Yamaha YST-SW800 active sub help needed

Hi everyone, I need to get in touch with someone who's familiar with Yamaha's active sub model YST-SW800. Well, this unit features a switchmode powersupply among with a high power class D output stage. It came into my repairshop with blown switching transistors in the primary (2SC4140). Replaced those and powered it up to find a high pitched self-oscillating sound coming out of the unit. Don't know if the sound comes from the smps or the speaker itself. Didn't dare to run it for more than 2-3 seconds in case I blow the switch.transistors again. So I'm looking for any advice regarding what could be the cause of the smps damage, like service bulletins or upgrades, or any explanation why the unit oscillates like it does. Any advice is warmly appreciated, best regards Stefan

Reply to
Stefan Toftevall
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Check for any high ESR or shorted caps. These do cause problems with the output drive tansistors.

The switching supply transormer may be damaged if the switching transistors were shorted. I have seen this before in other similiar units.

Unless you are an authorized service rep for these products, I doubt Yamaha will sell you the parts or service schematic. You may have to send them the unit to have it serviced. If the unit is not a very expensive one, it will pay to replace it.

Jerry G. ======

Reply to
Jerry G.

Once again, Jerry, you are giving advice based on general assumptions on equipment you obviously have no specific knowledge of and misleading another poster. Yamaha sells manuals rather inexpensively and sells parts to anyone. Dealers and ASCs get discounts, and only ASCs get access to training info and tech support, but you can buy nearly any of their audio manuals for about $18 retail and their people are very helpful about looking up part numbers.

It is unlikely that the unit is not worth repairing as it was one of Yamaha's more expensive subs.


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Leonard Caillouet


Mark Z.

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Mark D. Zacharias

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