Tuesday, December 06, 2011

Circuit board detail

Here's something that has been giving me grief. I had a computer come into the shop on multiple occasions with related failures, most originating with the power supply. It would cycle power on and off rapidly, flashing the front panel lights just like a Christmas tree. Now, I'm OK with festive and all, but an aircraft computer isn't supposed to do festive.

Replacing the usual suspects didn't help. There are four big electrolytic capacitors on the power supply board, and they're usually responsible for these problems. I replaced the three easy ones, but the largest is a 1.1 farad capacitor (think BIG!) and it's held tightly by a large piece of foam tape. To remove it, I heat the board to 140F in an industrial oven, and then twist the cap off the board with a huge pair of Channelocks. It's a PITA.

But there's one other component that causes power supply cycling, and that's the pulse width modulator chip. In a nutshell, a computer power supply is closely regulated by the PWM chip. It compares the output voltage against a reference, and alters the current on the input to control the voltage on the output. It switches on and off at a rate just above audio frequencies.

The chip is more readily changed than that big capacitor, so I decided to do it first. Yes, I'm basically a lazy technician taking the path of least resistance. I cut two of the chip's eight leads - and the chip fell off the board! That's a photo of the board and pads up above. The solder didn't wet to the pads. Notice the dull copper that indicates some tarnish and corrosion.

I applied flux and tinned the pads before installing the new chip. In the morning, I'll investigate the surrounding circuitry for any loose connections similar to this one. Chances are, the board was contaminated when the chip was first installed. Nearby ones may be contaminated too.

This may seem a small thing, but finding and eliminating an intermittent is cause for celebration.


Blogger lemmiwinks said...

Wasn't a 555 timer was it Ed?

Congratulations on finding it though. That type of troubleshooting, while potentially very frustrating is really rewarding when you get it.

3:31 PM  
Blogger Ed W said...

Nope, it's a pulse width monitor maybe an eighth the size of a postage stamp - or a US stamp anyway. The circuit feeds DC into one side of a transformer, with a transistor switch on the other side. The PWM turns the switch on and off, putting pulses of DC through the transformer primary. This has the same effect as AC, causing an expanding and collapsing magnetic field to induce a voltage in the transformer secondary winding. Five volts is fed back to the PWM chip, which compares it to a reference voltage from a zener diode. If the voltage goes too high, the PWM shortens the pulses and decreases the current in the primary. When the voltage goes too low, it widens the pulses to supply more current and lift the secondary output voltage. It does this thousands of times per second.

I'm training another tech on this unit, so I have my teaching hat firmly in place today.

5:03 PM  
Blogger Yokota Fritz said...

I think the largest capacitors I've ever seen are in the 50,000 uF range, so that 1.1F must be monstrous!

1:35 AM  
Blogger lemmiwinks said...

Oh, so the whole PWM is condensed into a single IC, neat! I've only ever built clunky PWM circuits based around the 555 :-)

3:45 PM  
Blogger alansaki said...

Thanks a lot for share superb info and details about PCB. I like to bookmark this post. Circuit Board

12:32 AM  

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