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Help. Replacement OPT sqrueeechhh.

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mfaughn

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Joined 2009
#1
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.
-Michael
 
#2
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.
 
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