MXR Analog Delay

[michael_krell]

I am working on repairing a MXR analog delay. I just purchased the schematic, have not recieved it yet though.

I am pretty sure the signal is not reaching the R5101 Reticon chip because i tested all the pins and there is no audio signal to be found. I tried tracing the signal through the input, but is difficult to do without the schem.


I have a feeling its either a coupling capacitor or IC.

Oh by the way, this is the problem. The pedal passes signal with the effect on but when the mix is turned up, there is nothing there.


THANKS

[cjlectronics]

First and foremost, check the power supply.  I've seen the regulators go on a few of them.  Hopefully this is the only thing wrong with it.  The R5101's are virtually impossible to find and if you do it will most probably be more expensive than the pedal is worth.

Good Luck,
CJ

[jimbob]

It would be nice to have a layout for an MXR Analog Delay

[Vsat]

I have a non-working MXR Stereo Chorus here... this particular unit has an SAD4096!!!
Hope the SAD hasn't gone South...
Cheers, Mike

[michael_krell]

checked the power supply already yesterday, i know that is working. i am almost positive the problem is that the signal is not making it to the Reticon. I dont need anyone telling me how rare they are and how easily they break, i have heard it 1000000 times.


Are all the 4558's for the signal chain?

[michael_krell]

i made a helpfull discovery. I measured the + inputs on one of the 4558's and it looks like the bias is off. The bias voltage should be somewhere around 6 volts, all of the other 4558's have that bias voltage, so i think there is a problem just before that input.


Please let me know if this is true.

[michael_krell]

anyone got any info???


also, has anyone ever purchased a schem. on schematic connection?

I cant get a hold of the guy.

[Vsat]

Michael,
I have one-half of the schematic for the E1010 if that is of any use. They probably have a lot in common. Unfortunately the other half on the other file on my floppy is unreadable. It was downloaded from somewhere however, perhaps it is still up. Other listmembers probably have both files.
Cheers, Mike

[michael_krell]

is that a yamaha?

[Vsat]

Ooooooops.......
That E1010 message was meant for a different thread (blush)

[Mark Hammer]

An SAD4096 in an MXR Stereo Chorus?  My schematic shows an MN3008.

[Vsat]

Yup.... an SAD4096!
My schemo shows the MN3008....

[michael_krell]

Yipee!!!!

hey Mark you have always been such a great help in the past, do you have any suggestions? should i pull the IC that is acting suspicious?

[puretube]

you could try & mail Kate

[michael_krell]

That link did not work for me.

[puretube]

strange - it`s a link to the archive, with Kate Ebneter offering a lot of schemos...

plus her e-mail address (at that time):
ebneter@ix.netcom.com

[Mark Hammer]

The only relevant schematics I have are for the MXR and Ross units using multiple SAD1024 chips.  Obviously these are an earlier generation than anything using an R5101 or any of the R-series chips.

That being said, the true innovation of the MXR delay pedal was its use of a tracking filter.  This harnessed a couple of CD4016 CMOS switching chips to the same master clock driving the BBDs, except that the clock had to be divided down a bit to nail the right filter frequency.  In this manner, however, the corner frequency of the lowpass filter could be adjusted so that the "right amount" of lowpass filtering could be applied to the audio signal for any given delay time.  Normally, contemporary pedals like the Memory Man or DM-2 used a fixed filter frequency that struck a compromise between eating up too much treble at short delays and not filtering the clock whine enough at longer delays.

I mention this because in between (in the sense of "standing a chance of interfering with) the clock source and the final audio output path for the wet signal are 4 CMOS chips (two 4013 flip-flops for dividing down, and 2 4016 Quad switches for the filtering).  A malfunction in any single one of these would interrupt the passage of wet signal, either because the 4016's are not being successfully switched for filtering purposes, or because one or more switching sections in the 4016's themselves are not passing signal.

The bad news is that's a lot of chips to test.  The good news is that they are all cheap and still widely available.  As you know, the same cannot be said for the Reticon chips.

Again, I am looking at the first issue schematic.  I have no idea how closely subsequent issues conform to this, but I think it is a fairly safe bet that they retained the same general approach to tracking filters since it worked well, since the same amount of low pass filtering would be useful despite change in BBD, and since this feature was a big part of what set their unit apart from the rest of the pack at the time.

[michael_krell]

Well, regardless of the schematic, I dont think the input signal is ever reaching the Reticon. Can you offer any advice in debugging the input signal path?

[Mark Hammer]

Well, the signal has to go through 4 CMOS switch sections to ever reach the BBD in the first place.  It then passes through another 4 switch sections to get out of the BBD to the compander.  Two of those switch sections pre-BBD are bottlenecks, meaning that there is no way for the audio signal to reach its destination if that chip is fried.  Same goes for the 4016 handling post-BBD filter duties.

If one or both of the 4013's that divide down the clock signal for filter-control purposes is fired, then NEITHER 4016 can go through its switching motions, since that divided clock controls both pre and post-BBD filtering.

In any event, pins 7 and 10 are the output pins of the NE570/571 compander chip.  There ought to be something coming out of at least one of them.

[michael_krell]

Mark, you are the man. I appreciate your help. I will check that when i get back home.

im going away tonight and tommorow so ill probably post on wednesday in regards to my findings.

Thanks again