Total Recall - Odd voltages on 4558 ICs

[zachlovescoffee]

I'll for sure get the app!

So, after getting my king of tone rolling, and almost done with a shin-ei Univibe clone, I've had a chance to poke around with the memory man again.

I plugged it into the amp tonight and I'm definitely getting bypass signal, though it's real low output (true bypass wiring) and when I engage the pedal I hear the delay working!

What isn't working is a nasty ass hum that overtakes the entire signal, whether bypass or engaged. Maybe pots touching the solder blobs. Dunno yet but I put down some electrical tape. I'll test it tomorrow and see what it looks like.

I also created a voltage testing spreadsheet with all of the references values, so ill do a full measurement run and post on google docs.

[thomasha]

That sounds like a good plan.

First, low signal in bypass mode means there is something wrong.

In true bypass, and with a 3PDT, the signal should go directly from the input to the output jack. You can check the layout from Brian's PDF and brown wires are grounded, while orange wire connects to jack's blue wire when the 3PDT connects the lower pins. At the same time, input of the board (green) is grounded by brown wire on the 3PDT. You can check this with a multimeter. Just check if you have a good continuity. If you abused your 3PDT it can also introduce some resistance, so try measuring the resistance between jack tips. It should be almost 0.

This test should be done with the pedal unplugged (no power), since in true bypass the pedal is just part of the cable. So there should be no hum in the signal.

If there is some short between the back of the pots and the board it could create some problems. Tape might be too thin. I had cases where the tip of the solder blob just perforated the tape. Since then I use a thick plastic sheet (0.5 mm? It is thicker than an overhead projector sheet).   

If you can lift the board, so that the pots won't touch it, it would be better. If we can roll out a short with the pots you can check the rest of the circuit. Most likely a lack of grounding somewhere(cold solder joint?).

[zachlovescoffee]

In this wiring diagram for true bypass switching are lugs 4,5 and 9 supposed to be wired together as shown? Or 4 and 9 to input tip and 5 to board?

[zachlovescoffee]

Hello folks! Wife out of town and kids in bed. So, I had time to do a full measurement on the board.

Audio-wise -- the hum in bypass or effect mode is still full on. It's even worse than before.

Included here is a link to my voltage tests with all ICs in, powered with an EHX 24v adapter. As far as I can tell, all of the voltages are pretty damn close.

If you have a moment to review I'd really appreciate the confidence check.

https://docs.google.com/spreadsheets/d/1_148Y8SX5kXWO9-Wrv2Y6N1t137AuvCW/edit?usp=sharing&ouid=115840715864376621355&rtpof=true&sd=true

[thomasha]

Have you given the audio probe a try, to identify where the noise begins? Sometimes correct voltages at the ICs is not enough.

Anyway, looking at your table I would check everywhere where there is a difference of almost 1V. For example, IC3A, pins 1 to 3. But it makes sense, since pin 7 of the NE570 is also different.

IC4B is also lower than expected. Weird, because its bias comes from IC4A, where the voltages seem OK. They both should have similar voltages on the signal pins of around -7.5 (VB). You can check for shorts, or wrong parts.
My mistake, IC3B comes after IC4A in the signal chain. IC4A is biased by the trimmer and matches the BBD input.

I'm guessing differences in IC6 and 7 (BBDs) are caused by the bias of the BBDs, which can vary.

PS. Some suggestions to rule out some common problems:
1. Bad Jacks or 3PDP: Test the pedal without the power supply. If the true bypass signal is strong and loud there is not problem here.

2. Dry signal with power supply, aka clean booster: The dry signal only goes through IC1A and the blend pot. With the blend on 100% dry and feedback/repeats on 0% check the output with the pedal engaged. You should hear the clean signal only, without delay, and the output can be controlled by the level pot. There should be no noise, since there is no feedback and no wet signal. If it still hums check all components that are nearby and go to ground.

3. Identify where in the wet signal the noise starts: Remove the BBDs from their sockets. If there is still noise it must be introduced after the second BBD. If it stopped, insert the 2nd BBD in its socket. If there is noise, it comes either from the BBD or from the previous gain stage (IC4B). If there is no noise, it comes from a stage previous to IC4.

