I'm fixing a broken green MXR delay, please help.

[Chris Goodson]

I was just given one of these but it doesn't work. It just makes a loud hum w/ no other sound coming through when the effect is turned on.  I've heard these aren't too bad sounding as far as analog delays go, so I hope I can get it going.  Anyone have the schematic?  Also, if you might have an idea what the problem is let me know.  Something with the power supply maybe??

[ExpAnonColin]

Poor guy.  The thing uses the R5101, which has a reputation of breaking and is about 100% impossible to find. SO, there's a pretty big chance that's what it is... given that you can't find anything wrong with the power supply or any broken wires.

-Colin

[toneless]

I think a ground wire is broken...
Check the jacks,too.
This big green pedal sounds very good and it's a little bit expensive...

[MarkB]

I've been promising for a while to take pics of the inside of mine.

Gotta run out for a bit - but when I get home, I'll take them and upload them.
"-)

[Chris Goodson]

How can I tell if it's the delay chip?

[Peter Snowberg]

Try building an audio probe and trace how far the signal makes it through the circuit. Even without a schematic.... if you get raw sound on one pin of the BBD chip, but you don't get an output signal on another, that points to trouble. It still doesn't mean the BBD is dead. It could still be the clock or a bias problem.

See the FAQ for more on the audio probe.

Good luck,
-Peter

[idlefaction]

btw, loud hum is often a sign that the power supply is hooked up the wrong way - check that out too!  

[smoguzbenjamin]

Bad grounding also causes hum. Maybe it's just the wire from the output jack that's loose I've had that problem more than once :x

[Gearbuilder]

Hi,
It could be a short on the supply  or  a bad/poor filtering  supply. If it's a 60Hz hum ,it's filtering problem like a dead filtering  capacitor.
Check all of them for a short  ,the diode too ,before trying to change any component in the audio path.
Bruno

[Mark Hammer]

Although perhaps later ones used the R5101, the original used several SAD1024's to achieve the delay length it did.  Could you perhaps pop the back lid, give us a listing and allow us to map your pedal onto a known schematic?

[Chris Goodson]

It has the R5101. :cry:

[Chris Goodson]

http://members.localnet.com/~cgoodson/images/DSCF0001.JPG
Here's a pic I took of the circuit board.

[MarkB]

you just saved me the trouble.. that's the same version I have.. exactly.
Same chip, too.
"-)

[ExpAnonColin]

Interesting.  Try the aformentioned possible problems, like a bad ground wire or a bad power supply connection.  Based on your description, it sounds as though if it's not a wire, it's probably a capacitor-capacitors DO get old.  You might want to test them all out, and if all else fails, you're going to need a new chip.*

*-Translation:  You're @#$%ed.

-Colin

[Chris Goodson]

I notice there's someone selling the boards on ebay for these that have been modded to run with another chip, anybody know much about that?

I guess if all else fails I could "profesionally remove" the ultra rare/magical Malaysian vintage 4558's and sell them on ebay.

[ExpAnonColin]

I notice there's someone selling the boards on ebay for these that have been modded to run with another chip, anybody know much about that?

I guess if all else fails I could "profesionally remove" the ultra rare/magical Malaysian vintage 4558's and sell them on ebay.

He replaced the R5101 with a PT2399.  It's doable, but I'm not sure if it's worth it.  The PT2399 will sound identical, though.

People are slowly coming around about those 4558's.  They really were in a lot more places then just the Tube Screamer.

-Colin

[Chris Goodson]

I don't think its the chip. I was listening to it through headphones last night and noticed its still making delay sounds, they just aren't very audible because of the loud hum.
It seems to me that maybe there's a capacitor somewhere not filtering out dc.  I checked the power-suppies cap ,I think it's okay, but I'm not absolutely sure because I don't quite understand it's markings.  Is it 400?
sprague
500d
400-25dc  
This pedal has some capacitor types I haven't worked with before.
I think it would be easier for me if I could find the schematic, but I can't find it on the internet anywhere, aside from being for sale for $8.  That's starting to seem tempting...

[Gearbuilder]

hi,
You don't need absolutly the schematic for this job .Take your ohm meter and check every capacitor ,you should read 1 Mohm  or more  on each capacitor,when you will find a short beetween the two lugs, change this capacitor.You can debbug  a lot of gear with this method if you haven't got a Scope meter.Try to begin by the first capacitor encountered after the battery or the wire from supply. Generaly the power supply filters caps are the bigger and the decoupling capacitor the smaller but it seems that the hum you descibe come from firltering caps .I hope this hepls
Bruno

[Chris Goodson]

I can't seem to locate a single bad cap in it. :?
I started thinking that the hum sounded a lot like 60 cycle, so I compared it with a note from my tuner(which will generate tones), and it seems to be somewhere between Bb and B. :idea:
So, I decided to hook it up to my variable power supply and goodbye hum!  But, my guitars signal doesn't make it through.  I then pluged my tuner in which is a good bit louder and sound gets through but doesn't sound delayed.  This is sort of confusing, when I tap on some of the op-amps inputs the click it causes delays perfectly and it does the oscillation thing nicely too.  Could someone possibly suggest what to try next?

[Maneco]

Don´t destroy this machine just for the opamps...if the reticon is destroyed,you can replace that part of the circuit using the pt2399 trick,it will sound almost the same...i downloaded the small pics from the ebay auction,and i'll try to complete the schematic using the picture posted...maybe i need some other pics...this circuit is interesting in that it uses tracking filters to adapt to the sampling frequency...