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VR-407 unbalanced supplies

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.

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

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

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Reply to
Franc Zabkar

Have you tested the bridge rectifier?

Disconnect the Regs from the output of the bridge. that remove all doubt with circuit load. Do some more testing.

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Reply to
Jamie

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
Reply to
Steve

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

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

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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Reply to
Franc Zabkar

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.

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Reply to
Jamie

I'll have to remove the bridge as a whole, it's a four terminal package. I was using a Fluke 87 III for the measurements. What you said about the center tap makes sense, that would explain everything, but it measures fine, .9 ohms between either winding & center, 1.8 total. I'll yank the bridge & caps tomorrow, it can't be too hard to find, this is a simple circuit for heaven's sake.

Thanks for the posts, Steve

Reply to
Steve

On Sun, 28 Sep 2008 19:03:28 -0400, Jamie put finger to keyboard and composed:

AIUI, a standard multimeter assumes you are measuring a sinusoid and estimates the RMS voltage based on the peak value. I can't imagine that the OP's waveform would be greatly distorted at a load current of

0.1A. In any case, how can you distort the waveform so as to increase its amplitude?

- Franc Zabkar

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Reply to
Franc Zabkar

On Sun, 28 Sep 2008 19:40:35 -0500, Steve put finger to keyboard and composed:

Could it be that the transformer's centre tap pin has a dry solder joint that closes up when you probe it? I always reflow the solder at coils and transformers for this reason.

- Franc Zabkar

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Reply to
Franc Zabkar

Definitely a loss of ground reference. I've run into this several times lately.

Mark Z.

Reply to
Mark D. Zacharias

Sorry to dissappoint, but it appears it was bad caps. Actually, probably just one bad cap, but I replaced both (I don't have a capacatince meter at home). One is 470uF, the other 2200uF. The

470uF is nested in with a couple of 12ohm 1w resistors, so it was probably running quite warm. Now oth supplies are balanced within a volt or so on the input, and the output is regulating beautifully. Unit powers up & doesn't go into protection. It's an interesting protection circuit they use. They put equal dividers in all three supply rails, the +-65V, +-35V, & +-12V. It looks like they are all summed & fed to a current mirror, and if any drifts off zero the mirror turns on the protection 2 circuit & shuts down the amp. Maybe this is common for amplifiers, I just never dug this deep into the protection circuits before. Usually I deal w/ blown output transistors or amplifier packs.

Thanks for all the help, I really appreciate it.

Steve

Reply to
Steve

I think you probably fixed the bad ground in the process. A bad cap on one rail isn't going to make another rail double with respect to ground.

Mark Z.

Reply to
Mark D. Zacharias

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