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Another half-working CMoy

post #1 of 11
12/31/09 at 10:36pm
Thread Starter 
I'm so close to being finished with my first project, a CMoy amp using Tangent's instructions. I had it working when it was just bare wires, but when I started to wire the panel components, I'm not getting sound out of the right channel.

Here's what I've done:

I've cleaned the board with 91% isopropyl alcohol.
I've made sure there were no solder bridges with the continuity tester.
Without the op-amp in the socket, I'm reading 9.26 and -9.33 volts on the 8 and 4 pins respectively (2 9V batteries in series). No voltage on the other pins. With the op-amp in, here are the voltages:
1: 10.4 mvLeft side out
2: 0.9 mvLeft side out
3: 0Left in
4: -4.36 vV- in
5: -2.309 vRight in
6: -2.071 vRi ght side out
7: -3.148 vRight side out
8: 14.10 vV+ in

It seems that it's pushing voltages back through the input on the right, but why would that be? It was working fine before, so I don't think it's the chip.

The source is good (Apple iPhone 3GS).
The test headphones are working.

Here are some photos. Let me know if there are others you'd like.
CMoy gallery

Any clues? What should I check next?

Thanks!
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post #2 of 11
12/31/09 at 11:58pm
Have you rechecked the wiring of your components? Made sure they were soldered securely and to the right jacks? And don't iphones have a proprietary 3.5mm jack?
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post #3 of 11
1/1/10 at 1:30am
Thread Starter 
Quote:
Originally Posted by shake View Post
Have you rechecked the wiring of your components?
I'm 90% sure everything's wired right. I'm going to recheck everything, though.

Quote:
Originally Posted by shake View Post
And don't iphones have a proprietary 3.5mm jack?
1st gen iPhones needed special long plugs. 2nd and 3rd gen (incl. 3GS) can use a "TRRS" connector, but is compatible with a regular TRS.
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post #4 of 11
1/1/10 at 1:34am
can you post detailed photos of the bottom of the board?

Perhaps the topside too.

It would be superhelpful if you noted where things go.
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post #5 of 11
1/1/10 at 1:54am
Thread Starter 
Basic Troubleshooting for Headphone Amplifiers

Using these instructions, I traced the AC voltage from the source (my laptop this time) from the input jack (same on both channels), to the input pins on the opamp (same on both channels). Now, the output pin on the left channel is about right (gain * input voltage), but right channel is 0. It seems like maybe the chip is bad now. I'll try and pick up a TL082 at RadioShack to see if it helps...
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post #6 of 11
1/1/10 at 2:05am
Thread Starter 
Quote:
Originally Posted by nikongod View Post
can you post detailed photos of the bottom of the board?
I've added notes to the bottom image. It's in the gallery:
https://www.dropbox.com/gallery/2542427/1/CMoy?h=f86ab1
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post #7 of 11
1/1/10 at 2:26am
Quote:
Originally Posted by petewall View Post
It was working fine before, so I don't think it's the chip.
Sorry, I skipped this part.
Opamps can go bad for a number of reasons.

Do you have another opamp to try? Maybe get one from radio-shack on saturday or something.
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post #8 of 11
1/1/10 at 2:38am
If it was me I'd be tempted to take it back to when it was working and then add the connectors again, taking a fresh look at it.
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post #9 of 11
1/1/10 at 4:03am
From your voltage listings, I would think that the feedback loop may have a problem on the right channel, though a bad opamp is a possibility as well. I would remove the opamp, and with it unpowered, ohm out all pins on the opamp socket to ground, and also to adjacent pins. Note the readings compared to the schematic (for example, pin 3 has a 100k resistor to ground, so that is what you should be seeing). For adjacent pins, ohming out pins 6 and 7 will show you the feedback loop (R4 + R5 if you used it). You should be seeing very similar resistance on both channels.

1: Left out
2: Left inverting input (in -)
3: Left noninverting input (in +)
4: V-
5: Right noninverting input (in +)
6: Right inverting input (in -)
7: Right out
8: V+

Note that in your readings, pin 3 is 0 volts, which is where pin 5 should be as well.
Take a look at Audio Designs With Opamps for some opamp background.
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post #10 of 11
1/1/10 at 1:20pm
Thread Starter 
Here's the resistance readings:

Pin to ground
1: 10.98 kΩ≃ 11kΩ
2: 1 kΩ≃ 1kΩ
3: 100.4 kΩ≃ 100kΩ
4: n/a
5: 100.3 kΩ≃ 100kΩ
6: 0.999 kΩ≃ 1kΩ
7: 10.99 kΩ≃ 11kΩ
8: n/a

2&3: 101.6 kΩ
5&6: 101.4 kΩ

Everything seems the same on both sides. I'm suspecting the opamp even more now... I'll pick up a replacement and test with that.
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post #11 of 11
1/1/10 at 4:42pm
Thread Starter 
I just swapped in a RadioShack TLO82 for the OPA2132 and I get both channels again. I'm going to assume that it's the opamp chip that's bustificated. I'll order a new one and cross the fingers.

Thanks for the help everyone!
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