Total Recall - Odd voltages on 4558 ICs

[zachlovescoffee]

Hello team MBP,

I'm in the middle of testing out my voltages on the ICs before I rock my box. I stated off with the 4558 ICs since there were so many of them. I'm not even sure I am taking these readings correctly. What I've been doing is putting my red probe on pin 4 and using my black probe to read across each pin. I.e., Pin 1 connected to Pin 4 gives me Xvdc.

My voltages seem to be all over the place on some of these ICs. IC1, IC4, IC5 all appear to be okay. IC3, IC9 are wozzed up.

IC3 P1: -9.3vdc
IC3 P2: -12.6vdc
IC3 P3: -12.8vdc
IC3 P4: -14.8vdc
5,6,7 all fine. pin 8 reads 0vdc to ground.

IC9 P1: Varies
IC9 P2: 7-8.6vdc
IC9 P3: varies
IC9 P4: -15vdc
IC9 P5: varies (7-9)
IC9 P6: varies (8-9)
IC9 P7: 8-9vdc

I could totally be doing these readings incorrectly. But any help would be appreciated. I did try to google the heck out of how to measure and test 4558s but all of the videos are in Hindi ...

[jimilee]

Welcome! To measure 4558s or any other passive, put the black prone on any ground point and measure with the red probe.


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[thomasha]

As said, you want the black probe at ground level. I use the input jack ground for readings.

Your readings are strange indeed, if you did if from pin 4 to pin x, why is IC3P4 -14,8vdc?  Shouldn't it be  p4 to p4=0vdc?

The 4558 is a normal dual opamp, so the readings are done as for a normal OpAmp.
Check a video of how to read circuit voltages and it will be the same. I guess the extra load of the voltmeter is negligible here.

[zachlovescoffee]

I measured the voltages again this time grounding my black probe to the ground lug of the jack. Now I have verified all of my voltages are within 5% of what's on the spec sheet provided by MadBean.

I tried hooking up my oscilliscope (Rigol 1052E, hacked to 100Mhz) but I have no idea what I'm looking for on the screen to measure the BBDs and timing chips. So I tried to just plug the pedal into my amp and all it produces is a loud hum, no signal. I may have hooked up the jacks wrong as they are stereo jacks and I always get the wiring backwards.

[thomasha]

You want to look at the signal after the BBD and see if the BBD is correctly biased (there is signal coming through, and the signal is symmetrical, and not distorted on one side).

You will need a signal generator, that produces a sine wave or strum your guitar while measuring. With a sine wave it is easier, because the wave is symmetric and periodic. Try 0.25V at 600Hz.

Ground probe of the oscilloscope should go on ground and the other probe goes at the first BBD input. If you get signal at the input, it is working until this point. Then, check if there is signal at the output of the BBD. It will have lots of clock noise. If there is no signal adjust the bias trimmer until the signal is visible. You will have to do the same for the next stage.

The gain trimmer is used to get unity gain between stages. 

[zachlovescoffee]

Ahhhh! Okay so I don't have a signal generator (yet) so I'll have to use my guitar. Here's to hoping it works! Will report back.

[Bio77]

There are app based signal generators that work well.  I use Function Generator PRO on my phone.  You can build a 1/8 stereo to 1/4 mono converter out of an old set of headphones.  If you have kids, you defiantly have a broken set somewhere 

[thomasha]

Yes, I like to use audacity (pc).
You can define the kind of noise or signal, freq., intensity and so on. You just need a cable that goes from your computer's audio output to the pedal input.

[zachlovescoffee]

Great idea! What would be a good signal generator to purchase for pedal and amp work like this? I have a Rigol 1052e hacked to 100Mhz.

[jimilee]

Ditto looper works fantastic.


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[zachlovescoffee]

I dunno if my scope is jacked or the pedal is jacked? When I put a signal from the looper across the input of the pedal and measure at pin 1 of IC6 I get nothing but junk on the scope. Any suggestions for what I'm doing wrong?

I have tried it set on AC and DC. I have 10x prob on. I'm using Channel 1. I've tried auto and manual capture. I messed with horizontal and vertical and trigger. I've adjusted the bias to max or min and I'm getting squat.

Also, maybe I wired my jacks incorrectly? Could someone take a peek?

[thomasha]

Ok, pin 1 of IC6 (BBD) is ground=0V.
The input of the BBD is pin 7, while the outputs are pins 3 and 4 (take a look at the schematic).

I am guessing here, that you already checked the voltages and the problem is with the continuity of the signal path.

If you are not sure how to use the oscilloscope, try using an audio probe first, you can easily build one yourself:

In my opinion, you need to know at which frequency you are looking at when using the oscilloscope, otherwise you could be checking it at a wrong configuration and it will be barely visible. The audio probe will give you an easy way to assess if the signal reaching the ICs.

Then play some music or use your looper at the input of the pedal and touch some points along the signal chain to check if you have signal, it will play through your amp if there is something. That will also help sorting out any problem with the wiring.

The input is the left jack shown in the drawing (j1, orange or green wires). Check the signal at every IC input and output:
IC    input output
IC1A:   2        1
IC1B:   5        7
IC2A:   6        7
IC3A:   3        1
IC6:     7        3 or 4
IC4B:   5        7
IC7:     7        3 or 4
IC4A:   2        1
IC3B:   5        7
IC5A:   3        1
IC2B:14/15    10/11
IC5B:   5        7

And check at both sides of the blend pot. With music or the looper playing, if there is signal you will hear it and you can move to the next point. The second half of the compander has 2 inputs and outputs. Check both of them.  Let me know where the signal vanishes and look around that point if you can find a wrong resistor/capacitor or a large solder blob (colder joint). You can check for colder joints with your multimeter (continuity or diode mode).

If there is no signal at the output of your IC it could be a bad IC or a problem with one of the resistors/capacitors that sets the gain of the stage.

[jimilee]

Looks like the jacks are wired wrong. Use you DMM to fund out which is the sleeve and which is the tip.


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[zachlovescoffee]

I've built an audio probe but am putting this one on the shelf for a week or two. I had a back log of pedals that were were ready be boxed up and decided I needed some time away from the DMM. I just finished a King of Tone clone with original MA858 and 1S1588 diodes and it sounds great. And I'm in the process of finishing a Univibe clone.

Once I get those two done I'll put a pin on building anything new so I can focus on the DMM and I'll resurrect this thread in a week or two if I run into trouble. Thank you for all of your help!

[Bio77]

I think for scope work, you want to get the app based signal generator or a computer based one.  A sine wave is much more informative than a guitar signal on the scope, and as Thomas said, you will want to set the frequency.  The "Function Generator PRO" I recommended, is a few dollars and IMO well worth it. 

To set up your scope, first thing is always to dial in on the input signal, right at the tip of your  input patch cable.  I believe the Rigol will have an AUTO button.  When you press that it should put the scope in a good freq and voltage ballpark, then you can fine tune from there.