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Old 17th December 2011, 09:04 PM   #1511
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@RS, I wasn't expecting the updates to the article so quickly. I was expecting a few days at least. Thanks for your continual support on this project. I'm in middle of work and trying to test what I can, but it seems we're narrowing down the problem. I swapped u3 and u4. Same channel still. I checked the voltages also. It looks like the issue is at u3. I wasn't sure which was pin 5 so I checked 5 and 6. One of them was 8.5v and the other started to climb just like it was doing on p2. I checked p2 and it seemed the voltages were climbing at the same rate. U4 had the same pin at 8.5v and the one that was climbing stayed at 0.0v. Fyi, this was tested with batteries this time. I'll test with wall wart later.

Once I get a moment I'm going to go through the article updates as well.

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Old 17th December 2011, 09:51 PM   #1512
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Quote:
Originally Posted by kmc52087 View Post
I did what you said and worked my way backwards. I found that the voltage was bad at C15 (8 volts) and good at the battery terminals (24 volts). Decided it was a bad connection on the switch, resoldered the pins and its working great. Perfect voltages everywhere.

Thanks for your help RS.
You're welcome. Glad you got it working!

@mathaios, Please ground your negative DMM lead to one of the two center battery terminals (i.e. the negative terminal of BT1 closest to the gain switch) and use the positive probe to check all eight pins of U3 (the bad channel) after the DC offset has developed. Please report each of the eight voltage readings when you've done that such as:

Pin 1 = x volts
Pin 2 = y volts
.
.
Pin 8 = 11.8 volts

Pins 1/2, 3/4, and 6/7 are supposed to be connected together. But please measure each of the pins anyway at the IC in the socket from the top side of the board. That will help show any soldering problems, a problem with the socket, etc.

Please also report the voltage at each side of R12 and R13 when the amp has the offset (four more readings).
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Old 17th December 2011, 10:03 PM   #1513
agdr is offline agdr  United States
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matthaios - one more thing to check in addition to all that RocketScientist posted. Check your connections on R12. Try re-heating them with the soldering iron. Also - with the power switch off - try a resistance reading across R12 and see if it is at least 40.2K.

In theory at least if R12 were missing, or a much higher value, the input bias current of U3 could start charging up C13. That would cause the non-inverting input voltage of U3 to ramp up over a few seconds and the output voltage would follow it in this configuration.

RocketScientist - those new troubleshooting and testing additions look great!
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Old 17th December 2011, 10:31 PM   #1514
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I think this is the best place to make this post; hopefully I'm not derailing the troubleshooting. Sorry if I am.

Would anyone be kind enough to look over the front panel I designed? It's a play off of what's offered by JDS Labs (don't really want their logo) and the front panel GB (I was too late).

By my eye, everything looks OK, but I've never done vector drawing before and never designed a front panel so it might be rubbish. I've included the .fpd along with the .plt files I used in a zip below. Thanks.
Attached Files
File Type: zip CustomFrontPanel.zip (11.2 KB, 33 views)
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Old 17th December 2011, 11:28 PM   #1515
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OK, so inhaled it plugged back in to the wall wart now and found something interesting when testing u3.

Pin1 - slowly goes up in voltage
Pin2 - same as pin 1
Pin3 - read below
Pin4 - 11.7v
Pin5 - read below
Pin6 - same as pin 1
Pin7 - same as pin 1
Pin8 - same as pin 1

Pin 1,2, 6, and 7 slowly go up in voltage like it's doing in p2. The interesting part is when I measure pin 3 or 5 it reads those same voltages and then instantly goes down to 0.8v. Once I measure one of the other pins again, the voltages start at 0.8v and start to climb again.

R12 and r13 both read 0.003v.


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Old 18th December 2011, 12:16 AM   #1516
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@agdr, I went ahead and checked the stuff you told me. The resistance on both R12 and r13 are 39.9k. I ended up resoldering in R12 and it looks to have solved my issue. The voltages in p2 were both 0.003v and u3 pin voltages match u4 now. Pin 4 and 7 are 11.7v and the rest are 0.003v. Testing it now to make sure.

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Old 18th December 2011, 03:24 AM   #1517
agdr is offline agdr  United States
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Quote:
Originally Posted by matthaios View Post
@agdr, I went ahead and checked the stuff you told me. The resistance on both R12 and r13 are 39.9k. I ended up resoldering in R12 and it looks to have solved my issue. The voltages in p2 were both 0.003v and u3 pin voltages match u4 now. Pin 4 and 7 are 11.7v and the rest are 0.003v. Testing it now to make sure.

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Hey that is good news! 3mV is right on the mark for the output offset voltages. That is what I get on my O2s.
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Old 18th December 2011, 03:40 AM   #1518
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@matthaios, agdr and I were both on the same track. That's why I asked you to measure both ends of R12 as I suspected it might be an open circuit. For the geeks in the audience, the 4556 was "sourcing" its bias current through the coupling capacitor from the input stage until. That only worked for a while and had the side effect of charging the capacitor to a DC value which was passed on to the output.

This is a good example of why it's important to follow the Initial DIY Testing. Those who skip it might end up with massive DC offset from a soldering error.
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Old 18th December 2011, 08:30 AM   #1519
Alexium is offline Alexium  Ukraine
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Will O2 work without battery terminals installed?
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Old 18th December 2011, 09:10 AM   #1520
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Originally Posted by Alexium View Post
Will O2 work without battery terminals installed?
of course
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