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Help.  Replacement OPT sqrueeechhh.
Help.  Replacement OPT sqrueeechhh.
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Old 15th May 2011, 12:50 PM   #1
mfaughn is offline mfaughn  United States
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Location: Sylva, NC
Default Help. Replacement OPT sqrueeechhh.

I have a Pioneer SA-400 with a dead OPT (posted schematic in another thread a couple of days ago but mislabled the thread as SX-400). My friend gave me a pair from an old fleetwood console (I think maybe they are actually Grundig). The original outputs measure 27.9:1 turn ratio (and are marked 6.5K on the frame) and the new set measures 30.75:1. That works out to ~6200 ohms and ~7500 ohms, respectively. I thought I'd give it a whirls and just drop one of the fleetwoods right in to see what would happen. I got loud feedback at a particular frequency. Any thoughts on where to go from here? Do I need to adjust the NFB? Can I just lift the NFB and try it out.

Notes: The donor fleetwood used 6BQ5. The patient uses 6BM8.

Thanks for any help.
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Old 15th May 2011, 01:02 PM   #2
SY is offline SY  United States
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Help.  Replacement OPT sqrueeechhh.
First, try reversing the primary leads, i.e., the connections to the plates (and screens, if UL) on one side should be swapped with the other side. You may well be inadvertently applying positive feedback because of the sense of the windings compared to the original.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 15th May 2011, 11:50 PM   #3
mfaughn is offline mfaughn  United States
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Join Date: Nov 2009
Location: Sylva, NC
That did the trick. Thanks.

No back to troubleshooting the original problem...very, very low output.
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