Help Fixing Old Green MXR Analog Delay

[pedalbob]

I have an old big green MXR analog delay with the R5101 bucket brigade chip.  It passes signal but when the mix knob is turned all the way clockwise (full wet signal) the output goes to nothing.  There is no delay coming thru with any setting.

I'm trying to figure out how to test whether the clock and switch circuits are working without a scope because I don't have one.  I've spent most of the afternoon surfing this site and others and although this seems to be common problem I can't find a lot on testing.  I figured there has to be a way to test whether the clock is bad or the bucket brigade chip has gone bad.

I've tested the power supply and it looks good at the voltage regulator and diodes.  Voltages on all of the OpAmp RC4558 chips (4) all look good.

Any thoughts or help would be greatly appreciated.  I know these R5101 chips are impossible to find without cannablizing a working pedal.

[ElectricDruid]

If you can tap a wire into the clock, you could run it out to your breadboard and connect it to a divider chip like the CD4060. Then you'd be able to listen to the clock (at least one of the outputs is bound to be within the audio range!) and you'd hear if it was sweeping correctly when you twiddle the delay knob.

Just an idea. I haven't tried such a thing with an old delay.

[Mark Hammer]

The unit uses a "tracking filter" that switches resistors in the lowpass filtering at a rate corresponding to the delay time (i.e., at a slower rate as delay time is increased) in order to get the best bandwidth at any given delay time.

I do not know for sure, but since the 4016 chips that do the switching use a divided-down version of the main clock, it is quite possible their switching frequency falls within human hearing range, and more likely so when delay time is maxed.  If you have an audio probe (I just use a guitar cable with an alligator clip connected to the shield and a 100nf cap connected to the hot lead), you might be able to hear variations in that clock at either pin 12 or 13 on any of the 4016 chips.  If you CAN hear it go up and down in pitch as delay time is varied, then that would suggest that the clock is working fine.

Of course, there are quite a few CMOS chips in that little green devil, and they can easily fry.  It could also be that the bias trimmer is off.  When the bias voltage is set either too high or too low, no delay signal will come out of the BBD.

[Govmnt_Lacky]

Hate to start out with this but, odds are that your 5101 is dead 

Just for grins, measure voltage at the 5101 Pins 8 and 12 (outputs) and let us know what you have. Also, measure at Pin 21 (clock)

[StephenGiles]

When you say "it passes signal", how do you know it's coming from the BBD output and not the bypass?

[Fender3D]

Use a signal probe and check signal does not stop at 4016 switches Mark was talking above...

[pedalbob]

Hate to start out with this but, odds are that your 5101 is dead 

Just for grins, measure voltage at the 5101 Pins 8 and 12 (outputs) and let us know what you have. Also, measure at Pin 21 (clock)

Thanks everyone for suggestions.  I got interupted by my day job ... hate when that happens but back working on this pedal.  Anyway here are voltages at the Reticcon R5101:
Pin 1 0.0; Pin 2 0.0; Pin 3 2.741; Pin 4 3.527; Pin 5 0.0; Pin 6 0.0; Pin 7 0.0; Pin 8 4.93; Pin 9 11.65; Pin 10 2.740; Pin 11 0.0
Pin 12 4.89; Pin 13 9.01; Pin 14 9.01; Pin 15 2.740; Pin 0.0; Pin 17 0.0; Pin 18 0.0; Pin 19 5.76; Pin 20 11.64; Pin 21 0.003; Pin 22 0.0

I will start poking around with the audio probe next.

thanks.

[Govmnt_Lacky]

Anyway here are voltages at the Reticcon R5101:
Pin 8 4.93; Pin 12 4.89

These seem to look ok-ish

Pin 21 0.003

This DOES NOT look ok. Looks like you are missing the clock signal.

If you have one, I would say swap out the 4013 chip and try again.

EDIT*
Also....

Pin 20 11.64

This looks a bit high. Probably have your Bias trimmer maxed to one side. Might want to set it to the mid position when testing and go from there.

[pedalbob]

Anyway here are voltages at the Reticcon R5101:
Pin 8 4.93; Pin 12 4.89

Pin 21 0.003

This DOES NOT look ok. Looks like you are missing the clock signal.

If you have one, I would say swap out the 4013 chip and try again.

EDIT*
Also....

Pin 20 11.64

This looks a bit high. Probably have your Bias trimmer maxed to one side. Might want to set it to the mid position when testing and go from there.

Thanks very much.  I don't have a 4013 but I will get one and give it a try.  Might be a week or so depending on shipping.
Stay tuned and Thanks.

[pedalbob]

So while I wait for the MC14013 chip ...

I still get 11.64 volts at Pin 20 when I adjust the Bias trimpot ... stays same regardless of where the trimpot is turned.

The schematic I have shows a 20K Bias trimpot but mine measures more like 8.25K (probably a slightly out od spec 10K).  It had a small solder blob on the frame to keep it from being changed similar to the other trimpots (looks like it was done during manufacture) so I removed it and tried adjusting.

Any thoughts ?  there are a couple of capacitors in that piece of the circuit between Pin 20 and trimpot leg and path to ground.  Should I try replacing those ?

[Govmnt_Lacky]

So while I wait for the MC14013 chip ...

I still get 11.64 volts at Pin 20 when I adjust the Bias trimpot ... stays same regardless of where the trimpot is turned.

The schematic I have shows a 20K Bias trimpot but mine measures more like 8.25K (probably a slightly out od spec 10K).  It had a small solder blob on the frame to keep it from being changed similar to the other trimpots (looks like it was done during manufacture) so I removed it and tried adjusting.

Any thoughts ?  there are a couple of capacitors in that piece of the circuit between Pin 20 and trimpot leg and path to ground.  Should I try replacing those ?

Reading resistance (in circuit) is always tricky. Only real way to confirm is to remove the trimmer from the board and measure.

The most concerning item is the lack of clock signal at Pin 21.

[pedalbob]

Thanks.  Yes I've been fooled by resistance of components intact in the circuit before.  Not sure why the voltage doesn't change at the IC pin though when I turn the trimmer though.  Positive I have the right one by tracing the board but I monkied with the other 3 too and nothing happens (at pin 20).

We'll see about the clock I guess when I swap chips.

I just remembered one of my friends has a working MXR Stereo Chorus (big yellow box) which has the same BBD chip in it.  I may see if I can borrow it to do some troubleshooting without frying his chip.  Thinking I could put my R5101 in his pedal and see if it works first.  I've always heard these chips are sensitive to static discharge but I have no idea in relative terms what "sensetive" is if I take standard precautions.  I don't want to owe him an unobtainable R5101 

[Govmnt_Lacky]

Thanks.  Yes I've been fooled by resistance of components intact in the circuit before.  Not sure why the voltage doesn't change at the IC pin though when I turn the trimmer though.  Positive I have the right one by tracing the board but I monkied with the other 3 too and nothing happens (at pin 20).

Because Pin 20 and Pin 9 are tied together. Pin 9 goes through a 10 ohm resistor directly to +12V