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Ideas on troubleshooting Worng LRMSMSLR
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Ideas on troubleshooting Worng LRMSMSLR
batch
I got a full DIY kit from Thonk which I built last night. Really easy build. Plugged in and nothing. On examination found no issues with solder joints (have built many, many more modules) and so started testing with the oscilloscope.

What I am finding is as follows

1. The signal is actually passing through - it’s just extremely quiet and needs to be boosted by over 20db in my mixer to hear it, but it’s there
2. There is a -2V and -1V reading on the M and S outs with nothing plugged in.

Am thinking I have duff op amps. I might have some cheap spares I could swap in to test it.

Anyone have any ideas?
search64
Post pictures.
ucacjbs
Yep, pictures will help. Also, with power on, check the voltage at

1. The Euro connector
2. The supply pins on each IC.
batch
Here is what I am getting for voltage readings with power on.

1. Power -12.01 and +12.06 on each rail
2. At each socket

L< -0.613
R< -0.613
M> -1.090
S> -2.090
M< -0.608
S< -0.609
L> +2.428
R> 0.000

3. Across the op amps going right to left in picture

4 to 3 -11.75
4 to 2 -11.16
4 to 1 -10.37
4 to 5 -11.74
4 to 6 -11.14
4 to 7 -10.38
4 to 8 -9.98

4 to 3 -11.75
4 to 2 -11.08
4 to 1 -10.52
4 to 5 -8.95
4 to 6 -8.91
4 to 7 -9.65
4 to 8 -9.98

4 to 3 -11.73
4 to 2 -11.11
4 to 1 -14.44
4 to 5 -10.52
4 to 6 -11.75
4 to 7 -10.51
4 to 8 -9.97

4 to 3 -11.75
4 to 2 -11.75
4 to 1 -14.44
4 to 5 -11.13
4 to 6 -11.13
4 to 7 -11.79
4 to 8 -23.78

Here are some pictures:





What do you think?
batch
The IC's are NE5532P
ucacjbs
Something is odd -- on the last IC, you have a 4->8 voltage measured of -24. I assume you've got the negative probe of the multimeter on pin 8, and the positive on pin 4? If so, that's OK. Pin 4 is -Vcc (-12V here) and 8 is +Vcc (+12V) so, if you have the probes 'backwards' like that then -24 makes sense. But in general, it's easier to interpret these voltages, if you just put your -ve probe on a known ground, and then move the +ve probe to the measurement points.

Now, you'll see that for all the other ICs, the voltage across pins 4->8 is not the same as it is for that final IC. It should be. Disconnect power, remove the ICs from their sockets (a good idea to either mark each IC somehow, or stick their legs through a piece of paper on which you've noted some labels -- will help keep things straight if you have to later figure out which, if any, IC might be in bad shape).

With the ICs out, and power off, measure resistance between pin 8 of the IC where you measured 4->8 voltage of -24, and pin 8 of each of the other ICs. Should be 0. Next, measure resistance between pin 4 of the '-24' IC, and pin 4 of each of the other ICs. Should also be zero. If it's not, then try just retouching all the solder joins on the sockets, and measure again.


p.s. final sanity check, but just to confirm that your pin-numbering in the above measurements is:

1 (notch) 8
2 7
3 6
4 5

(Apologies if this seems like a rude question, but always good to check conventions)
batch
Thanks for this, I’ll try this tonight after work and report back. I’ll also go back over al the joints.

Yes am using the pin layout you have above.
batch
OK, so I removed all the IC's and went back over all the joints. Then I measured the resistance from Pin 4 to Pin 4 and Pin 8 to Pin 8 in all directions and combinations (i.e., socket 1 to 2, 1 to 3, 1 to 4, 2 to 1, 2 to 2, etc.).

On Pin 4 the readings are always 0Vs

On Pin 8, the reading are 0Vs on 3 out of 4 sockets. On the one where we got the 24V reading its infinite. This is true between it and all other sockets. I did notice when doing the reading it starts at 0 and then goes to infinite. If I do the reading again within a minute then its just infinite. Wait a little longer and its then zero, and rises to infinite. Think this suggest a capacitor is doing something?

Side note, I ran an audio signal through the module without it powered up and found it works. Turn the power on and it does still work, but the volume drops a lot.

Second observation is that I swapped out each IC for a spare TL072 Op Amp I had lying around and tried it again. When the TL072 was in socket 1, I did not have to boost the signal quite so much to hear it. Probably 3-6db instead of 20db. Was still quiet though.

Any thoughts?
batch
BTW - the black things to the left and right of the power. I don't know what they are, but they look diodes or similar to protect against reverse polarity. However there was nothing on them to say which way round they should go. On the component itself or on the PCBs so I just put them in.

Do you think maybe I put one in the wrong way round?
Revok
Armstrb wrote:
BTW - the black things to the left and right of the power. I don't know what they are, but they look diodes or similar to protect against reverse polarity. However there was nothing on them to say which way round they should go. On the component itself or on the PCBs so I just put them in.

Do you think maybe I put one in the wrong way round?


Those are ferrite beads. The have no polarity.
Revok
Everything appears to be where it should be. The solder joints don't look so great though... I would clean the flux residue and give it another go. Readings instantly changing could be due to a bad joint.
batch
I've been over the joints a couple of times since that photo was taken. Am pretty convinced its not that.

Another observation. Pin 4 is the +12V rail and Pin 8 is the -12V right? Well checking the resistance from the +12V rail into Pin 4 on each socket gives ~4 ohms. Whereas checking the -12V rail to Pin 8 gives ~3 ohms on the left/ bottom socket and infinite on the other 3.

When I power it up and check the voltages at Pin 4 I get -12V at each one. But at Pin 8 on one of them its +12V, but at the other 3 its low/ nothing.

I think this suggests there is a break in the -12V power rail somewhere and therefore 3 of the op amps aren't working properly because they are not getting powered.

Could this be just a duff board?
Revok
I'm not as to whether the break is in the +12 or -12 from your description but here's a drawing for both. You could try tacking on a jumper wire like the drawings below to see if that's the case. The pads might be lifted on the circled joints depending on which rail is broken.

+12



-12

batch
Let me try that and see what happens.
Revok
You can do a simple continuity test from joint to joint to pinpoint it as well.
batch
I put the jumper on the +12V rail as shown in the first photo.

That fixed it!!!

Works like a charm now.

Thanks so much for all the help on this, really, really appreciate it.
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