Question about White dot

[bandersen]

Originally Posted by old_tv_nut

A design that allowed spot burn should have been corrected quickly, but sometimes a problem escapes engineering until the last minute, or happens only in marginal cases where all the component variances are in the wrong direction....

I think it's more an issue of the image collapsing to a dot 100s of times and the damage slowly accumulating.

I've mostly seen it in 21" sets from the late 1950s. CRTs like the 21CEP4, 21ESP4, 21FDP4, etc.

[old_coot88]

Originally Posted by Aperna1985

Would capacitor 36 and Resistor 44 be my bleeder circuit?

If by "bleeder" you mean High Voltage bleeder, it would mean adding it between the CRT HV anode and ground. It would be a very high value resistor (tens of megohms) such as used in a HV probe (as Electronic_M mentioned in post# 6).

A 'spot killer' circuit would work by making the CRT control grid (pin 2) more negative relative to the cathode (pin 11), thus turning off emission. Same result by making the cathode more positive than the grid (as Notimetolooz mentioned in a previous post).

[Aperna1985]

Originally Posted by Notimetolooz

No those are in the focus network.
The grid and cathode voltages determine whether the CRT gun is emitting the beam. To cut off the beam the grid would have to be more negative than the cathode.
You didn't say that you have finished re-capping the set. I know opinions differ but you cannot expect a set to be working as it should if it still contains some old caps.
Possible paper caps in the grid and cathode circuit would be C32, C33 and C34.
The grid is indirectly biased from the 245V supply, which also supplies the sweep.
The cathode is indirectly biased from the 135V supply which comes from the audio output cathode, that is a common "split supply" circuit.
The values of C2 and C3 could be intended to hold the cathode more positive while the grid supply decays. Those caps would be suspects.
Of course a resistor might also cause the problem.

Thank you I will check them. To be honest I just haven't recapped the set yet because I'm so excited to actually have something that works. I've watched it for about 8 hours so far in the last 2 days. If you don't mind explaining how can you identify these circuits I'm really trying to learn. It's pretty cool that you were able to pick them off quickly

[Notimetolooz]

Well first of all it isn't a sure thing that I got it right, its a idea.
You have to understand how vacuum tubes work.
The rest is reading schematics and knowing how resistors and capacitors work in circuits. Recognizing sections of circuits I've seen before, etc.
There are some good vintage books available for free download on the web.
RCA tube handbooks have a section explaining a bit about how tubes work and for a more detailed explanation is in books like "Radiotron Designer's Handbook" also available.
Actually it seems to me that the way that C2 is connected it may make the problem worse. It may have been connected the way it is so as to reduce C2's needed voltage rating.

[old_coot88]

Since this particular design injects video via the CRT's cathode, I wouid be disinclined to mess with the cathode circuitry at all. But rather concentrate on how to make the grid go negative at switch-off. A dedicated negative supply of maybe 16V or so, charging an electrolytic, would kick the grid negative thru a diode reversed-biased. Or sumpthin' similar or other, just musing.

[old_tv_nut]

Originally Posted by old_coot88

A 'spot killer' circuit would work by making the CRT control grid (pin 2) more negative relative to the cathode (pin 11), thus turning off emission. Same result by making the cathode more positive than the grid (as Notimetolooz mentioned in a previous post).

Caution! A spot killer that works by biasing OFF the CRT has to hold the negative bias until the cathode cools or the high voltage decays, whichever comes first. Read my post about the burn problem on Motorola's first small transistor sets.

Many color sets killed the spot by turning the beam ON fully while the raster had not yet collapsed completely. You would get a short bright flash of a small raster, discharging the high voltage through the CRT before the raster had shrunk to a dot.

[Aperna1985]

Originally Posted by old_tv_nut

Caution! A spot killer that works by biasing OFF the CRT has to hold the negative bias until the cathode cools or the high voltage decays, whichever comes first. Read my post about the burn problem on Motorola's first small transistor sets.

Many color sets killed the spot by turning the beam ON fully while the raster had not yet collapsed completely. You would get a short bright flash of a small raster, discharging the high voltage through the CRT before the raster had shrunk to a dot.

Would that short bright flash put excessive wear on the crt?

[old_coot88]

Interesting discussion. Back in the day, the bright dot was never known to be a problem, service wise (at least in our experience). But musing on fixes nowadays is good mental exercise anyway.

[old_tv_nut]

Originally Posted by Aperna1985

Would that short bright flash put excessive wear on the crt?

No, it's equivalent to having a bright image for a short period. So for the cathode, it's no worse than having a bright scene for a fraction of a second. For the phosphors, it's not enough to produce visible darkening even after many cycles. The only time visible darkening happens is in studio monitors that run 24/7 with a test pattern on, or a message display that has text 24/7, or a video game in constant used with permanently stationary image elements / menus, etc.

[Electronic M]

With relays being inexpensive these days I'd be tempted to power a 6VAC relay coil off the tube heater winding and use it to gate a voltage to one of the CRT when the set goes off....A 4 pole relay could probably switch a lytic to charge when the set is on, and to bias the tube off when the set is off.

[Blast]

In the mid-late 90's Funai built a great number of TV/VCR combos for themselves and many brands including Magnavox. I remember a Magnavox service bulletin regarding "the spot" in some models and a fix for it because it was so bright and long lasting that it had the potential to burn the CRT.

[Notimetolooz]

I finally tracked down where I had seen a "spot killer" using a dual section on/off switch. A 19" RCA from 1969. It may be there for customer satisfaction rather than protect the CRT.
Basically the cathode bias is removed and goes to near ground. I suppose if you have a set where it bothers you you could add a relay powered by the filament supply (as mentioned) or the switched power to the set.

I wouldn't get overly concerned with this issue. In a properly operating set it seldom is a problem. There would be a lot of CRTs around with a small area damaged in the center of the screen if it was really a problem and there isn't.

[Blast]

Originally Posted by Notimetolooz

There would be a lot of CRTs around with a small area damaged in the center of the screen if it was really a problem and there isn't.

You're right, it's probably NOT a real problem and I didn't mean to make it out like it was.