Vintage MXR Analog Delay (SAD1024 version) trimpots

[disorder]

I have a vintage MXR Analog Delay here, the big green one with SAD1024 chip and only a single output.
It turns on and delays still but it could use a proper setup. Right now the max delay setting seems too long (well over 300mS) and setting the feedback pot anywhere past half way leads to runaway delay.

https://imgur.com/a/P3tyP3l

Does anyone have a proper schematic for this or know what each of the trimpots do? I have a signal generator and a scope.

[Scruffie]

One schematic https://hosting.photobucket.com/albums/v474/mhammer/mxr_analogdelay.png

R51 controls the clock frequency and thus delay time, test point at pin 8 of the 4069, 5kHz will give 300mS

R25 controls the wet frequency level, that's the only point of control for the feedback.

[disorder]

Thank you very much! R44 trimmer seems to be a bias or cancel of sorts for the BBD line? and do you know what R50 is supposed to adjust?

[Scruffie]

R50 adjusts the PWM output that controls the 4016 'resistors' that adjust the filter cut off points in conjunction with delay time. Problem with that control, is knowing where it should be set, which I don't precisely... so I'd leave it be.

R44 is the bias for all 3 BBD's and R38 (I think, hard to read) is the clock balance/cancel trim.

And before you ask, R10 & 11 are for fine tuning the input filters which would work in conjunction with the PWM setting.

Without a proper set up procedure, a lot of the adjustments would have to be educated guess work.

[Mark Hammer]

R10/11 can be set to max resistance, for starters, to reduce the bandwidth of the wet-input signal, and can be dialed back after other adjustments are made.  This will let you hear the clock noise more easily.  The 2k trimmer on the output of the last SAD1024 half (can't read the number clearly) sets the balance between the two complementary paths, and is set so as to minimize the audibility of the clock.

R44 is set like the bias control on any BBD circuit.  There will be an optimal bias voltage setting.  A bit higher and lower than that will be a range where the delay is distorted and at a slightly lower volume.  HIgher and lower than that you won't hear any delay at all.  Being able to set the Mix to all wet will make adjusting R44 for clearest delay sound easier.  R25 sets the overall wet level.  Set Regeneration to max, Mix to 100% wet, and R25 to min, then adjust R25 until the howling stops.

Sadly, I have no advice for setting R51 and R50 optimally.

[Scruffie]

R51 controls the clock frequency and thus delay time, test point at pin 8 of the 4069, 5kHz will give 300mS

Ignore that, I forgot that it gets divided 4 times afterwards! 80kHz

R10/11 can be set to max resistance, for starters, to reduce the bandwidth of the wet-input signal, and can be dialed back after other adjustments are made.  This will let you hear the clock noise more easily. 

Surely you mean minimum if you want to hear the clock noise more easily  Think you've given mistaken advice like I just did 

R25 sets the overall wet level.  Set Regeneration to max, Mix to 100% wet, and R25 to min, then adjust R25 until the howling stops.

Nice idea in principle but then the wet & dry levels might be unbalanced. That control is for setting the wet level to match the dry level, the feedback should then automatically be at the intended factory level.

[Mark Hammer]

Nope, I meant that turning the trimmers up brings the rolloff lower, which ought to allow the whine of the clock to be heard more easily, instead of being inadvertently masked by the treble in the audio.  At least, that's the theory I'm working on.

[disorder]

Nope, I meant that turning the trimmers up brings the rolloff lower, which ought to allow the whine of the clock to be heard more easily, instead of being inadvertently masked by the treble in the audio.  At least, that's the theory I'm working on.

This is how the unit behaves... trimmer clockwise = clock is more audible.

R51 controls the clock frequency and thus delay time, test point at pin 8 of the 4069, 5kHz will give 300mS

Ignore that, I forgot that it gets divided 4 times afterwards! 80kHz

R51 gets me 500kHz to 1.2MHz on pin 8 of the 4069. From what I can tell 800kHz get's me close to 300mS. Will need to investigate further tomorrow.

[Scruffie]

Nope, I meant that turning the trimmers up brings the rolloff lower, which ought to allow the whine of the clock to be heard more easily, instead of being inadvertently masked by the treble in the audio.  At least, that's the theory I'm working on.

Ahhh, I get ya, to try and isolate it from general noise.

R51 controls the clock frequency and thus delay time, test point at pin 8 of the 4069, 5kHz will give 300mS

Ignore that, I forgot that it gets divided 4 times afterwards! 80kHz

R51 gets me 500kHz to 1.2MHz on pin 8 of the 4069. From what I can tell 800kHz get's me close to 300mS. Will need to investigate further tomorrow.

I was really not on the ball yesterday, you have to set the delay time control to max to set it to 80kHz. Assuming the pin numbers match up to the schematic that is.

[disorder]

So thanks to Scruffie and Mark I got this pedal running pretty well.

Max delay is right around 300mS, it's not too noisy.
Still don't know how the two filter trimpots and the PWM trimpot should be set. But they don't seem to have a huge change on the tone.

I have one lingering issue however... when DELAY TIME is set to minimum I get a bad hum. Sounds like a DC offset or ground hum noise.
If I bring DELAY TIME  up just a touch it goes away. Any ideas what might be causing this?

[Mark Hammer]

Weird.  Usually analog delays are on their best behaviour when the delay time is at its shortest, and misbehave at longest delays.

[Rob Strand]

I have one lingering issue however... when DELAY TIME is set to minimum I get a bad hum. Sounds like a DC offset or ground hum noise.
If I bring DELAY TIME  up just a touch it goes away. Any ideas what might be causing this?

Maybe your supply filter caps (or some other DC filter caps) are on their way out?    High frequency operation has a tendency to increase the current draw   More current draw means more supply ripple and that means more hum.

[Scruffie]

I have one lingering issue however... when DELAY TIME is set to minimum I get a bad hum. Sounds like a DC offset or ground hum noise.
If I bring DELAY TIME  up just a touch it goes away. Any ideas what might be causing this?

Maybe your supply filter caps (or some other DC filter caps) are on their way out?    High frequency operation has a tendency to increase the current draw   More current draw means more supply ripple and that means more hum.

That's what I'd have gone with.

I think unsurprisingly, as we get further from the 70's we might have to start giving cap replacement as default advice for complex effects.