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Old 8th March 2010, 08:43 PM
Richard Richard is offline
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Join Date: Dec 2005
Location: Notts
Posts: 5,203
Default Re: Phono 3 with 5751 valves

Originally Posted by bearded fiddler View Post
Hi Richard,

I need some assistance with the trim caps. I tried to adjust them, thinking there would be end stops, but now it turns out the trimmer can be turned indefinitely, so I don't know where the trimming range starts and ends. I'm looking for the middle of the range, to adjust for ECC83's. It will be clear by now I don't know anything about the blessed things. When using 5751's I understand the trimmer should be a little past the middle of the range. Your help will be much appreciated.

Hi Jan,
Check the pic below carefully.
See the slightly protruding flat end of the white ceramic base.
See the brown mica disc with silver semi-circle that the screwhead is soldered to.
When the silver is turned to be fully at the flat end of the ceramic base that is max cap.
When the brown is at the flat end that is min cap.
When it is halfway with the screwslot aligned leg to leg it is half cap.
Thus, one complete turn of the screw will go from max through halfway to min then through halfway again and back to max. (So the full range is covered in half a turn of the screw, either way, and no end stop so no harm done.)
The full range measures 5-70pF.
The pic is set for 5751 at about 40pF (see the silver is more nearly at the flat end so is greater than half cap).
For Ecc83 you need about 20pF on this cap so wind it to a similar position but with the brown semicircle nearest the flat on the base to make less than half cap.
Hope this makes sense!

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