, what does that mean...

loss of sound (lights are still on but no sound)?
the unit goes dead, no lights, no sound, no clean signal?

It sounds like a heat problem. If it goes totally dead, then I would resolder all of the connections in the power supply section. Make sure the voltage regulator's heat sink is attached well. If there is no heat sink, verify that it is in a position where air can flow all around it (i.e. standing straight up or in a 45 degree angle)

CJ

[Tav]

When you say turns off by itself, what does that mean...

loss of sound (lights are still on but no sound)?
the unit goes dead, no lights, no sound, no clean signal?

It sounds like a heat problem. If it goes totally dead, then I would resolder all of the connections in the power supply section. Make sure the voltage regulator's heat sink is attached well. If there is no heat sink, verify that it is in a position where air can flow all around it (i.e. standing straight up or in a 45 degree angle)

CJ

Yeah, it turns completely dead.

[Tav]

Any other suggestion?