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Old 16th May 2012, 12:33 PM   #1
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Default problem with Chipamp.com 1875 kit

Hello all:

I have a problem with two Chipamp LM1875 amplifier boards. I emailed Brian with the question, but haven't heard back. Not sure if he is still supporting these kits or not, so I'd like to post the question here:

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The output goes to the + power supply value. I built a second board and have the same problem. There is one capacitor (47 uf, I believe it is C6) on the board that appears to be an electrolytic but the board shows no polarity for installation. So I am suspicious of this. Is the circuit the same as shown on the first page of the National data sheet for the LM1875?
=================

The power supply is fine (it is the board that came with the amp kit). Without knowing the exact schematic it is hard to figure out diagnostics. Also, a few of the components are labelled as "optional" without any information on what they can do for you.

Any help would be appreciated.

Thanks,
Mark
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Old 16th May 2012, 04:09 PM   #2
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Hi mbohn.

This was a picture of the very first Chipamp.com 1875 boards they posted on this forum. The picture links are broken in the build thread now unfortunately.

http://i703.photobucket.com/albums/w...s/ampboard.jpg

You can see the positive symbol for C6 marked on the board. Compare your board to the photo before you act on it though as there could have been revisions since then.

Hope it helps.

John
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Old 16th May 2012, 04:25 PM   #3
AndrewT is offline AndrewT  Scotland
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What schematic are you using?
Then we can compare to the National version and recommend tests to find the error.

A close up pic of both sides of the PCB and of the PSU with cable to PCB would be very useful in debugging.
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Old 16th May 2012, 09:57 PM   #4
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John, Andrew:

Thanks much for your help. I'll post what I find.

Mark
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Old 17th May 2012, 01:14 AM   #5
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Quote:
Originally Posted by john blackburn View Post
Hi mbohn.

This was a picture of the very first Chipamp.com 1875 boards they posted on this forum. The picture links are broken in the build thread now unfortunately.

http://i703.photobucket.com/albums/w...s/ampboard.jpg

You can see the positive symbol for C6 marked on the board. Compare your board to the photo before you act on it though as there could have been revisions since then.

Hope it helps.

John
My board does not have the 0711 (date?) stamp on it but I do now see the polarity indication and I do have C6 installed correctly on both boards.

I now see that it is C2 that is shown on the schematic as polarized but on the board polarity is not shown. C2 is also one of the optional components. I've got it installed in one board and not in the other.

Last edited by mbohn; 17th May 2012 at 01:16 AM.
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Old 17th May 2012, 01:34 AM   #6
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Quote:
Originally Posted by AndrewT View Post
What schematic are you using?
Then we can compare to the National version and recommend tests to find the error.

A close up pic of both sides of the PCB and of the PSU with cable to PCB would be very useful in debugging.
photos:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
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Old 17th May 2012, 10:09 AM   #7
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Hi Mark.

There is a snapshot of the original parts listing here.

http://i703.photobucket.com/albums/w.../forumpage.jpg

C2 is listed as a non polarised capacitor and as optional on the BOM. I would be tempted to reflow the soldering on your boards and maybe look at getting bigger heatsinks once you have it working. I found the little 1875s run hotter than the 3875s I have used previously.

On your photo, follow the trace down from the SG pad. Do you have an unsoldered wire in the next hole down?

John
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Old 17th May 2012, 10:27 AM   #8
basi is offline basi  Croatia
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Greetings! C4 is same as C6 47uf,probably is better to put there 100-220uf.You must put C2 22uf,C7 0.1uf and R6 jumper or 10 ohm resistor and that should work.
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Old 17th May 2012, 11:09 AM   #9
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Quote:
Originally Posted by john blackburn View Post
Hi Mark.


On your photo, follow the trace down from the SG pad. Do you have an unsoldered wire in the next hole down?

John
That hole is for R6 listed as

"jumper (optional resistor between input ground and output ground)"

I was not sure how a jumper could be a resistor so I left it out. Could the option be between a jumper and a resistor, not between a resistor or jumper or leaving it out?
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Old 17th May 2012, 11:18 AM   #10
AndrewT is offline AndrewT  Scotland
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Why so many components omitted from the PCB?

If I was going to test drive a car. I damn well would be sure before I set off that the steering wheel was not omitted.
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