Cleaning out the shed. Old Vizio SV470XVT1A shows orange standby logo. And no other signs of life.
(measurements with DSO doing the math for me)
No toasted components, no visibly bad caps, etc. hence no attached photos (see below)
+5VSB at 5.14 "DC"
+3.3VSB at 3.32 "DC" (sourced by U22)
All other regulators downstream of the switched (U14) +5VSB are out of wack.
U14 is 49536M (dual 30V, 5A, P-channel) wired in parallel as a single switch.
U14 source is +5VSB. Drain shows ~3.2V, with a 300mV, ~25Hz square wave riding atop it. Gate mimics drain. U14 is warm suggesting it's actually passing current with an IR drop across it (recall, source is 5.14V).
Gate should be ~0V -- Vce(sat) of Q2 driving it behind a ~100K R7.
Bad FET? Or, is R7 actually open and the 100K a misread of other impedances in the circuit? (I'd have expected R7 to be closer to 10K)
[I'll tear open another Vizio and see what the equivalent circuit topology is, there, and possibly get a better feel for what "R7" should be]
Does this look like a typical failure mode? I'll look for a U14 cross and replace the thing as that seems a straightforward course of action.
(sigh) I think any future TV purchases will be conditional on manufacturers having schematics available -- even if hard to find!
Thanks!
(Please reply to forum as my PM is munged)
(measurements with DSO doing the math for me)
No toasted components, no visibly bad caps, etc. hence no attached photos (see below)
+5VSB at 5.14 "DC"
+3.3VSB at 3.32 "DC" (sourced by U22)
All other regulators downstream of the switched (U14) +5VSB are out of wack.
U14 is 49536M (dual 30V, 5A, P-channel) wired in parallel as a single switch.
U14 source is +5VSB. Drain shows ~3.2V, with a 300mV, ~25Hz square wave riding atop it. Gate mimics drain. U14 is warm suggesting it's actually passing current with an IR drop across it (recall, source is 5.14V).
Gate should be ~0V -- Vce(sat) of Q2 driving it behind a ~100K R7.
Bad FET? Or, is R7 actually open and the 100K a misread of other impedances in the circuit? (I'd have expected R7 to be closer to 10K)
[I'll tear open another Vizio and see what the equivalent circuit topology is, there, and possibly get a better feel for what "R7" should be]
Does this look like a typical failure mode? I'll look for a U14 cross and replace the thing as that seems a straightforward course of action.
(sigh) I think any future TV purchases will be conditional on manufacturers having schematics available -- even if hard to find!
Thanks!
(Please reply to forum as my PM is munged)
]
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