XG-8 Mk3s, Gas problem 2

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Mike Burns2004-02-23 22:23

Paul, I do have the XG8 Mk IIIs now at home. I was able to get each to play independently. There seems to be a fault somewhere in the ST-300A Transformer/Bias supply. One channel will not work. Is there any possiblility of getting a schematic for this unit? I have opened it and there are no obvious burns spots, cold solder joints, missing parts etc. Also, I am having difficulty getting a small enough amount of SF6. The logical places (medical and industrial gas companies) all sell it in 70lb plus cylinders with huge psi that must be reduced by a valve / actuator sysytem that costs as much as the 70 lb bottle of gas. I have been quoted as much as $800.00 for the gas alone. Do you have ANY ideas about how to get this gas in a usable format that will not leave me with a ton of extra gas and equipment? Other industries that use it? Any help is much appreciated. I will post photos of these beauties if I ever get them running.

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Paul Young2004-02-23 22:23

I do not have a schematic for the ST-300A, only the IM-10 (approx. 1 MB, due to size available on request) My memory tells me that the ST300 was very similar. You can use this schematic to create a dwg of how yours is different. Be sure to check for corrosion of the pins and sockets in the connectors at the base of the speaker. Be careful that all HV is discharged - under the correct conditions these power supplies can actually kill you!

Re getting the SF6 - I am not too much help. I do know from personal experience that most gas supply companies in North America prefer to sell gas via a std. practice of using cylinders that are approx.. 5 feet tall and 10 inches in diameter that they continue to own. This means that while you buy the gas, you rent the cylinder for a small fee per year. The fee is usually less than $100 per year. The reason for this is that they are required by law to inspect 100% of all cylinders prior to refilling (and scrap them if they are damaged or too old). This law is to protect them (and you) from being killed by a cylinder that fails due to damage, rust, or fatigue. When you get one of these cylinders, you have to supply the regulator that drops the pressure from approx. 2200 psi to 10-20 psi to work with the gas. These regs cost around $100 min. - getting a used one is a good idea - possibly from a welding supply shop. Make sure that it will fit the threads of the SF6 cylinder - there are different threads to keep people from using the wrong regulators for the gas.

The big cylinders are the common size (called a ”J” size I think - they use alpha characters), but there are also smaller sizes. I own (not RENT - more later) an ”N” size (I think) cylinder of welding gas that is about 18” tall and about 4” in diameter. It was a real test of my endurance to buy this cylinder as no one wanted the business - too much nuisance for the money. I think it cost about $120 to buy - and I have my own welding regulator. I just went from store to store begging and hassling them until someone finally gave in. I don\'t know who else uses it except for the people that make huge industrial circuit breakers that are filled with it.

Good luck. PS - you can run you speakers with just air inside - just don\'t run the bias high enough to arc them.

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