4. With the audio probe you could just check different positions of the circuit without having to remove the BBDs to see if there is Hum or not. If all of them have some hum, it is probably something like a missing ground of the power circuit.

5. Every time I had a loud hum in a circuit it was a cold solder joint on a ground pin. Check the continuity between the ICs/resistors/capacitors ground pin and the jack ground. If there is continuity move to the next one. Do not measure it directly on the solder blob, sometimes the solder makes contact, but the pin of the component don't.

Hope it helps you in some way.

[zachlovescoffee]

Hello! I haven't had time to use the audio probe yet. I did do all of these tests below. Here are my results:

1) Bypass mode w/ no power -- dead silent, no hum, pure signal goodness. Sounds amazing.
2) Dry only -- lots of hum
3) No noise/output with the BBDs removed. Insert one, hum. Insert two, lots of hum. On power or not.

It really seems like when using the blend pot to bring in more wet signal, obviously, the hum gets way worse.

Seems like the wet circuit is totally jacked though I'm not sure why. I have gone over every blob 2x at this point.

I haven't done the continuity checks yet but I will do those when I have some time and report back. I appreciate all of your patience. I'm definitely swimming in the deep end here, but just from your last post I have learned so much!


**When pulling an MN3005 out of the socket I accidentally broke off the small/thin part of the leg. Can I just solder a good lead onto that to fix it?

[jimilee]

You can do that with the 3005, you might want to replace it eventually.

We definitely need voltages and very clear well lit close up photos of both sides of the board and off board wiring, please.


Sent from my iPhone using Tapatalk

[zachlovescoffee]

As requested!

[thomasha]

Well, without the BBDs the dry side should work at least...
It seems the second BBD might be creating a problem here(if you added it first)?

I made a mistake here, now I see that after IC4A comes IC3B and not IC4B. IC4B comes before the second BBD and is biased by the trimmer. It also matches IC7 pin 7, so no problem here, my bad.

Thanks for the pictures, there are some suspicious solder points. Look for the ones that look like a large round drop or where it looks like there is a separation between the component leg and the solder. I would test all with a multimeter, cause sometimes even good looking solder blobs are can be bad...
You forgot the Jacks, but I guess they are right, since it works without power.
Check the continuity of the small black wire from xs to xr too. I guess they are good, otherwise there would be no repeats at all.

[zachlovescoffee]

Face palm moment of the day. I didn't test with the second BBD in first. I did first BBD and then second BBD. Will re-run that test and report back!

[zachlovescoffee]

News! I tried the step again where I removed the BBDs. I put the second BBD in first and VOILA! I have relatively clear dry and wet signal. The hum I'm getting now appears to largely just be from running the pedal un-shielded and outside of a box.

Then I plop the first BBD in and BOOM! Nasty hum is back!

[thomasha]

Wait, what?
If you remove one of the BBDs there should be no or very weak wet signal. The wet signal path is broken. Unless there is some power line that is not correctly decoupled (VB?, which could work as a path for the signal).

Try bypassing the first BBD: With only the second BBD in the socket, put a capacitor in the socket of the first BBD(between pin 7 and pin 3, any value that you have on hand higher than 100nF). That will create a path that jumps over the first BBD.

If the noise is there, it probably enters the circuit before the first BBD stage and the BBD only allows it to pass. If there is no noise, you should check connections around the first BBD, because it might be causing the problem.

With the signal probe, it would be much faster to identify where the noise is introduced, because you would be able to check the signal at the input of every IC.

I only get very loud and strong hum when there is a missing reference (resistor to ground), for example when you turn on a high gain amp and the cable is still not connected to the guitar and you touch it.

[zachlovescoffee]

What I mean is that I am getting good dry signal and when I click on the pedal, I am getting very low output, barely audible, but no crazy hum.

Does the polarity of the cap matter? I'll test tonight.

[thomasha]

Ideally you would use a film cap, without polarity. Otherwise I would have to check where the voltage is higher in the circuit...

[zachlovescoffee]

Nasty hum is still present with a 220nf film cap jumped between 7 and 3 on BBD1.

So the hum must be coming in before the BBD. I'll start poking around. I'm going to order a honey tone amp to put on my bench for the audio probe work.