MXR 126 DOubler/Flanger dry signal only

[BrianJSiegel]

MXR 126 Doubler/Flanger goes silent the more I turn the dry/wet knob to wet. All the way to 100% muted when fully wet. Otherwise everything looks to be working, all the LEDs are responsive, just no wet signal. I'm new to working with opamps in schematics and trying to understand the circuits and I'm having trouble tracing to where the problem could be. Any help would be very much appreciated. thanks fellas!

 

[JohnRoberts]

see if the audio signal is making its way into and out of U4,,,, if that's an old reticon CCD they are long obsolete. Confirm the U4 is getting proper clock signals (that is relatively easy and possible to fix).

JR

 

[mjrippe]

Dead Reticon SAD1024 is most likely. I might have some spares...

 

[BrianJSiegel]

Dead Reticon SAD1024 is most likely. I might have some spares...

I was afraid that might be that problem. If you have spares, I will gladly buy them off of you. That would be fantastic!

 

[BrianJSiegel]

see if the audio signal is making its way into and out of U4,,,, if that's an old reticon CCD they are long obsolete. Confirm the U4 is getting proper clock signals (that is relatively easy and possible to fix).

JR

Excellent. I'm on it, I will report back with what I find. Thank you so much

 

[JohnRoberts]

The schematic says CCD, IIRC that was a reticon (5106?) long obsolete CCD (charge coupled device). The 5106 was a sweet part and I used in an a commercial Bozak design, but that part went obsolete in the 80s. SAD 1024 is a BBD (bucket brigade) also obsolete but perhaps findable. There are still chinese BBDs made but different process so circuit would have to be modified to use.

JR

 

[NOON]

I've fixed a couple of Reticon based devices (where I thought the chip might be gone) by tuning the clock signals using the trimpots. There was a very narrow window of combinations where it worked. You could also have dirty or damaged trimpots, look at this first before changing the chip.

 

[JohnRoberts]

I've fixed a couple of Reticon based devices (where I thought the chip might be gone) by tuning the clock signals using the trimpots. There was a very narrow window of combinations where it worked. You could also have dirty or damaged trimpots, look at this first before changing the chip.

There are generally two important trims for BBD devices. A DC bias for the input audio, and an output clock noise cancellation trim. Inside the BBD it only passes one sample per up and down clock, so the BBD repeats that one sample twice. The output trim reduces DC between those two subsequent samples since that DC is at the clock frequency and has to be filtered out.

JR

 

[mjrippe]

Hmmm, U4 is marked "CCD" but that is a 22 pin IC. U5 is a 16 pin with "BBD BAL" written below it. It has been a minute since I was inside one of these flangers, but the SAD1024 is a 16 pin DIP, while the 5106 is 8 pin DIP.

 

[JohnRoberts]

I have never been inside a MXR efx, but I have worked with BBDs and CCD a bunch over the decades. FWIW BBDs are actually charge coupled devices too, so arguably CCDs, but Reticon's family of devices they called CCDs were a different technology with different behavior (actually some pretty nice parts but with a few weird behaviors).

I don't have the interest to dissect that schematic rigorously but it could use a CCD for longer delay (doubling) and BBD for short delay flanging... or not....

You should be able to read part numbers off the devices.

JR

PS; Over a decade ago I shipped about 5-10 pounds of my old BBD/CCD/etc literature and application files to some gearslutz puke who said he would scan them for prosperity. So I can't even look it up. Good luck finding them.

 

[BrianJSiegel]

There are generally two important trims for BBD devices. A DC bias for the input audio, and an output clock noise cancellation trim. Inside the BBD it only passes one sample per up and down clock, so the BBD repeats that one sample twice. The output trim reduces DC between those two subsequent samples since that DC is at the clock frequency and has to be filtered out.

JR

How do I properly set the trimmers? What am I looking for or comparing? There is a BIAS and a BALANCE for both the BBD and CCD - I just don't know how to adjust them correctly
-Brian

 

[beatnik]

The SAD1024 IC is static sensitive mind that when you touch it

It could be other opamps that are dead not necessarily the BBD

As said before make small variations to the BIAS trimpot and listen if the BBD sound comes alive

Before replacing components you should try feed a test tone in the unit and probe the signal with oscilloscope that way you can be sure of the section that needs a component replaced.
The scope would be useful to check clock signals.

 

[JohnRoberts]

How do I properly set the trimmers? What am I looking for or comparing? There is a BIAS and a BALANCE for both the BBD and CCD - I just don't know how to adjust them correctly
-Brian

With a scope look for symmetrical clipping on top and bottom of a waveform. If setting it by ear with a smaller than clipping signal you want to center bias trim between distorting the bottom of the waveform and distorting the top of the waveform. The clock balance trim is adjusted for minimum clock frequency present in the output, by observation with a scope. By ear listen for lowest HF noise floor.

JR

 

[BrianJSiegel]

Had a bad 4016 chip. Unit works great now. Thank you guys for all the help. Much appreciated
-B. Siegel