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Old 19th May 2006, 10:18 PM   #1
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Join Date: May 2006
Default noise (feedback) runaway.. Help

I'm in the process of repairing a Kicker ZR1000 amplifier. Im stuck with a noise problem that amplifies itself (feedback), acting like thermal runaway. Any help would be appreciated.!


What I have already repaired:

Shorted rails:
When the amp was brought to me there was a dead short from rail to rail in the secondary transformer. Its labeled transformer but technically its a mutually coupled opposed inductor. I desoldered the inductor and found where it had arcwelded from coil to coil. From vibration the Bdot output had rubbed against the A coil. I unwound it, repaired the inside coil with varnish, and put heatshrink over the legs of the Bdot leads up to where they soldered to the board. I repositioned the inductor so that it would no longer rub and mounted it down with silicone rubber.

RC filters on the O/P:
There is a series RC filter on both outputs to the signal ground composed of a 4R7 2watt resistor and a metal poly .1uf 63v cap. The resistors and capacitors showed signs of overheat. One of the resistors was reading low (4ohms). Both caps read very low (.05/.04uf). I replaced both ckts.

Open on the Right channel signal lines between crossover card socket and line to the gain pot. Resoldered.


Where Im stuck:
The power supply ground and the signal ground are isolated from each other. They use an NEC photocoupler PS2701-2 for feedback to the SG3525. With the scope I can see SMPS noise on the input channels. If I leave the amp turned on the idle current will ramp from 1.6A up (until I shut it off). On the scope, the noise signal gets larger and larger. I checked the opto-coupler and it seems to read correctly acrossed the diode and PN junction. It does this "noise feedback runaway" with the crossover module installed or not. It is apparent on both channels, though the left seems to be affected a little more. I'm thinking it has to be something in common with both and not their individual preamps.

Any help would be appreciated.

-Ken
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Old 23rd May 2006, 01:52 AM   #2
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Join Date: Nov 2003
Location: Louisiana
Is the surface mount fuse on the bottom of the board open?

The attached photo is from a smaller amp but the design should be the same.
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File Type: jpg kickersmfuse.jpg (60.7 KB, 96 views)
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Old 24th May 2006, 03:41 AM   #3
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Perry,

Nice site btw. I'm going to order your disc. I have a couple "problem child" amps on the shelf that are missing pieces. Those pictures could sure come in handy.

I dont recall if there is a cap there on the bottom, Ill check that out tomorrow at the shop (at home now). There are a pair of large THT poly caps on the topside located between the RCAs and the TL072c.

Further investigations.. and a hiccup..

I was starting to think that perhaps it was a problem with the SMPS side of the equation. Perhaps the capacitors were not able to filter the supply well enough. I also saw a bit more ringing on the mosfets than I would like. They drive the SMPS mosfets directly from the SG3525 on this design. I thought possibly one or more of the internal transistors on the output was going bad. I disconnected the gates and connected 100ohm 3W resistors in place from driver to ground and checked on the scope. Square waves looked great. I checked the oscillator frequency. Approx 58khz which seems a bit low, but I suppose they could be compensating for gate capacitance and lack of external drivers with a slower switching frequency.

Anyway... I desoldered the timing capacitor to read it and melted the poop out of it. Apparently they use a reflow only type of capacitor. Well, now I broke it and I don't know its exact value. I couldn't get a reading on it!! Assuming that the oscillator was probably working correctly before, I reverse calculated Ct using the oscillator hz=58k, rt=4.99K and rd=150 and got Ct being 4.37 which is closer to a standard cap value of 4.7n than to 3.9n. So thats my guess. Any confirmation on that would be great.

-Ken
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Old 24th May 2006, 11:59 PM   #4
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Perry,

There is no fuse there. The layout is similar but its a solid signal ground plane to the RCA ground. I had done a continuity check from the RCA shield to the signal ground. I just checked it again and its good.

-Ken
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Old 25th May 2006, 05:41 AM   #5
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Location: Louisiana
Since my last post, a zr1000 came into the shop. The oscillator on this one is 54k at the timing cap. The cap has no markings and I didn't want to chance pulling it since there are no problems in the power supply. I tried to measure the value of the cap in the board and got a reading of ~350pf. Something on the board was likely causing some error. Doing the math with 54kHz, I get 4.7nF.

If you need to know the value of some of the components on your amps, post the model and part. If I have a photo (or an amp), I'll post the photo or the value of the component.

On the amp I have here, there was a metallic screw in the hole near the secondary center tap. This would create a ground loop if the amp was mounted to a grounded surface. Did your amp have a nylon screw in that location?

On your amp, is there oscillation on the output of the op-amps?

Do you have your power supply grounded to your signal source? On the amp I have here, the secondary is completely isolated from the DC power source. It may require that the signal source ground be connected to the power supply ground.


I'll have the amp for at least another week so let me know if you need any other information.
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Old 26th May 2006, 05:30 AM   #6
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Quote:
On the amp I have here, there was a metallic screw in the hole near the secondary center tap. This would create a ground loop if the amp was mounted to a grounded surface. Did your amp have a nylon screw in that location?
Yeah, its supposed to according to kicker. They ground the chassis to signal ground. It says in the owners manual not to bolt the amp to metal or it will create a ground loop.

Quote:
On your amp, is there oscillation on the output of the op-amps?
If I plug my function generator in, yes I can see the sine wave. I can see noise there also, riding on the signal. With the RCAs unhooked I can see noise there also.

I measured the bias pots (so I could put them back to factory if need be) and then lowered them. The runaway problem is reduced but still happens just not as fast.

I was thinking, it might not be a feedback to the O/P section thats causing it. Perhaps its the filtering of the power supply. As the "noise" is amplified it draws more from the power supply which now cranks out more power which makes the filtering worse.. and so on. Tomorrow Im going to solder in some big caps to see if thats the problem. It has some smaller 1uf caps that filter some of the higher frequencies, but I pulled those and tested them and they were dead on.

I did make schematics of various sections of that amp if you need them for the one you are working on.

-Ken
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