Anyone ever encounter a leaky Bridge Rectifier?

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beaver

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Apr 16, 2007, 1:59:33 PM4/16/07
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Short version: I have a hum from my Pinbot that may be caused by the
BR. Anyone else seen this?

Long version: When I power up my Pinbot from a cold start, there will
be a soft hum. It goes away after 2-3 minutes. The problem is
completely gone if I swap out the power supply board with a tested
one. I initially thought the caps were dry (higher ripple than
expected), and replaced them. No change.

Then I looked at the waveform on the 12V, and saw that perhaps one of
the diodes on the BR were bad. I checked them with my DVM and sure
enough, one of the diodes showed open. To check my theory, I tacked
on a new BR in parallel with the existing one. Still no change. The
waveform is now better, but still hums for 2-3 minutes.

Currently, once the hum is gone, I can get it to come back by hitting
the old BR with freeze spray. It looks like I have to remove the old
BR, but since it is soldered in so well, I would need a very hot
iron. Before I do that, I wanted to check with y'all. I suspect the
bad diode is leaky, and causing the noisy power supply. After a
while, it warms up (due to the leakage current), and then stops
leaking -> hum stops.

So has anyone had humming due to a leaky BR?

Edward Cheung CARGPB26

Lloyd Olson

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Apr 16, 2007, 2:05:08 PM4/16/07
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Yup, I've had lots of funny things happen with BR's. They don't always just
short out. LTG :)

"beaver" <e...@edcheung.com> wrote in message
news:1176746373.2...@b75g2000hsg.googlegroups.com...

A...@team-em.com

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Apr 16, 2007, 2:18:07 PM4/16/07
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On Apr 16, 10:59 am, "beaver" <e...@edcheung.com> wrote:
> So has anyone had humming due to a leaky BR?
>
> Edward Cheung CARGPB26

Ed,

Try teaching it the words so it won't have to hum from now on. ;-)

Actually, in my experience, failing semiconductors can display all
kinds of odd symptoms. You have already proven that you have a failing
device by seeing changes in performance due to accelerated thermal
changes. I suspect if you hit it with a hair dryer, when cold. you'd
stop the hum as well. The bridge components, their internal or
external connections, any of which could be suspect.

As entertaining as these VooDoo symptoms may be, IMHO it's time to
replace the bridge.

AL CARGPB 33(1/3)
www.Team-EM.com

Pint...@gmail.com

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Apr 16, 2007, 4:16:05 PM4/16/07
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"Peak clipping" is a notorious bad feature on bridge rectifiers. Plays
havoc with reset circuits and low voltage cut out regulators.

Mario
Pinthetic

beaver

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Apr 17, 2007, 8:19:23 AM4/17/07
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Ok, got it fixed, and here it is FTR.

The hum was not caused by the BR. After pulling it out (by clipping
the leads because the joints were so well heat sunk), I verified the
BR was bad and one diode leaking. I then put in a new BR, but the hum
still persisted (!) Just as before, it starts out with it, and goes
away after 2-3 minutes.

The hum was also not caused by the Pinbot not knowing the words (Al's
theory). I will never tell how I verified that.

It was caused by......C8 on the power supply board. This little cap
filters the supply to IC1, the voltage regulator. In circuit, its
value tested to 15 uF (should be 47uF). This low value caused lots of
ripple (10V peak) on IC1's supply, and then a ripple (small) on the 5V
supply. THAT is how the hum got into the audio, not the 12V as I
thought.

Anyway, I replaced C8 with a 100uF unit (the only one I had on hand),
and it is now soooo quiet. I can't even hear any background noise.
Pinbot now comes through loud and clear.

Now that the electronics are 100%, I will do the cosmetics next.
Touchup translite and clearcoat the playfield. Fun!

Edward Cheung CARGPB26

RonKZ650

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Apr 17, 2007, 9:34:36 AM4/17/07
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Ha, I was going to say yesterday that I doubt the bridge will fix it.
I've spent more money on freeze spray and sprayed stuff only to find
the heat sensitive part is not actually the problem. Kind of after the
fact, but I'm glad you got it. a Capacitor causing this makes a heck
of a lot more logical sense.
> > > AL  CARGPB 33(1/3)www.Team-EM.com- Hide quoted text -
>
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