MXR 118 Weak Delay Signal

[DrT]

Hi Everybody,

I've purchased a nonfunctional MXR 118-3001-104, not the SAD chip, has a second delay only output.  The issue I am having is that the delay signal is very weak.  It can be heard in the delay only output or with the mix turned fully clockwise.  Tracing the audio signal back from the output, I found a strong clean signal on pin 7 of U3 (4558 op-amp) and a weak signal on pin 3 of the same.  I thought it might be the chip, but checking voltages makes me think it is a resistor in the voltage divider.

I've had trouble finding a legible schematic.  I've been working off a few found here:
https://music-electronics-forum.com/showthread.php?t=4059&styleid=13

The power section appears to match the Ross schematic, but other than basic layout, I can't get much from that document.  The schem2.png file is a little more readable and appears to have the correct ICs, but the power section doesn't appear to match.  I've attached the schematics with voltage readings on the op-amps for reference.

In all cases, I am getting close to 12 volts on pin 8, but only 2.2 volts for the bias.  I'm assuming that the bias voltage should be 6 volts.  The issue I am having is finding the resistors used in the voltage divider.  The Ross schematic appears to use 470k resistors for this purpose, but the other one uses 10k.  I've found three 470k resistors on the board.  I get the 2.2 volts on one side of each but I can't trace any to ground.

Does anyone have ideas on where I should look.  I've also included a gut shot.

Thanks,

DrT

[Fender3D]

Bias resistors should be R30 and R36, 10Keach, I don't think they're the issue though... I would check the tantalum connected there C25.
Parts are just above the footswitch in your picture.

[ElectricDruid]

Personally, I'd be suspicious of the CD4016 switches used as PWM resistors in the pre- and post-delay filtering. Those are not the most reliable chips long-term and these ones have spent their entire lives switching at 10s of KHz all day every day. Tiring!

If you've got a scope, check the on/off switching clock signals going to them, but check the switches themselves too. If they're socketed, socket corrosion is a possibility. If they are soldered, switch failure is possible. If any of those went south, the delay either doesn't get all the signal it should, or the signal it produces doesn't reach the mixer as it should.

Disclaimer: This is just a hunch based on experience with old synth circuits where there are a lot of those switches. I have no other reason for supposing that's the cause, and there are other possible causes

[DrT]

Fender3D, thanks for the help.  There is a short across R30.  I will see if I have a replacement cap in my supply and update once the new one is in.

DrT

[Mark Hammer]

I vote for replacing the 4016 chips.  The design used a very clever tracking filter. 

You can see that there are a pair of 2-pole lowpass filters before and after the BBD.  You can also see two little grey blocks - one per 4016 switch - in each of those 2-pole filters.  The 4016 switches are slaved to the master BBD clock, but divided down.  As the delay time is shortened, that master clock goes faster.  The clock steps the audio signal through the BBD, but it also turns the 4016 CMOS switches on and off, at a rate proportional to the delay time.

Each clock pulse from the clock is divided down by a 4013 flip-flop and turns the 4016 switches on and off.  The average resistance of the switch section and adjacent fixed resistor gets lower, the faster the master clock is running.  That has the net effect of changing the corner frequency of the 2-pole lowpass filter section.

Many analog delays would use a fixed filter scheme, aiming for as much delay bandwidth as could be tolerated at the longest delay time.  So if the clock driving the BBD was well within the hearing range at the longest delay time, and quite audible in the absence of steep filtering, then the unit would filter out hi-freq content to make that clock whine inaudible, without sacrificing too much mids and highs.  Some would, however, inevitably be sacrificed.  Ironically, it is the "dullness" of analog delay that often makes it so highly prized.

The genius of the MXR/Ross approach was that the tracking filter provided as much bandwidth as could be tolerated at any given delay time, offering up more bandwidth at short delays and less at longer ones....automatically.

Why am I telling you this?  Because as you can plainly see, if any one of those CMOS switches is fried or otherwise dysfunctional, you simply won't hear much wet signal at all.  Those switches are brilliant, but they are also a bottleneck.  Install new ones and you should, in theory, be okay.

[DrT]

Hi Mark,

Thanks for your insights.  I've read your previous post regarding the 4016 chips on this pedal.  Before I go through the trouble of replacing those chips, I would like to ensure that the power section is properly sorted out.  I replace the 15uf C25 cap with a 22uf I had available and also replaced both 10k resistors.  I am still getting a 2.2v bias voltage.  It doesn't make sense to me that I was getting a 2.2 bias voltage when C25 was reading as a short and I get the same voltage now that C25 isn't reading as a short.  Maybe I was shorting out my multimeter leads on another part of the circuit and didn't notice.  With the new cap and resistors I am no longer getting a signal out of the delay only output. 

Would bad 4016 chips cause the bias voltage to be off?

Thanks,

DrT

[DrT]

Here is an update after doing some audio probing this evening.  It appears that Mark is right, I can find a good audio signal through the whole circuit up to pin 8 on A4.  It looks like the very last CMOS switch in the chain is the bad actor. 

Would this TI CD4016BE be a suitable replacement?
https://www.ti.com/store/ti/en/p/product/?p=CD4016BE

DrT

[Slowpoke101]

Would this TI CD4016BE be a suitable replacement?
https://www.ti.com/store/ti/en/p/product/?p=CD4016BE

DrT

It should be fine.

[DrT]

So I replaced the 4016 chips.  After replacing A4, I could get a really good mix of clean and delay signal, as long as I used short delay times.  At longer times, the pedal would just produce a high pitch squeal.  Thinking it might have something to do with the filtering done by the other 4016, I replaced it as well.  The result was that the delay was a bit weaker and the squealing was still there.  Would any of the trim pots needed to be adjusted for the new 4016 chips.

Also, after fidgeting with the knobs for a bit, I lost all signal.  Clean, wet, on, off, all gone.  I think that is a separate issue.  I didn't have time this evening to go back through with the multimeter or sound probe.  I will update once I have a chance.

[Mark Hammer]

Change to the CMOS switch chips should not, in principle, require any re-adjustment of the trimmers, since they do not provide anything that needs to be compensated for.

Can we confirm that all new chips were inserted at the proper orientation and that no pins were unintentionally bent under the chip, rather than fitting properly into the socket?  I've had to grapple with that it-certainly-LOOKS-right error too many times myself.

RE: whine
There are several things one needs to do to minimize audible whine.  The whine is heard when the clock is running too slow for the filtering to keep out, and when the balance of the two interleaved outputs of the BBD - what I like to call "tick" and "tock" - are not equal.  The two BBD outputs and balance control function in a manner more or less analogous to a humbucker pickup.  When the two outputs are perfectly balanced, the clock whine ("hum") is effectively cancelled for the most part.

There is also the "clock" trimmer, which is used to set the maximum delay time.  Adjust the Balance trimmer to achieve as much whine-reduction as you can.  Set the panel control for longest delay, and tweak the clock trimmer to gradually reduce that maximum to make the whine barely audible or even inaudible.

[DrT]

Thanks Mark.  It seems pretty obvious that I should have thought of the balance trimmer after replacing the first chip.  I remember noticing that humbucker effect when I was doing my initial audio probing, I just got too excited and didn't think it all through.  I think both chips are installed correctly, but I will recheck this evening.  I'm hoping I didn't overheat some other components while removing and installing the second chip.

[DrT]

After adjusting the trim pots for the balance and clock I could get most of the whine out. It is barley auditable at the highest delay time, but I think that is as good as I can ask for.  Thanks for the help.

[Mark Hammer]

Glad it's working out.