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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Help: NEC PJ STK conv IC fault
Problem is blowing Red channel output transistors in the STK392-040 IC
in the convergence board of my 6PG. The first IC blew a short to both +/- VCC pins, the replacement (still in place) to the + VCC rail. This is then burning the Neg FB / conv yoke return resistor. First thing I did was run Red conv yoke from Green channel with no obvious problem. Simple circuit and I have been right though it unable to find anything wrong so far. The red signal has DC offset (normal) and Red VCC is separate, so was wondering if partial short might affect Red worse, so have turned attention back to yoke. Measured resistance at .2 ohm which is same as other yokes, but at limit of meter range and I know its the Q that counts anyway. Baffled - any ideas - I have spare yoke - assume I can run it off the tube. I noticed preceding weeks to the original fault, the PJ came on with a slightly blooming blank blue screen for a while before operating as normal, this might be a red herring though. PS. resistor I mentioned is axial 1/2 inch long of 3W, sky blue - am I right in thinking it is std metal oxide type? TIA. Clive. reply by NG. |
#2
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The red signal has DC offset (normal)
What do you consider normal, remember this is an inductive load. One volt might be 5 amps, at full Vcc for that side of the IC. Anything over =BD a volt is too much, but would not fry it immediately. Weeks maybe. If it's 2 volts it needs fixed, 2 volts will fry it in minutes This is not good. It's about time you get the pinouts and start chcking it from there, disconnecting certain things. Don't float an input pin to these ICs, short to ground or a resistor to ground if you need to, floating won't work in some ICs and I just don't remember if those ICs are OK with a floating input. I think not,, but I'm not sure, gotta be grounded or something. Make sure to disconnect it and not blow what's driving it, some of those drivers are really low impedance and can fry. Not usually but it can happen. JURB |
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