Franc Zabkar wrote:

On Sun, 28 Sep 2008 16:27:43 -0500, Steve put finger to
keyboard and composed:


On Mon, 29 Sep 2008 06:51:58 +1000, Franc Zabkar
wrote:


On Sun, 28 Sep 2008 15:11:30 -0500, Steve put finger to
keyboard and composed:


I have a Kenwood VR-407 which keeps going into protection. The ±12VDC
supplies are unbalanced. The schematic for the circuit is posted in
a.b.s. It's just a basic circuit, it uses a center tapped transformer
feeding into a full-wave bridge, with the center tap grounded, with a
positive and negative three terminal 12 volt regulator. No rocket
science here. However, the supplies are unbalanced, pretty severely.
The input to the bridge is 14.24 & 14.15 VAC. The output of the
bridge is -25.28 & 10.23VDC with .685 & .240VAC ripple, respectively.
I pulled the board from the unit and powered up each regulator
individually . With -18VDC in across the negative, there was .1A draw
& -11.90VDC out. With +18V in across the positive regulator, there
was .1A draw & 11.83VDC out, so the reguators are fine and neither has
excessive current draw down the line. When I put an external supply
across both bridge capacitors I saw the same unbalance. With 35VDC
across the caps, there was 25.94 across the negative cap and 8.99V
across the postitive cap. I connected a capacitor in parallel across
both caps individually, but it didn't create any significant change.
Is it probable that one of the caps is bad causing the unbalance?

Thanks, and I'll provide any more information if necessary.

You can interchange the capacitors to see if the unbalance goes with
the cap. However, it looks to me like you may have an open centre tap,
or the trace between the centre tap and the junction of the caps may
be open.

- Franc Zabkar

The center tap checks good, as well as the trace between center tap to
the capacitor center point. Also, the bridge checks out ok. I guess
I need to pull the regs & put voltage across the caps & see if I have
unbalance then. Also, the caps are different sizes, it looks like
they may have designed the circuit to have higher current out of the
12v reg, which makes perfect sense, I guess the caps could have aged
at different rates then being dramatically different sizes. I'll do
some more testing, thanks for the replies.
Steve


How can you get 25VDC across a cap when the AC supply is only 14Vrms?
The maximum voltage should be only ...

14 x 1.414 = 19.8 - Vf(diode) = 19V

True how ever, if he does not have a True RMS meter and the wave form is
abnormal, then I guess it would stand the reason he'd be under reading
the AC voltage?
Which is why, the old fashion analog meter was good out side a true
RMS meter that used thermo technology.

Try desoldering each bridge diode one at a time. That way you'll be
able to test each half of the bridge on its own.

- Franc Zabkar



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