I have a Huang Chang: Protek 6502 20mhz oscilloscope that has developed arcing across D1012 in the power supply next to the fly-back transformer. I found 3, ISS83 diodes had failed in the circuit so I ordered 3 Ultrafast UF4004 diodes by post because I couldn't get the ISS83s? The UF4004s are twice as fast and have the same capacitance according to the datasheets. The thing I would like to know is what could have caused these diodes to fail? I don't want to replace these diodes only to have them fail again? My knowledge of circuit theory isn't up to scratch by a long way? So I thought the best place to ask this is at the AAC circuit gurus hang-out? I bought the scope through the Trading Post online,when it arrived it had a broken plastic voltage adjuster for 230v,120v,100v etc... I put it back together using heatshrink so that its on the 230volts it needs here in Western Australia.(until I get a new one) All the voltages appear to read OK except for the 1900volts that I haven't measured yet? I'm waiting for a high voltage measuring kit to arrive in the post so I can do that. Ive uploaded the service manual for you, its a 3 in one schematic that includes 40mhz and 60mhz models.
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