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My First CMoy: No sound from left channel, no amplification in right channel

post #1 of 14
1/24/12 at 9:35am
Thread Starter 

I'm getting back into DIY electronics for the first time since my 7th grade science fair project (over 2 decades ago), and having a lot of fun getting reacquainted with a soldering iron. I'm not very good at it though.

 

I've been following the Tangent CMoy tutorial. Testing after Stage 1: The Power Supply went exactly as expected.

 

After Stage 2: Adding the Amplification (Signal) Section, I'm not getting any sound from the left channel and don't appear to be getting any amplification in the right channel (sounds the same as when I have the headphones hooked up directly to the source).

 

I tested with my multimeter as I was adding components to try to make sure I didn't short anything with solder bridges, and to make sure I didn't have any bad solder joints, because my soldering technique could definitely use some work. I think things were ok - didn't observe any continuity between pads that should be unconnected or lack of continuity where connections should be present.

 

What are some things I should be testing for to determine where the problem lies?

 

I've uploaded an album of pictures that show how things currently look.

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post #2 of 14
1/24/12 at 1:29pm

First suggestion would be to get more solder on your joints for the opamp, the issue might be that you're not making a connection, you shouldn't see the trace that is surrounding the pin, but you also don't want to bridge your traces with solder. 

 

RAwy4h.jpg

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post #3 of 14
1/24/12 at 3:17pm
Thread Starter 

Thanks, I'll try resoldering those joints after work tonight.

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post #4 of 14
1/24/12 at 3:21pm

it is hard to trace stuff with pictures, but from what i can see it doesn't look like the batteries are connected in series. (because both the reds are together.

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post #5 of 14
1/24/12 at 3:28pm
Thread Starter 

Sorry, I probably should have zoomed the picture out a bit more. The two sets of wires are for a 9V battery and a power LED.

 

FYI, I have also tried using two 9V batteries in series, with the same results.

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post #6 of 14
1/24/12 at 3:48pm

Trace the missing signal using an older set of ear/headphones

 

at the 3.5mm plug ground the shaft and use the tip to trace the signal, it should be at the input socket then at the input capacitor then at the opamp, at some point you will loose the signal thats where the hole will be

 

good luck

cheers

FRED

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post #7 of 14
1/24/12 at 3:55pm
Thread Starter 

Sorry Fred, I'm a complete newbie when it comes to DIY electronics. How exactly do I trace the signal? Do I just hook up the headphones  to the input socket, then the input capacitor, etc and see at what point I no longer hear sound from the headphones?

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post #8 of 14
1/24/12 at 4:04pm

pretty much

 

take the earphone plug connect the shaft part of the plug to the ground on your amp using a piece of wire, you can then touch the tip of the plug to various points on the circuit and hear the signal, if you look at the schematic you can trace the signal from the source

 

at some point you will stop hearing the signal, that will be where the problem lies

 

cheers

FRED

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post #9 of 14
1/24/12 at 8:15pm

The earphone test that Fred suggested should help.

 

Another thing I always advise people to do is with the power off, remove the opamp from the socket, and ohm out all socket pins to ground, using the schematic. For example, the input pins (3 and 5) should show 100k if you were using 100k resistors to ground. Additionally, ohm out each adjacent pin to the next. This will show you if the feedback loop is connected. If you are using the stock gain of 10, you would see 10k from pin 1 to 2 (assuming you have jumpered R5, which you have). If you see any really low readings, you probably have a short... open readings means a bad solder joint somewhere close.

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post #10 of 14
1/25/12 at 8:02am
Thread Starter 

Thanks all for your suggestions. I did not get a chance to resolder any joints yesterday, but these are the measurements from following Pars' suggestions:

 

pin 1 - ground: 1M

pin 2 - ground: 1M

pin 3 - ground: open - I guess this is the problem?

pin 4 - ground: 4.72k

pin 5 - ground: 99.7k

pin 6 - ground: 1M

pin 7 - ground: 1M

pin 8 - ground: 4.72k

 

pin 1 - pin 2: 9.95k

pin 2 - pin 3: open - this and the next also seem to point at the problem being with the connection to pin 3, right?

pin 3 - pin 4: open

pin 4 - pin 5: 104.4k

pin 5 - pin 6: 1.1M

pin 6 - pin 7: 9.93k

pin 7 - pin 8: 1.01M

 

Thanks again for all the help!

 

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post #11 of 14
1/25/12 at 8:53am
Quote:
Originally Posted by gjritter View Post

Thanks all for your suggestions. I did not get a chance to resolder any joints yesterday, but these are the measurements from following Pars' suggestions:

 

pin 1 - ground: 1M   probably ok

pin 2 - ground: 1M   should be R3

pin 3 - ground: open - I guess this is the problem?  part of it, should be 100k (R2)

pin 4 - ground: 4.72k  should be voltage divider resistors with electrolytic caps in parallel

pin 5 - ground: 99.7k  OK (R2)

pin 6 - ground: 1M  should be R3

pin 7 - ground: 1M  probably ok

pin 8 - ground: 4.72k  should be voltage divider resistors with electrolytic caps in parallel

 

pin 1 - pin 2: 9.95k 

pin 2 - pin 3: open - this and the next also seem to point at the problem being with the connection to pin 3, right?

pin 3 - pin 4: open

pin 4 - pin 5: 104.4k

pin 5 - pin 6: 1.1M

pin 6 - pin 7: 9.93k

pin 7 - pin 8: 1.01M

 

Thanks again for all the help!

 

 

Pins 1-2 and 6-7 are your feedback loop, and should show R4 + R5. It appears that you have problems with each channel's feedback loop (though different problems). Most of the other pin to pin measurements won't mean much, if anything. Pins 3 to 5 would show both R3s, but you can see from your pin 3 to gnd that there is a problem.

 

 


SUBMIT

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post #12 of 14
1/25/12 at 5:24pm
Thread Starter 

It looks like the R3 resistors that I ordered - which were supposed to be 1K - were actually 1M. I just compared the resistors that I got in my last shipment to ones that I received today, and sure enough, they've got different colour coded stripes.

 

The voltage divider resistors are 4.75K, and the resistance that I measure for pin 4- ground and pin 8 - ground actually starts at 0 and climbs to ~4.72k. Is this what I should expect to see?

 

If that's the case, I guess replacing my R3 resistors with the correct ones I received today, and resoldering the pin 3 connection should resolve the problem. I'll try later tonight.

 

Thanks again for all the help!

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post #13 of 14
1/25/12 at 8:15pm

Regarding the voltage divider resistors, yes, that is the behavior I would expect. You are seeing the caps charging (DMM is doing it), and when they do and the resistance hits 4.7k, it will stay there.

 

EDIT: with the 1M resistors in, your gain is 1.01 (R4/R3 + 1), so the one channel is working fine, just not amplifying at all.


Edited by Pars - 1/25/12 at 8:20pm
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post #14 of 14
1/25/12 at 8:16pm
Thread Starter 

I replaced the R3 resistors with the correct ones, and resoldered pin 3, and it's working exactly as I expected now!

 

Thanks everyone for your suggestions!

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