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QMX Failure #qmx

 

Hello,
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My QMX had low output power (2W vs original 5W), so I have replaced the BS170. When I switched on, high supply current and a burning smell. I have detected a failure in one of the new BS170 (Chinese supply ... ) . I removed all the BS170, and the two power supply boards, but now I have a short between Vdd and ground (Vcc is OK). I started to desolder the VDD pin of IC501, IC502, IC407 ...... no change .... Before starting to desolder the CPU, have you perhaps an idea ??
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Best 73,
Nico
 

I'd start by checking the parts you originally replaced - the BS170s with Chinese ones.? We recently had a report that Chinese BS170s had the source and drain pins reversed.? Even without that there's no guarantee that they are real BS170s, they could be some random small signal N type MOSFET, even some random TO92 device.
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Naturally run at reduced voltage and a current limited supply.? The disadvantage is that this doesn't allow using device destruction as a diagnostic tool, the advantage is that failed devices are less likely to take out their companions.
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Chris
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