20-04-2013, 03:09 PM
If you can determine that all is ok component wise in the circuit associated with the 12AU7 then I guess it's down to no or insufficient feedback.
This is usually one of two things, either the feedback loop voltage is not enough or non existant or it is being overly damped.
From what you say the continuity of the primary windings are ok, the 1nf across the primary is ok as is R501.(6.8k) C501 is a ceramic, if it was leaking it would be evident, I have known ceramics to go o/c, might be worth eliminating that prospect.
If all that is ok then it would normally point to damping of the transformer either by internal leakage, overcurrent draw on the secondaries or the dreaded shorted turn on any winding, here's hoping it's not the latter.
Lawrence.
This is usually one of two things, either the feedback loop voltage is not enough or non existant or it is being overly damped.
From what you say the continuity of the primary windings are ok, the 1nf across the primary is ok as is R501.(6.8k) C501 is a ceramic, if it was leaking it would be evident, I have known ceramics to go o/c, might be worth eliminating that prospect.
If all that is ok then it would normally point to damping of the transformer either by internal leakage, overcurrent draw on the secondaries or the dreaded shorted turn on any winding, here's hoping it's not the latter.
Lawrence.
