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Audio and Video Talk
The Vintage Audio Section
Quad 303 restoration
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<blockquote data-quote="marantz123" data-source="post: 899864" data-attributes="member: 16849"><p>Your diode tests look ok (4wd volt drop),. The b-e /b-c junction resistance combinations are questionable. The c-e switched resistance, I assume those results are without anything connected to the base, ..if thats the case then it should be open resistance if good, but all these junctions have conductivity. ...which is questionable. Small signal characteristics hfe should be within a decent margin......some measure strangely here. Given this situation, I would pull the transistors/diodes in the driver stages (use heat shunts to protect the these components when desoldering) and check those in reverse bias as well. With all these pulled, then compare resistances (between channels) across the cct at various points for anything irregular. Dont leave any stone unturned..if theres problems, your new power transistors can blow. I suggest putting some current limiting resistors in the collector lines and starting up with a dim-bulb tester when you are ready for switch-on and setup meters to measure biasing before/at switch on. Shouldn't have problems finding 3MhZ TO3 packages...theres plenty to choose from. Good luck and well done thus far !! :thumbs:</p></blockquote><p></p>
[QUOTE="marantz123, post: 899864, member: 16849"] Your diode tests look ok (4wd volt drop),. The b-e /b-c junction resistance combinations are questionable. The c-e switched resistance, I assume those results are without anything connected to the base, ..if thats the case then it should be open resistance if good, but all these junctions have conductivity. ...which is questionable. Small signal characteristics hfe should be within a decent margin......some measure strangely here. Given this situation, I would pull the transistors/diodes in the driver stages (use heat shunts to protect the these components when desoldering) and check those in reverse bias as well. With all these pulled, then compare resistances (between channels) across the cct at various points for anything irregular. Dont leave any stone unturned..if theres problems, your new power transistors can blow. I suggest putting some current limiting resistors in the collector lines and starting up with a dim-bulb tester when you are ready for switch-on and setup meters to measure biasing before/at switch on. Shouldn't have problems finding 3MhZ TO3 packages...theres plenty to choose from. Good luck and well done thus far !! :thumbs: [/QUOTE]
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Audio and Video Talk
The Vintage Audio Section
Quad 303 restoration
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