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Old 12th August 2006, 09:27 AM   #1
316a is offline 316a  England
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Default Low output from one panel

Hi ,
One of my DIY panels has decided to go quiet on me after only intermittant usage . Output from the other panel is much louder . I've given the panel the once over and re-shrunk the diaphragm as a precaution , also vacuumed the panel both sides , checked stator wiring continuity and swapped over the stator connections . The diaphragm EHT is the same as the other panel (shared EHT) so I know there is not an internal short . I am now concerned that the problem is the diaphragm coating , or maybe the diaphragm charge ring which consists of copper tape with conductive adhesive . What further testing can I carry out before a rebuild is in order ?

cheers

316a
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Old 12th August 2006, 11:44 AM   #2
Calvin is offline Calvin  Germany
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Hi,

it could be the coating, or it could be serious leakage (maybe just a fingerprint at the right point).

IŽd suggest You try an indicator circuit first.
It consists of a small flash bulb (example. 90V ingnitiontreshold) paralleled with a small capacitor (foil type, ~100nF and more than 100V) and a series connected resistor of ~10meg.
Connect this indicator circuit between the output of the HV-polarizing supply and the diaphragm connection.
The number of flashes/sec indicates how ogften the diaphragm is charged. It might flash quite often at the start, and when thereŽs a leakage, but ideally flashes only every couple of seconds when leakage is low and everything ok.

jauu
Calvin
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Old 12th August 2006, 12:14 PM   #3
316a is offline 316a  England
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Hi ,
Thanks for the advice , unfortunately I don't have a flashbulb to hand , would a neon bulb be any use ? Earlier I cleaned the sides of the panel with a cottonbud and isoprop , there was slight residue of graphite powder . At power on there was an arc on the outside edge between the stator and the middle point of the spacers which seemed odd as I'd just cleaned the thing . Cleaned again and the arc is gone . I'm beginning to suspect that the problem may be loose particles of graphite powder inside the panel causing a partial short , probably too high for my meter to pick up . I've had this problem before , although the symptoms were a slight hissing sound rather than a loss of output

cheers

316a
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Old 12th August 2006, 12:34 PM   #4
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Hi 316a,

you have experienced the most famous issue with graphite powder. If your stator is not insulated residual powder particles cause local shortcuts.

maybe you try to clean the inside with air pressure. But be careful not to destroy the mebrane. Graphite powder should be blown away even at moderate pressure.

BTW: i cannot understand, that people still mess around with graphite, while excellent and reproducible coating is avaiable !!!!

Good luck, capaciti
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Old 12th August 2006, 01:43 PM   #5
BillH is offline BillH  United States
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Hi 316a. Have you checked you HV bias supplies to make sure they're both still working?

Quote:
would a neon bulb be any use ?
Yes. It works very well.
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Old 12th August 2006, 01:57 PM   #6
316a is offline 316a  England
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Hi ,
Each bias supply is -2400V , that was the first thing I checked and this is mentioned in the original posting . This afternoon I've blasted a bit of compressed air through the panels , vacuumed and retested but the same problem exists . I think that the problem is either the diaphragm coating or a very remote possibility , the contact ring . The diaphragm contact is made with thin copper tape with conductive adhesive and unfortunately I've run out of the tape which rules out a rebuild today . This is a real pain as recently I not only built a dedicated direct drive amp but also new frames for the panels , also another dedicated amp to drive Jordan JX150 running as bass-mid. Up until then the panels worked fine . Doh !

cheers

316a
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