low 1176 power supply voltage (rev. j mnats board)

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drumminkiger

Active member
Joined
Sep 3, 2010
Messages
36
Location
Seattle, Washington, USA
Well, i had this posted on the general 1176 help thread, but i never got a response on it so i figure it may have gotten a little burried, so i figured i would repost it in the lab instead where it was a little more visable.

I built two channels of 1176 rev J with IC input and the mnats power supply. Now, when i fire up the power supply it gives me about 29.5 volts on the +30 rail, when i plug it into one of the channels it works just fine and supplies the same power, the unit looks ready to calibrate. but when i unhook it and then plug it into the other unit the output drops by about 5v (even at the power supply itself). -10 seems to stay about the same from board to board. I tried to check over my board for shorts and couldnt seem to find any, though there always could be one im sure. What got me looking at this  is that Q8 and 9 are getting really hot, and its eating 250ma fuses for breakfast. Ive bumped up the fuse size to 500ma so i can at least turn it on and grab a voltage and then turn it back off. I kept my build really straight forward, used the BOM off the mnats page and a hairball output transformer. Any ideas where i should start looking for fixes? Ive been crawling the form looking for a similar issue but havent come across it yet. Thanks for any input, I really appreciate it!
 
Stagefright13 said:
but this is not the rev J thread. Please post there.

This has already been done, no replies. Most of the threads that i have searched and read on here that i hoped would give me some insight as to what might be the issue ended with a highly generic response, and instructions to repost on another thread. To me this seems like it will lead to a lot of orphaned threads and a lot of un answered questions. Thats just my opinion.

I did however look at all the caps, the orientation on each board appears to be the same unless something has been labeled backwards. While i was building the boards i tried to install each component on each board at the same time and in the same direction (even non polar parts)

I did a little PMing and got this response to the problem, I'll try and test for it today:
Gotta be a bad resistor around Q8 and Q9.  seems like they are sucking too much current, getting hot, and dropping the voltage.  Check the resisitors between the 30+ rail and those resistors.
 
Well, i checked every value of all the resistors on the board with the BOM from mnats and though i found a couple components that needed replacing, there was no change after replacement/ rechecking the value to make sure it was all there. Anything else i should check out? is it likely that Q8 and Q9 are faulty?
 

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