Help. Replacement OPT sqrueeechhh.

mfaughn · 15th May 2011, 12:50 PM

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

SY · 15th May 2011, 01:02 PM

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.

mfaughn · 15th May 2011, 11:50 PM

That did the trick. Thanks.

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

15th May 2011, 12:50 PM	#1
mfaughn diyAudio Member Join Date: Nov 2009 Location: Sylva, NC	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. -Michael

15th May 2011, 01:02 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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15th May 2011, 11:50 PM	#3
mfaughn diyAudio Member 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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