Deluxe Octave Multiplexer (4 Knob) Muddiness

[drummer4gc]

Hey all, I came across an old Deluxe Octave Multiplexer that was having some issues - fuzz was working, but not the octaves. I was able to get the octaves going again by replacing IC3 in the schematic below. However, it sounds super muddy with the tone turned up. I'll be happy to do all the voltages if that would be helpful, but I audio probed first and hope that it might be easier to get an idea of what is going on by reporting on that.

The signal out of the 3098 is a LOUD square wave octave down, very Nintendo sounding, tracks well. I get one octave and two octaves respectively from the 4013/IC5 section (measured at the -2/-1 Oct pot), but they do sound very muddy. It's hard to find great sound clips from this pedal, but what I've heard suggests that it should be cleaner than it sounds. Can someone help me understand if this section of the circuit is supposed to smooth out the octave sound?

Again, happy to grab voltages or probe more if that would be helpful - just figured this might be an easier place to start than number crunching.

Schematic is a pdf., not totally sure how to post as an image: http://www.dirk-hendrik.com/temp/EH_1325D.pdf (thanks Dirk!)

[Scruffie]

If everything seems to be working apart from muddyness, I would suggest just to rule it out and also to give the pedal a 'service' that you try replacing all the electrolytic and tantalum capacitors, no it's not very scientific debugging but the pedal is nearly 40 years old, it's quite likely some of them have gone a bit dry and once you've done it you'll at least know one wont go bad down the line even if it doesn't solve the issue.

However, posting all the IC & Transistor/FET voltages can't hurt but do be careful probing pin 5 of the CA3094 and pin 2 of the CA3098 as they're sensitive and replacements are very hard to source.

Also the NE5554 do tend to go bad over time from heat so it may be wise to attach a heat sync.

[drummer4gc]

I appreciate the advice - a recap is definitely on the list of to dos, but right now I'm equally interested in the "why is it doing this" as I am in the "how do I fix it".

I've read this thread a number of times - http://www.diystompboxes.com/smfforum/index.php?topic=52725.0 . What I get from it is that the circuit works similarly to the Boss OC2 and creates an octave sound by converting the input signal into a square wave, dividing it, and then using that to manipulate the original signal. Through probing, I definitely get the divided square wave, and the clean signal coming from the input buffer sounds fine, but it's a buzzy mess at the oct1/oct2 pot. I guess I'm making an assumption that it should be much cleaner here - by looking at the schematic, can anyone venture a guess as to how the signal should sound after the 4013/FET section? Should it sound similar to the Boss OC2 octave?

If it helps, voltages:

IC1
1: 0
2: 0
3: 0
4: -5.2
5: 0
6: 0
7: 0
8: 5.3

IC2
1: 0
2: 0
3: 0
4: -5.2
5: 0
6: 0
7: 0
8: 5.3

IC3
1: 0
2: 0
3: 0
4: -5.2
5: 0
6: 0
7: -3.72 (is this odd?)
8: 5.3

IC4
1: 0
2: 0
3: 0
4: -5.2
5: 0
6: 0
7: 0.12 (another weird pin 7?)
8: 5.3

IC5
1: 0
2: 0
3: 0
4: -5.2
5: 0
6: 0
7: 0
8: 5.3

IC6
1: 0
2: 0
3: 0
4: -5.2
5: 0
6: 0
7: 0
8: 5.3

IC7 (there is not an R6 trimmer or R72 on my board as in the schematic)
1: -.08
2: -.02
3: -.02
4: -5.2
5: -.02
6: -.02
7: -.08
8: 5.3

IC8 CA3094
1: -3.96
2: 0
3: 0
4: -5.2
5: -4.61
6: -5.2
7: 5.3
8: -.02

IC9 CA3094
1: -4.02
2: 0
3: 0
4: -5.2
5: -4.61
6: -5.2
7: 5.3
8: .04

IC10 doesn't exist


IC11
1: 18.98
2: 0
3: 0
4: -18.28
5: 0
6: 0
7: 0
8: 0
9: 0
10: 0
11: -5.21
12: -5.21
13: 0
14: 5.34


U1 4013
1: 5.3
2: -5.2
3: 5.3
4: -5.2
5: -5.2
6: -5.2
7: -5.2
8: -5.2
9: -5.2
10: -5.2
11: 5.3
12: -5.2
13: 5.3
14: 5.3

U2 CA3098
1: -.02
2: -4.6
3: 5.35
4: -5.2
5: 5.24
6: 5.35
7: -.02
8: -.02

[StephenGiles]

You can test if the front end is working OK (similar to the EH Rack Guitar synth) by listening to the audio at pin 8 of IC 9, where you should hear a perfect sine wave. IC7 and U2 square and condition the signal for octave dropping by the 4013, so you can audio test it along the way.

[drummer4gc]

Thanks Stephen. I don't think I'm getting a perfect sine wave at pin 8 of IC9 - sounds very muffled, like a note played with the tone knob rolled all the way off.

