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Old 17th October 2002, 12:18 PM   #1
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Unhappy Help for Acoustat Spectra 33

I hope someone can help with a problem of a friends Acoustat Spectra 33.

The problem started with a blown 500megohm resistor. This has been replaced but the same channel is now producing a shorting sound when the volume is turned up. It is fine at low volume.

What should I look at?

Thanks Mark
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Old 17th October 2002, 04:00 PM   #2
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What exactly is a "shorting sound"?

Where was the 500M resistor in the circuit- was it at the output of the bias supply?

500M is pretty hard to check with your average, run of the mill, DMM... How do you know it was "blown"?

What was the 500M resistor replaced with?

MR
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Old 17th October 2002, 06:14 PM   #3
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Is the 500M resistor of the same or higher voltage rating of the one it replaced? A quick check would be length of the element... is it as least as long as the original?

cyclotronguy
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Old 17th October 2002, 10:31 PM   #4
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Resistor was replaced with the same type, no worries there.

The shorting sound is like a crackle or like an arc welders rod is scraped over metal.

Mark
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Old 17th October 2002, 11:33 PM   #5
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Noise from the panel itself or the power-supply???

Electrostat panels are like electrostatic precipitators (electronic air cleaners), they attract all manner of air borne stuff. If they get dirty enough they will start arcing.

Could you be hearing the panels arcing over?? Large arcs can burn membranes. Little burns don't seem to cause too much damage..... large ones are a different story.

Cyclotronguy
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Old 18th October 2002, 12:04 AM   #6
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Default Noise and cleanliness

Very high voltages and or high impedance circuits often suffer from leakage currents on the PCB. Did the resistor leave any carbon traces on the surrounding circuit. I would also chip away solder flux around the replaced part/s. Even a fingerprint can cause problems here. If you clean the circuit with any solvents it had better be real clean or you will make matters worse. I would hesitate using solvents unless you are willing to REALLY ( like a dozen steps of solvent and drying) clean it. I went though a couple real character building exercises defluxing a tube phono stage board and getting electrolytic fluid from a failed cap off a tube power amp PCB. Check for other components that may have been stressed during the resistor failure. It would be very interesting to figure out why the resistor failed to begin with as it probably did not fail by itself. Let us know what you find out.

Val
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Old 18th October 2002, 04:29 PM   #7
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Val,

Absolutely, correct!
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Old 18th October 2002, 09:01 PM   #8
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The type of sound you describe could be caused by a number of things. I have experienced such a sound when I had intermittent contact between the step-up transformer leads and aluminum stators. The aluminum oxidizes when exposed to air, and so the electrical connection, which looks fine, might not make good contact. In my case the problem was solved by cranking up the drive to the speaker until the arcing at the poor contact burned away the oxide and allowed clean metal to metal connections.

Another possibility is that a short is occuring inside the step-up transformer. At high levels you might get some arcing which will make nasty noises such as you describe.

The 500M resistor (seems awfully high resistance to me) in your speaker is probably at the output of the bias supply. You can't burn up a 500 Megohm resistor by running too much current through it (you would need mega volts!), so if the resistor failed, it would die due other causes. If it is a carbon film resistor, the film can be destroyed by the high voltage if the film is exposed to air. I suspect the high voltage causes some small corona discharge which ionizes air/creates some ozone and the film gets oxidized. Whatever actually happens, the result is that the rsistance increases until it finally seems completely open. I had a bias supply work for a couple months and then slowly the the sensitivity of the speaker descreased until it finally stopped working. The carbon film resistor at the output of the speakers was the culprit- it had opened up completely over a period of a few months with about 3kV applied.

I don't know if the speakers have dust covers, but if they don't, try vacuuming the speaker with a brush attachment. Pay particular attention to the areas where anything comes into contact with the diaphragm, especially the bottom edge of the driver. Dirt and crud will tend to accumulate there.
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