27" magnavox B&W focus problem

[DaveWM]

focus adj has some effect, but not much, checked the voltages on the two grids pin 3 580 and 4 at best focus 580 with the focus pot turned all the way from ground. the sams says 545 and 365.

I was wondering if something else would cause an out of focus condition?

other than that the picture is pretty good.

[jpdylon]

If it uses a separate focus divider or trippler it may be dying. Also check the focus Spark Gap (if used) for corrosion on the lead that goes to the CRT board.

[kx250rider]

Does the focus get good when you turn the brightness and contrast way down? If so, you either have poor high voltage (or poor B+ causing poor high voltage), or a weak CRT. It's very rare to see a focus problem on electrostatic focus black & white sets.

If it does focus nicely at low brightness, measure the high voltage and see if it drops off a lot at max brightness. A few hundred volts down is probably OK. Also, the picture will bloom or grow in size, as it dims, if the high voltage is falling off. If the high voltage is OK and the picture stays the same size, then unfortunately, you will need to suspect the CRT.

Charles

[DaveWM]

HV 20kv no change when adj brightness

turned down brigness, some improvement.

I think the biggest thing is is seems to have a slight ghost, that makes it look blurry.

played with the adj some more, it seems that I can get better focus now more in the middle. I may be expecting too much. I will try for some screen shots later to show you what I am seeing.

[kx250rider]

Originally Posted by DaveWM

HV 20kv no change

That's awfully high!!!! You did say it's a black & white set, right? I can't remember what it's supposed to be on a 27" Magnavox, but it shouldn't be higher than 13kv in most cases on any B&W set. A color set of that era should have 21kv-25kv usually. If someone has the schematic handy, maybe they can comment.

A good screen shot would help bigtime. If it's a ghost or outline ("ringing"), it's likely an IF problem, such as a bad tube or leaky cap. If it's a smear (like somebody "wiped" a rag across a newly painted picture), it's a video detector problem. A lot of those 60s B&W sets used a 3-legged germanium diode as a video detector, and those go bad.

Charles

[DaveWM]

I just checked my sams 16-19.5 then 17kv so I guess they want it to be 17kv but anywhere from 16-19.5 is ok?.


I will check the cathode current on the HOT. I was planning on putting a fuse in line with that cathode (.2 amp fuse) as a precaution to a burned up flyback. That would do it right?

I figure I can also use the fuse holder as a easy way to check the cathode current (pull fuse, hook up mA meter).

I am going to play with it some more esp now that I have a handle on the vert control for the interlacing (per my other thread, I think this one could use some adj as well).

I will get a screen shot soon.