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Quiescent current no less than 670mA

beta22

Quiescent current no less than 670mA

Postby strogg » May 19th, 2019, 3:53 pm

Hello. I'm having trouble with one of the boards I'm building. I have 4, of which 3 are setup and tested to be working. The fourth one (scarily the first one I tested) didn't turn out so well. I was going through the initial setup, got the power plugged in correctly, got VR1 dialed into 4.5V, but somewhere, something screwed up, because when measuring R34, I would get no less than 315mV with the trim pot dialed all the way down. All the components look OK, but its difficult to confirm that 100% due to the small print and rotated components. Hopefully someone has seen this before and knows which component(s) to look for. And yes, as you would expect from that amount of current, the heatsinks get piping hot very quickly.
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Re: Quiescent current no less than 670mA

Postby amb » May 19th, 2019, 4:25 pm

Check to make sure R25, R26, R27 and VR2 are the correct values, and the trimpots are installed in the correct orientation. Also make sure you have the correct MOSFETs installed (i.e., don't mix a IRLZ24N in with a IRFZ24N).
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Re: Quiescent current no less than 670mA

Postby strogg » May 19th, 2019, 8:58 pm

Thanks, amb. It indeed was VR2. I accidentally swapped VR1 and VR2, which makes total sense in hindsight. Derp. I'd feel stupid about that, but that pales in comparison to the next thing that happened.

I swapped the two pots back, and that fixed everything for all of 1 minute. Unfortunately, something slipped, a short occurred on the bottom side of the board, and Q26 exploded in a very spectacular manner. As expected, with Q26 gone, the whole board didn't function at all. I replaced Q26 and got the input side to somewhat work again, but something else is still dead on the board, as the output side is not functional. I'm suspecting Q20 also died in the process. I'm going to try replacing that tomorrow night. If that isn't successful, I may end up cutting my losses and ordering another board. Oh well. Mistakes happen.
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Re: Quiescent current no less than 670mA

Postby strogg » May 27th, 2019, 7:47 pm

Update for all who may find this helpful or interesting.

Q20 wasn't the issue. I swapped it from another board, and the entire board was still nonfunctional. Rather than troubleshooting, ordering parts, troubleshooting more, ordering more parts as something else on the board breaks other parts, I decided to cut my losses and purchase another full board of parts. I assembled it and it worked flawlessly.


At that time, I noticed something interesting happen. Two of the other boards started developing symptoms of incredibly bad dc offset that couldn't be adjusted. I checked VR3, and it was functioning properly. All of the mosfets were functioning properly based on the 16Mohm resistance test. Voltage along the diode points were fine. Then I realized on both the offset was probably coming from the input side. There was odd voltage readings along Q5 and Q6 on both boards. Voltage on CR1 showed higher than normal on both. Odd. After eliminating everything else I could, I went to my bad board (now my spare parts board) to grab all the BC550Cs and proceeded to match them. I've read on here that sometimes they may be out of spec from the factory, so I may as well match them if I was going to check. One of the BC550Cs from the spare parts board was shorted. Well, that explains that. After matching the remaining ones, I pulled Q5 and Q6 from the malfunctioning boards to test them. Well, there we go. One of them has an Hfe well out of spec. Another from the other board was just dead. No current was going through it at all. So both got chucked. I put in the new BC550Cs and presto. Back to normal.

The boards are now mounted in the enclosures and working flawlessly. There's still a lot of work to be done, though. I'll post pictures in a new thread once completed.

Thank you Ti for all your work in the community, along with everyone else who posts on this message board. The wealth of information here is amazing, and I've learned a lot in the process.
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Re: Quiescent current no less than 670mA

Postby amb » May 27th, 2019, 10:12 pm

I'm glad the problem is solved. Looking forward to your pictures!
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