J
JURB6006
Guest
It's me again;
This is not for the faint of heart.
Now it's blowing the LA7838 in about a minute. Same deal, there is some DC on
the output, it has leakage to ground, but it does not discharge the boosted
Vcc. Now I have disconnected the shutdown, and of course the filament supply.
There was no raster even with the vertical running, but it has the CM kit so
whatever isn't right is probably triggering the CM ckt..
Now this is your typical current feedback output compared to a ramp. The ramp
appears about right and the top of the yoke output appears alright on the
scope. Any variance in the impedance curve of the yoke load will cause a change
in the output voltage as the chip tries to match the current to the ramp. Right
?
The output voltage waveform matches the picture on the print except for one
thing, there is a bowtie waveform superimposed on it. Many times this is
normal, however I simply can't know everything so I thought, OK the fault is
not properly bypassing the HF component of the feedback, as such it's screwing
with the slewing rate of the outputs. I added a 1000pf cap to that pin and the
bowtie component is still there. In either case it wasn't that big, I'd say the
HF component peaked at about 15V P-P. Still blew it, no change.
This time I saw it fail, I made it a point to sit there and watch the scope. It
simply became a flatline at about 10Vdc (I think). I didnt see a spike or
anything. Of course at that slow sweep time it might not have been displayable.
I have also changed the NS PIN transformer to no avail.
What are the chances of it being a rogue yoke ? To test that theory I think
I'll run the set with the red and blue yokes, or all of them disconnected from
the vertical. I think I can manage to turn the plugs around to make it so. Of
course the filament supply will remain disconnected and the LA7838 will put out
square waves. If it runs for hours like this, now does that really <u>prove</u>
that it needs one of the yokes ? Before when I looked into the tubes the sweep
seemed pretty normal, without the mirror in place I can't say it was absolutely
correct but there were no glaring abnormalities. I couldn't quite see the top
of the rasters, but the bottom was not bowed or compressed. Even if one of the
vertical windings was a dead short it woudn't do this, but if it was an
intermittent short to the H winding it would.
As of during the day today, I had replies and both have been dealt with, I
changed the "boost" diode and the 11V is present on pin #1 even when there is
no sweep. Thanx for your help, but it didn't work this time. Anyone else
replied since this afternoon, I'll read it in a few minutes.
One thing's for sure, if it blows with the yokes disconnected, the problem is
on board for sure, it should be able to put out those square waves for quite
some time, right ? Or should I find a yoke somewhere and hook it up ? Maybe
both ways.
This thing has good enough tubes and it has been on one of my benches for way
too long so any ideas are appreciated, what would you do ? Thanx in advance.
JURB
This is not for the faint of heart.
Now it's blowing the LA7838 in about a minute. Same deal, there is some DC on
the output, it has leakage to ground, but it does not discharge the boosted
Vcc. Now I have disconnected the shutdown, and of course the filament supply.
There was no raster even with the vertical running, but it has the CM kit so
whatever isn't right is probably triggering the CM ckt..
Now this is your typical current feedback output compared to a ramp. The ramp
appears about right and the top of the yoke output appears alright on the
scope. Any variance in the impedance curve of the yoke load will cause a change
in the output voltage as the chip tries to match the current to the ramp. Right
?
The output voltage waveform matches the picture on the print except for one
thing, there is a bowtie waveform superimposed on it. Many times this is
normal, however I simply can't know everything so I thought, OK the fault is
not properly bypassing the HF component of the feedback, as such it's screwing
with the slewing rate of the outputs. I added a 1000pf cap to that pin and the
bowtie component is still there. In either case it wasn't that big, I'd say the
HF component peaked at about 15V P-P. Still blew it, no change.
This time I saw it fail, I made it a point to sit there and watch the scope. It
simply became a flatline at about 10Vdc (I think). I didnt see a spike or
anything. Of course at that slow sweep time it might not have been displayable.
I have also changed the NS PIN transformer to no avail.
What are the chances of it being a rogue yoke ? To test that theory I think
I'll run the set with the red and blue yokes, or all of them disconnected from
the vertical. I think I can manage to turn the plugs around to make it so. Of
course the filament supply will remain disconnected and the LA7838 will put out
square waves. If it runs for hours like this, now does that really <u>prove</u>
that it needs one of the yokes ? Before when I looked into the tubes the sweep
seemed pretty normal, without the mirror in place I can't say it was absolutely
correct but there were no glaring abnormalities. I couldn't quite see the top
of the rasters, but the bottom was not bowed or compressed. Even if one of the
vertical windings was a dead short it woudn't do this, but if it was an
intermittent short to the H winding it would.
As of during the day today, I had replies and both have been dealt with, I
changed the "boost" diode and the 11V is present on pin #1 even when there is
no sweep. Thanx for your help, but it didn't work this time. Anyone else
replied since this afternoon, I'll read it in a few minutes.
One thing's for sure, if it blows with the yokes disconnected, the problem is
on board for sure, it should be able to put out those square waves for quite
some time, right ? Or should I find a yoke somewhere and hook it up ? Maybe
both ways.
This thing has good enough tubes and it has been on one of my benches for way
too long so any ideas are appreciated, what would you do ? Thanx in advance.
JURB