MXR Series 2000 model 205 Stereo Chorus

[armillary]

Hi friends. I got an inoperative MXR 2000 series Stereo Chorus (model 205) pedal to learn from and fix.

I believe this was the last of the original MXR products before they went out of business in 1984. It has the heavy cast metal black case. It has labeled interface pins on the PCB which aren't used by the pedal, but I suspect these 2000 series PCBs might have been used as pluggable modules in the MXR Omni rack system. Anyway it's a shame MXR went out of business back then.

My pedal powers up and passes clean dry signal whether engaged or disengaged. It has a red LED for power and a separate green LED for engaged. The LEDs work and the footswitch switches between them. Changing the controls doesn't change the sound. (I read that the depth knob controls the wet/dry mix.) So the problem is, no wet signal.   
 
Here are the major components on the PCB:
NE571N (1) Compander
TL061 (1) JFET Input Op Amp
SCL4103BE (1) Dual D Flip Flop (triggers to SAD4096)
Reticon SAD4096 (1) BBD for chorus.
MC4558P (2) Dual Op Amps 
MC1458P (1) Single Op Amp
CD4007CN (1) Dual Complementary Pair Plus Inverter
2N5088 (5) Transistors Q1, Q2, Q3, Q6, Q7 
2N4126 (1) Transistor Q5
J113 (1) JFET N-Channel Switch Q4 for momentary ground foot switch. No mechanical switch.       

I can post pictures if anyone wants to see.

With a 9.03V, 500mA power supply applied to the battery contacts and the pedal engaged with signal input, the battery contacts read 9.03V.

The Reticon chips have a reputation for going bad, so I started measuring there:
Pin 10 (Vdd) of SAD4096, 8.3V
Pin 3 (Vbb) of SAD4096, 8.3V
Pin 14 (Input) of SAD4096 has very loud audible dry signal.
Pins 8 & 9 (Output) of SAD4096 have very soft audible signal (~50mV). On the scope it shows a sine/triangle shape and responds somewhat to changes in Speed and Depth controls.
Pin 16 (clock 2) has squared sine wave of +4V to -4V that reacts to Speed and Depth knob settings. 
Pin 2 (clock 1) has the same signal as pin 16.

So I'm hopeful the SAD4096 and the clocking circuit is OK, assuming the weak output is normal.   

Is the drop from 9V to 8.3V sign of a possible problem? 
Measuring the power pin of the other chips, they all read 8.3V too, except for the TL061. The TL061 reads 5.3V. Its output is an alternating positive and negative voltage that goes to zero. It reacts to the Speed control.   

Moving on to the expander side of the NE571, there's no input signal at pin 14 or output signal at 10 or 11. Between the SAD4096 and the NE571 is where the
wet signal likely stops. I've looked for a schematic but can't find one. Anybody have one?

I'm thinking this schematic is probably pretty close, in terms of the SAD4096 to NE571 path (and also the DOD 680 schematic):
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/mxr_time_delay_175.jpg.html?g2_imageViewsIndex=1

With no schematic and the double-sided PCB I'll have to up my game to trace this. Any pointers for tracing undocumented circuits?   

I can see the output of the SAD4096 goes to the base of a 2N5088. I'll follow the Troubleshooting page and start measuring BCE voltages.

And there's a 4.7 uF electrolytic at the input pins to the NE571. I replaced it, but no difference. The old one measured OK.   

Comments appreciated. I don't know much, but I'm learning!

[Mark Hammer]

A 4096-stage BBD for chorus?  Talk about overkill!
Actually, the only other chorus design I've ever seen that uses an SAD4096 for chorus is the Anderton Chorus shown in this document:  http://hammer.ampage.org/files/Anderflange1.PDF  although, to be fair, Craig also endowed the circuit with the capacity to provide up to 100msec delay.

My instinct is to first check the 4013 to see if it is okay.  The other CMOS chip - the 4007 - is likely used for the switch, and by all evidence provided in your post, it is 100% healthy.  So the question is whether the last link between the HF clock and the BBD is sending a viable clock pulse.

[armillary]

Thanks Mark! I'll look closer at the 4013, but the clock pulses appearing at pins 1 and 16 of the SAD4096 do swing 8V and are pretty square.   

I agree the SAD4096 is overkill for a chorus, and who would try to use one in a 9V application? And who would be using a Reticon SAD chip in 1983? But MXR and Reticon were both out of business by 1984, so who knows.             

[armillary]

Today I swapped the questionable SAD4096 from the MXR chorus with a known good SAD4096 and a known good DOD 680 delay.

The MXR's SAD4096 wouldn't make the DOD 680 work. I tried several bias settings.

And the DOD 680's SAD4096 wouldn't make the MXR chorus work.

So the MXR chorus has a bad SAD4096 and some other problem. Maybe I can dead bug a MN3205 in it.   

Fortunately, the DOD 680 still worked when I put its original SAD4096 back in. So no harm done.

[Mark Hammer]

That makes me SAD.