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TKO956  
#1 Posted : Wednesday, May 1, 2019 8:32:50 PM(UTC)
TKO956

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Hello,

I just FINALLY finished my Sympatico and was doing some testing before inserting it into the rest of my system. When checking for DC at the speaker terminals I found the left channel had 2uV but the right channel had 14.5V! I grounded the inputs and the left channel rose to 4mV but the right was even higher at 29.7V!

I bought bare boards and soldered everything myself so I double and triple checked for any solder bridges or bad joints. I didn't find anything that worried me but I did reflow the connections for the LM4780. Checked the DC again and got the same results. That high of a DC voltage leads me to believe that something is very wrong. I left the amp powered on for 3 or 4 minutes and nothing on the right channel amp was even warm.

Has anyone seen something like this or can somebody point me in the direction that I need to look?

Thanks in advance,

Ty
Brian Donegan  
#2 Posted : Wednesday, May 1, 2019 10:06:43 PM(UTC)
Brian Donegan

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Check the feedback loop from the LM4780 to the OPA1632. Make sure those comps are fully soldered and correct.
TKO956  
#3 Posted : Wednesday, May 1, 2019 10:46:11 PM(UTC)
TKO956

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Thanks Brian, I'll take a look

I'm guessing that you mean caps not comps though, correct?
TKO956  
#4 Posted : Thursday, May 2, 2019 12:19:37 AM(UTC)
TKO956

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I ended up just replacing C1 and C2. Checked again and still at 14.5V DC at the output. Is it possible that my LM4780 is bad?

Thanks
Brian Donegan  
#5 Posted : Thursday, May 2, 2019 4:50:33 PM(UTC)
Brian Donegan

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Originally Posted by: TKO956 Go to Quoted Post
Thanks Brian, I'll take a look

I'm guessing that you mean caps not comps though, correct?


I meant comp(onents). Resistors and capacitors.

It is possible that the LM chip is bad, but just as likely there is another bad component. Can you post a picture?
TKO956  
#6 Posted : Thursday, May 2, 2019 6:01:26 PM(UTC)
TKO956

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I ended up reflowing everything on the board and the DC is now jumping around between 7V and 10V. It does look like something just has a bad solder joint. I'll hit everything again later today when I have time. I did check all of the resistor values and they are correct.

I attached the picture. Please let me know if you see anything obvious other than the solder bridge between R1 and R11. I figured that it wasn't that big of a deal since they are attached by a trace anyway.

Thanks.

amp1.jpg (106kb) downloaded 6 time(s).

Edited by user Thursday, May 2, 2019 6:03:34 PM(UTC)  | Reason: Not specified

TKO956  
#7 Posted : Saturday, May 4, 2019 1:21:18 AM(UTC)
TKO956

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Reflowed all the components again and the DC on the right channel is now 9mV.

Thanks for the help, Brian.

Edited by moderator Monday, May 6, 2019 5:14:36 PM(UTC)  | Reason: Not specified

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