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September 30, 2020, 06:38:25 PM

Author Topic: Voltage check isn't checking out  (Read 260 times)

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Online Paul Birkeland

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Re: Voltage check isn't checking out
« Reply #15 on: September 13, 2020, 10:54:16 AM »
So, from your photos I can tell that you added solder from the top side of the board.  That's not really my recommended approach.

One of your R2 resistors has a similar solder joint that looks insufficient.  Please try reheating these from the bottom side of the board, all the joints.  You don't need to add solder from the top side.
Paul "PB" Birkeland

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Offline brianbuntz

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Re: Voltage check isn't checking out
« Reply #16 on: September 26, 2020, 01:36:47 PM »
Hi there. I added solder from the bottom, but added a little extra on top. Maybe that wasn't necessary.

Anyway, the problem is likely not the solder joints but one of the 2.49k Ohm resistors. I went through checking all of the resistors and the one on the A side read 7.3 Ohm. It still has the right color - red, yellow, white, brown, brown. Going to order a new one and swap it out and see what happens.   

Online Paul Birkeland

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Re: Voltage check isn't checking out
« Reply #17 on: September 26, 2020, 02:06:03 PM »
Or it could be the TL431 transistor that has two legs across that resistor.

In either case, there will be an associated cause with that failure, and not finding the cause may lead to repeated destruction of whatever part is no longer in spec.
Paul "PB" Birkeland

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Offline brianbuntz

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Re: Voltage check isn't checking out
« Reply #18 on: September 26, 2020, 05:21:05 PM »
So I removed the resistor and it measured correct when not in the circuit, so the TL431 is definitely a suspect.

Offline brianbuntz

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Re: Voltage check isn't checking out
« Reply #19 on: September 26, 2020, 05:37:29 PM »
So the 431 transistor has a short circuit according to a tester I have.

Re: what might be causing it to short, I don't know. I went through all of the instructions this morning again, reflowed solder everywhere and performed all of the tests indicated in the manual and it passed everything up until this step.

Online Paul Birkeland

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Re: Voltage check isn't checking out
« Reply #20 on: September 26, 2020, 05:42:11 PM »
A shorted TL431 won't allow you to pass the voltage check on the PC board it's mounted to.

They are somewhat sensitive little components.  They will last a long time in a properly built circuit, but are often the canary in the coalmine when there are issues.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man