Weak CRT or not?
 

Hi Everyone,

I've written on here about the Magnavox CT-220A that I have up and running. I've changed all of the capacitors, some resistors here and there, and the video detector diode. I'm pretty impressed. The set is stable and plays fairly well. The picture tube, I think is weak. I don't have a CRT tester but I really can't turn up the brightness that much. The contrast has to be full on, and when I turn up the brightness, the focus disappears, the picture blooms while the image enlarges and the high voltage drops. I've changed the horizontal output and high voltage rectifier with the same results. Should I just assume the tube has low emission?

Bloomig wouldn't be caused by a weak CRT. Rather, the HV supply is being lugged down trying to supply beam current to the CRT. Normally, the HV rectifier tube is the culprit. Is the replacement rectifier known good? If so, it's possible the filament isn't getting hot enough. It should be glowing dull orange. Can you get a visual on it in a darkened room?

RE:CRT Weak or Not?
 

I can’t confirm for sure if the replacement HV rectifier is better. I really should invest in a tube tester. Here’s some pictures with the brightness all the way up and back to where I keep it. One can see that contrast and focus disappears as the brightness is increased.

I guess you don't have a HV probe :o

I have a CRT probe. It measures just under 7kv when the brightness is turned down. It drops in half with the brightness all the way up. Riders calls for 8kv.

My Admiral wouldn't focus no matter what, and it turned out to be the focus pot was open in the middle. Because the focus coil is sort of in circuit with everything else, it threw everything else out of whack. I wonder if your set has a similar problem.

NOT good :/


might have a bad resistor in series with the CRT anode output and the HV rect, that seems to happen a lot :(

I checked the resistor but in circuit. I measured 1 meg.

Is the Dag coating on the bell of the tube grounded?
Does it have a Doorknob cap in the HV?

Even 7k sounds low for a 10" or 12" tube, 9kv is more often the spec.

There is a doorknob cap. It reads 500pf. The dag is grounded.

If the doorknob were leaky, it would drag down the high voltage irrespective of the brightness setting.

As mentioned previously, could you darken the room enough to get a visual on the HV rectifier's filament? It should be glowing dull orange, and if it's not glowing, there's your problem. If it is glowing, possibly the replacement rectifier is bad.

Hi,

The rectifier filament is glowing. I ordered another one that is supposed to be a JAN new old stock from Sylvania. I'll wait until it arrives and go from there.

A weak HV rectifier is the single biggest reason TVs bloom at middle brightness in my experience. Hope your new tube fixes it.

Does the flyback run hot after about 20 minutes?

John

Hi John,

The flyback doesn't run hot. Interestingly this chassis has an interlock; if the high voltage cage is opened the TV power is disconnected. A tab on the cage cover slides a switch that's in series with the line. Were they protecting us from the high voltage?I can't imagine that the rectifier would pump out soft X-rays at 7kv?

Probably HV protection. My CTC4 shorts the HV if the back is removed. The 21CT55 would short the HV if the cage was opened...

That 1M resistor should be shorted or replaced to prove its innocence.... Those old carbon comp resistors can do strange things at HV. For instance the carbon resistive element can go open but arc over at the open can make a faux 1M resistance that a meter sees yet it can't support normal HV current. May not be the case but better to rule out than have hanging...