Roundie left edge purple

[DaveWM]

no effect at all, at least it still works.

Still under scanning.

The adj calls for zero beam current, adj horz bias for 25.3kv.

Not sure how to get to zero beam current, what I have been doing is turning brightness all the way down, screen drives all the way down, and CRT bias all the way down to get as dark a screen as possilbe. With the Horz bias turned fully CW I can get the cathode current to 220ma which is in line with the sams. I am reading this with a simpson 260 bypassed with a .1uf cap.

when I turn up the brightness to a normal viewing setting (screen up and crt bias up) the cathode current goes up to about 320ma max (it varies with the signal).

that just seems high to me. AND I still lack full horz sweep.

voltage reading look good now that I am using my VTVM, the DMM was just not giving me good readings. There is a 4200 ohm 2watt resistor in the yoke (Y-107) but I cant imagine that being an issue.

Next up will be the drived voltages to the HOT on the scope.

[DaveWM]

I read somewhere that you need to make sure your equip is accurate, well the VTVM was way off, I adj the calibraion.

I get about -40v on the HOT grid, should be -60 (yes it was that far off).

My B+ voltages are off some but no more than 10 on the various B+ amounts.

Considering that it should be able to fill out the raster at 110 and its short at 120 makes me feel its GOT to be the horz drive.

I have tried mucho HOTs all about the same all NOS.

So after much fiddling I will get the scope out, set it up (have to set the voltage so I can measure the P to P) and see whats going on.

IF the grid drive is weak I will have to see why. I have tried half a dozen horz osc tubes, replaced all the coupling and bypass caps, and any out of spec resistors. The voltages all look right. the only thing I am not clear on is a 18k and 680pf 5% resistor in series that come off the plate of the horz osc. I assume this is some kind of wave shaping network.

[DaveWM]

FIXED, AND i AM AN IDIOT


after consulting with some folks over at fun with tubes, I check the line voltage AT THE CHASSIS, turns out it was low, I still had the set on the variac and thought I had 117v at the chassis, wrong, voltage drop across the line/switch must have been doing me in.

The orig problem of under scan was prob fixed when I replaced the osc driver tube, as this improved the scan width, but by then I had the set on the variac.

[DaveWM]

added some "active cooling" as a precaution on the fly, will monitor its effect on overall temps.

[DaveWM]

Not so fast. the concern for the hot fly had me recheck the HOT grid voltage, should be -60 was -40

I scoped it, 120v pp correct waveform, but should be 190vpp

checked the osc tube all forms and PP looks good on that tube.

on the osc tube a 6BL8 screen volts look good cathode voltage looks good so short of a bad tube I cant see why the wave form would be down (but I will try subbing some tubes to see the effect anyway).

[DaveWM]

I am going to check the wave shap network cap, its a 680pf tube precision cap, comes off a resistor that is at the plate of the osc tube. If leaky I assume it could drag down the output.

I subed some other tubes (all test good on my hickok 533) for the horz osc, best I could get was 140pp. Low drive would sure explain the hot flyback and the fact that I have to turn the horz out bias all the way down to get the cathode current in check.

I have a local tube guy I may go to his place and check a bunch of nos 6BL8's and pick the strongest he can find (if the 680pf is ok).

[DaveWM]

hmmm lifted one leg of the horz wave form shaper network, very leaky and would not even open the eye of the cap tester at all. going to remove it completely and check again. This could be the answer i hope.