The input signal to IC8 doesn't sound right to me either. I gather the section before the 3094s is for compression - if that's right, it's definitely working. The signal out of IC1b is amplified and compressed. Just gets weird at the 3094s. Any way to test if they're shot? Unfortunately they're a bit pricey to just purchase and swap, id like to be more sure.

Thanks!

[StephenGiles]

"Thanks Stephen. I don't think I'm getting a perfect sine wave at pin 8 of IC9 - sounds very muffled, like a note played with the tone knob rolled all the way off. "

It should sound muffled like a note played with the tone knob rolled all the way off. What you are getting is the Fundamental of the note played, which is formed by the 3094s in an Adaptive Filter configuration.

[drummer4gc]

Oh! Then I guess all is good there.

Can you help me understand then what the signal should sound like coming out of the 4013, and what the FET/IC5 section then does? I'm assuming it's in here somewhere that I should be getting a more pure signal than what I'm getting, so I'm wondering if there is something to address in this part of the circuit.

Thanks so much to everyone that's contributed something so far!

[StephenGiles]

Oh! Then I guess all is good there.

Can you help me understand then what the signal should sound like coming out of the 4013, and what the FET/IC5 section then does? I'm assuming it's in here somewhere that I should be getting a more pure signal than what I'm getting, so I'm wondering if there is something to address in this part of the circuit.

Thanks so much to everyone that's contributed something so far!

Isn't it just an impedence changer buffer ?

[slacker]

The outputs of the 4013s should be square waves 1 and 2 octaves below the note you're playing. The one octave down fet and IC5 stage flip the phase of the input signal to give an output that looks a like a one octave lower lower version of the input. There's a rough explanation of how it works here http://www.valvewizard.co.uk/uboat.html. I'm not sure how the two octave down part works, I can't figure out in my head how it chops up the input into something that looks like it's two octaves lower.

[StephenGiles]

Does it sound like it's 2 octaves lower?

[drummer4gc]

Does it sound like it's 2 octaves lower?

Definitely. To me it looks like the first 4013 section divides the signal and outputs an octave down, which feeds the first FET/IC5b as well as the second section of the 4013, which divides is again for the second octave before sending it off to IC5a. But I'm teetering on the edge of my understanding here...

The one octave down fet and IC5 stage flip the phase of the input signal to give an output that looks a like a one octave lower lower version of the input. There's a rough explanation of how it works here http://www.valvewizard.co.uk/uboat.html.

This is super helpful - the visual of the "Boss method" definitely helps me understand how it's working to affect the input signal. In previous posts, I've read that the phase switching method does add some grit to the input, but I'm assuming this should be minimal, which it certainly isn't in my case. Time to replace IC5? or the FETs?

[drummer4gc]

IC5 swap didn't help. Still sounds buzzy/farty, and the tracking is pretty poor too.

Am I overthinking this and expecting too much?

[StephenGiles]

If the tracking is poor, then I'd put money on a problem with the adaptive filter. My reason is that when I built the EH Rack Guitar Synth which includes this filter in the signal conditioning, the tracking was perfect.

[Scruffie]

If the tracking is poor, then I'd put money on a problem with the adaptive filter. My reason is that when I built the EH Rack Guitar Synth which includes this filter in the signal conditioning, the tracking was perfect.

Look again at the schematic for this and then the guitar synth, this doesn't use the same fundamental extractor as the 5 knob version and guitar synth.

[StephenGiles]

The compressor is different, the adaptive filter is the same - just drawn double Dutch, christ knows why - shame to spoil the original work of art!!! and the IC7/CA3098 network is an earlier version of what the 311 does.

Ah ha - just found my EH Delux OM circuit as drawn above dated 2/1/78, so it was not drawn upside down - the original EH one was!!

[Scruffie]

Similar topology yes, but I wouldn't expect them to operate exactly the same way, especially without the 'muting' circuitry.

A quick google of some reviews show this unit is called 'gurgley' by people so it may well be functioning normally.

[StephenGiles]

Ah the TL601 muting circuitry around the 4098, I found that a bit of a bugger to get working - a few 4am bed times if I remember back in the 1980's!

[drummer4gc]

Here's a vid to share some of what I hear. You'll probably need speakers/headphones to hear it well enough. Forgive the cell phone video and guitar "playing."

It starts out clean, then demonstrates the best setting I can find. Not bad, definitely some gurgling on the D string, not much effect on higher strings. Some odd tracking, although I'm not sure how much to expect. As I adjust the settings though, check out the heavily distorted tone when the Tone knob is wide open. This is what worries me.

[StephenGiles]

That's working fine.

[drummer4gc]

Dang. For real? Even with that growl when the tone knob is full on?

I'm sorry if anyone feels like I wasted their time, but I really appreciate the replies and engagement - I learned a lot about how the particular circuit works and how similar effects are achieved in other circuits. I guess this will just provide a reference for the next person that comes around with a non-functioning (or perfectly fine...) DOM to check out!

Thanks again!