Bottlehead Forum
Bottlehead Kits => Stereomour II => Topic started by: CalPolyDude on April 10, 2021, 09:41:35 AM
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Schematic states voltage at Terminal 7 (right audio channel) and Terminal 14 (left audio channel) should be somewhere between 170 VDC and 230 VDC. I'm reading 206 VDC at Terminal 14 (which is output OB on the printed circuit board), but reading is 245 VDC at Terminal 7 (output OA on the printed circuit board). I've checked all relevant solder connections and they appear to be "good," but am interested in ideas as to cause for the high reading. Can't proceed with hum balance testing until this issue is resolved.
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What's your IA voltage? Does this voltage move when you switch the tubes around?
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IA and IB are both 405 VDC, so DC supply is good. FYI, house voltage is 122 VAC. Switched the 12AT7 tubes per your suggestion and tested again. Now, OA (Test Point #7) is 219 VDC and OB (Test Point #14) is 201 VDC, so it appears that I'm good to move on to hum balance, correct? Should I be concerned that the voltages differ by about 10 percent? Is that "normal?"
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That voltage variation is OK. You may well have a solder joint that isn't 100% that initially caused the high voltage at terminal 7. It's also very common for the middle solder joint on the MJE5731A to be a little difficult to flow out all the way and often more heat is needed there.
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Touched up a few points in the relevant circuitry, even though they looked good. Now reading 210 VDC at Terminal 7 (Right Channel). For reference, 201 VDC at Terminal 14 (Left Channel). I expect that being within 5 percent is okay, so I'm moving on to hum balance. Thanks for your advice/assistance.
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even though they looked good
Looking good without actually being useful really only works for Instagram influencers. Good on you for reflowing the joints to be sure they were functional.