GE roundie

[kvflyer]

I too got a Heathkit vectorscope from the auction site. It came with the manual and I have not used it yet. I got it for the minimum bid of something like $10.00 plus very little shipping. I could not pass up the item for that price even if it sits on the shelf. I would guarantee that it was almost never used (if ever used!).

[DaveWM]

they are fun to play with, have a built in dot/cross/grey scale/purity setting, and shorting switches for the grids. the main thing is it give you a quick check of the output from the chroma demodulators, so if they are not right you don't have to spend anytime messing with CRT setup trying to fix problems that are not real.

[DaveWM]

had a little set back, fly got hot, started to lose sweep and HV, turned it off, fly very hot.

checking a few things, the line voltage is high (125vac) and I did not yet switch to the high line tap, so that will be 1st. double checked the HOtube, about 215ma, at 115vac its about 200ma, playing with different 6JS6s got it down to about 195ma @115vac.

the eff coils slug is way high (mostly out of the form) to get a dip. I am going to replace the cap and maybe the coil associated with that circuit. and will replace the boost filter caps as well (if I did not to that already).

the service manual has a not about changing the HOtube screen resistor to 18k from 13k so will do that as well.

there is a diode that is connected to the aux winding on the bottom of the fly (agc/burst/hor afc... winding) thru a cap. the neg of the diode connects to the grid of the HO tube thru a 1.5meg resistor.

My guess is this is a safety circuit that cuts off the HOtube in case of hv runaway, sort of like the RCAs that use the neg voltage from the blanker tube as a safety in the same way. Besides the diode there is a cap and a couple resistors. The grid bias of the HOtube was a bit high (too low a neg) by about 10v (was -56vdc vs -67vdc) could be the vtvm calibration I suppose but that is big enough to look into. I will have to get the scope out to be sure. The scope is the only way to see if the neg bias is correct when that backup safety circuit is used (the neg reading is a composite of the two, the rectified drive from the horz osc and the rectified drive from either the diode mentioned above or the blanker tube).

[DaveWM]

nothing definitive, the horz osc plate load resistor had drifted up some to 92k shb 82k not much but then again, maybe just a tad less drive. While I was in there I went ahead and replaced the .01 coupling cap, it was a small brown drop rated at 400v, it sees a pretty good sized pulse so I put in a .01 1kv dipped. those boost caps are all disc, I really doubt they are leaky, and they had the spark gap cardboard spacer on the lead that I did not want too fuss with.

the diode used in the bias circuit of the HO tube tested fine, and the resistors that form a voltage divider (1.5 meg and 12 meg) were right on the money. There is a small 680 pf disc cap that supplies them from the flyback, it looked fine I just can see it being bad, will leave it for now.

did switch to the HV tap, so before doing anything else will see what it looks like now (couple cap,plate load resistor, and PWR Xformer tap).

I still don't like the looks of the eff coil slug being up so high, but I dont like to make too many changes at once without testing.

Oh forgot the 18k I will have to get one of those from the surplus store tomorrow, I want to to that before testing again as the service man said that should always be replaced.

[DaveWM]

found a sams on it (I was working from the service manual) so started with the resistance chart on the horz osc, hmmm another plate load resistor off a good bit, this time a 12k that was down to 8.8k. found a 12k that tested 13k in my stash of 2 watt carbons so I put that in. It also called shows a cathode current of 215ma, which is where I was, just seems a tad high.

I am starting to wonder if something in the HOTube started to short or get gassy or some other failure that may have pushed it over the top when the melt down started. oh well anyway press on and see what happens.

[DaveWM]

I put a 3.3k power resistor in series with the existing 12k, put it all back and touched up the eff coil again, now its about 187ma had to switch to the strong HOtube as the weak one resulted in too narrow a sweep (and about 165ma).

I have to double check the HV and make sure it regulating, the color seemed a bit off and the pic was not as good. I suspect my HV is down. If so I will undo the screen resistor change and check again.

[DaveWM]

HV holding right at 25kv regardless of brightness or scene changes. The tint is a bit odd, it seems to change a bit from left to right. I suspect its the odd ball GE color circuit (no color osc). Maybe the lower voltage from the tap is effecting it, but I did not notice it on lower line voltage. hmmm have to look at the schematic some more. Maybe a weak burst amp or something like that.

The eff coil is still WAY out nearly out of the form to get the peak. I did not change those caps (the one across the coil and two that go to ground from each end of the coil).

I noticed a very slight foldover on the right edge that was not there before. I was going to adj the coil just a bit to see if I could correct that.

Oh one more thing, the HOtube grid voltage is now about -64v increased the neg voltage about 8v.

what I may end up doing is wiring up the chassis so I can switch out the extra resistance on the screen. I would like to add a low/high switch on the back, I have this on some of my sylvania sets (for the PWR xfromer tap).