CapnDenny1
Student Of The Blues
OK, time to do something else for a bit. But here is what I found out today. It is still a mystery, but I am not crazy.
I put a dmm on the hv secondary and set it to AC volts. When I powered it up it was rock solid at 250vac.
I put a second DMM on the B+, and while I watched the AC hv stay constant I could see the B+ steadily rise.
I put a scope probe referenced to chassis ground on the point where the hv secondary connects to the diodes. One channel on each of the leads. I could get a similar result by adding a DC offset into the output of the transformer output in my pspice simulation. I captured it after a few seconds and then waited a few more seconds and halted the scope and took a picture. You can see the earlier voltage levels on screen in white.
IMG_2806 by Dennis Kelley, on Flickr
The scope trace in yellow was the one connected to the same side as the bias circuit coupling cap.
Here is the schematic of my pspice sim.
bias sch by Dennis Kelley, on Flickr
And what the scope traces are supposed to look like.
bias good sim by Dennis Kelley, on Flickr
I reasoned that what appears to be happening is the AC hv output of the transformer, while staying at 250 vac between them, is getting a DC offset. So I added a cap on the bias ckt side, and it kind of does it. Not quite. But notice here the pspice output looks more like my scope shots.
bias sim by Dennis Kelley, on Flickr
Of course that cap does not exist in the circuit, or at least it isn't supposed to. I am thinking perhaps the power transformer has some sort of issue?
I put the variac on it, thinking perhaps it only happens at high voltage. But not, it drifts the same way with the AC line at 60 vac. It does stop eventually, at low voltage and full voltage.
So I am going to go stew for a bit, maybe play guitar or watch a little TV. My brain is tired of trying to understand things that don't make sense. You would think I would be used to it by now? But I just can't stand not knowing why.
I put a dmm on the hv secondary and set it to AC volts. When I powered it up it was rock solid at 250vac.
I put a second DMM on the B+, and while I watched the AC hv stay constant I could see the B+ steadily rise.
I put a scope probe referenced to chassis ground on the point where the hv secondary connects to the diodes. One channel on each of the leads. I could get a similar result by adding a DC offset into the output of the transformer output in my pspice simulation. I captured it after a few seconds and then waited a few more seconds and halted the scope and took a picture. You can see the earlier voltage levels on screen in white.
IMG_2806 by Dennis Kelley, on Flickr
The scope trace in yellow was the one connected to the same side as the bias circuit coupling cap.
Here is the schematic of my pspice sim.
bias sch by Dennis Kelley, on Flickr
And what the scope traces are supposed to look like.
bias good sim by Dennis Kelley, on Flickr
I reasoned that what appears to be happening is the AC hv output of the transformer, while staying at 250 vac between them, is getting a DC offset. So I added a cap on the bias ckt side, and it kind of does it. Not quite. But notice here the pspice output looks more like my scope shots.
bias sim by Dennis Kelley, on Flickr
Of course that cap does not exist in the circuit, or at least it isn't supposed to. I am thinking perhaps the power transformer has some sort of issue?
I put the variac on it, thinking perhaps it only happens at high voltage. But not, it drifts the same way with the AC line at 60 vac. It does stop eventually, at low voltage and full voltage.
So I am going to go stew for a bit, maybe play guitar or watch a little TV. My brain is tired of trying to understand things that don't make sense. You would think I would be used to it by now? But I just can't stand not knowing why.