Vintage MXR Flanger that makes a metronome noise

[docSanchez]

So I have an old MXR Flanger with the 3001-102 PCB, so it's from '77 or around then; grey box. I plug it in and this is what happens:

All of the following symptoms show up whether the pedal is bypassed or not. When the pedal is engaged, the volume drops a little, but behaves the same.

- I can hear the guitar, but it is pretty quiet.
- Lots of noise and fuzz
- A beat. It goes around 3 to 4 beats per second at it's fastest and when it beats the guitar and noise go away. The beat it louder than either of the other two.
- If the Manual knob is turned all the way down, the pedal goes silent.
- The Regen knob acts like a tone control, like an LPF. With it all the way down everything is very muted and dark.
- If the Width knob is turned all the way down, the pedal goes silent.
- The Speed knob does what it's expected to do; it controls the speed, or rate, of the beats. It maxes out at about 3 or 4 beats per second and all the way down it is something like 1 beat per minute. Sounds like a freakin' metronome.

What I know:
- The power supply is fine. All power is at the correct voltages.
- The switch is fine.
- When I assemble/disassemble the pedal, the symptoms sometimes change.
- I measured the voltage coming out from the effect, and it was about 9.8v DC. I can't remember if it was negative or not.

So my thoughts: (for what it's worth)
- The clock/timing chip is working. Woo.
- Something is wrong with the grounding (the switch tested fine, so the fact that the problem exists even with the pedal disengaged means something with the grounding is wrong)
- The bucket brigade chip may be toast/having issues.

Unfortunately, the delay chip is a Reticon SAD 1024, which I understand is very, very difficult to come by these days. Not only that, there is no pin for pin replacement that will cut it.



Note: this schematic is for the 4001-102 series. The polarity of all the major caps is opposite. Also, the voltage regulator looks like a regular little transistor; not one of those with a heat sink hole as pictured in the layout. There may be other differences; but almost everything is the same.

I have an idea, but I wanted to run it by somebody with a little more know-how and experience before I tried it, for risk of damaging something. The delay chip is socketed; could I confirm a delay chip problem by removing the delay chip testing whether it continues to make it's annoying metronome?

So... anybody out there?

P.S. Thanks in advance, everyone I've met on this forum is the best. I greatly appreciate your help. To say the least, learning this way beats school :P

[cjlectronics]

I've seen this problem and the SAD1024 can cause this. With all of the testing you've done, I'd say the BBD is the problem. You can pull the BBD an run it that way but you won't get any effect... just the blended clean signal.

I knew there would come a day when the SAD1024's would dry up. Looks like that day has come and gone. I wonder if its worth trying to retrofit an A/D - D/A arrangement using a FIFO or memory chip to store and delay the data? I've seen some systems on a chip ( I think Maxim has one) which would make the A/D-D/A conversion easy. Now, cramming all of that into a space the size of a SAD1024 is the real trick.

CJ

[docSanchez]

OK, I can see how a defunct SAD1024 would be an issue, but how does that affect the clean/bypassed signal? Why won't my pedal bypass? Is it just the clock ticking through the ground?

[docSanchez]

What I mean is, you said pulling the BBD would leave me with a blended clean signal. What do you mean 'blended'? I'm certainly not getting a clean signal out of the pedal.

[Jazz P Bass]

The first thing is why will it not bypass.
I would remove the very first cap C16 & see if the bypass works.
Unless the schematic is incorrect, S1 should take the whole circuit out when bypassed.
If as you said, it is a grounding issue, removing C16 will be a good start.
Start at the first ic & look for any Vdc on the output pins (1 & 7)
Then try to pickout where in the circuit the tick starts.

[cjlectronics]

What is the voltage at U2 pin 5? Should be 7.5vdc
Do you have a clock signal at U3 pins 1 and 2?
Is there a triangular wave at U5 pin7? Does the frequency change as you adjust the speed control?

[docSanchez]

So I removed C16 and did indeed get a perfectly clean bypassed signal, and no signal at all with the pedal engaged.

As for the ICs, I took the following measurements:

IC1 pin 1: 7.63Vdc
IC1 pin 7: 7.63Vdc

IC 2 pin 5: 7.30Vdc

With Width control all the way Down:
IC3 pin 1: 7.67Vdc
IC3 pin 2: 7.67Vdc

With Width control all the way Up:
IC3 pin 1: 6.2V - 7.67V
IC3 pin 2: 6.2V - 7.67V
(Both pins appear to put out a triangle wave with occasional spikes up to 8.2V; I am unsure if this is indeed a triangle wave or if the clock ticks are just too fast for my multimeter. Turning the speed knob up results in more stable readings: holding around 7.4V, which leads me to believe my multimeter just can't keep up.)

IC 5 pin 7: 2V - 12.7V triangle wave: the frequency of the wave adjusts with the speed knob as expected.

So... I am no good at ICs so I don't know what this means. I should also note that I performed all these tests with the BBD (and C16) removed.

[cjlectronics]

OK, those voltages sound OK for the clock signal.

Can you verify that it is wired correctly, i.e. all of the jacks are wired properly as well as the bypass switch and potentiometers?

[docSanchez]

Well, now that I can cleanly bypass the pedal, I assume the jacks are wired properly. However, I can double-check that in a little bit; mostly the grounding of the jacks, but especially how they ground to the chassis. The bypass switch and pots are all good to go.