What am i doing wrong?

[jbivy]

So ive a 1956 packard bell (i think its a model 24s as thats what the back says, the interior says its an 8851 tube layout). Ive recapped it, did them one at a time, so its correct. Checked the tubes and replaced the ones that were low. Had my uncle check it out, he used to be a tv repairman, he saw no solder spalshes, nothing.

When i turn it on it gets a picture for about ten seconds, crystal clear one at that, then it goes to a horizontal line and then a dot and dissapears.

Im a novice so.. what should i be checking? Ive yet to find a sams for this or well..much on it at all.

[Penthode]

We need more clues.

Does the set totally lose power? A total collapse of the rater to a dot suggests a power supply fault?

Any detail, no matter how trivial, is useful.

[stusnyder]

Loosing high voltage someplace?

[leadlike]

That sounds like the entire television is shutting down. Do you still get audio when this happens?

Sounds like you'll need to invest in a decent multimeter. Head down to the Kent Harbor Freight and buy the multimeter that isn't a clamp-on, and certainly not the one that is usually $3:


http://www.harborfreight.com/ac-dc-d...ter-37772.html

This meter has everything you'll really need for tv repair work. 20 bucks is a steal for this meter, too. At any rate, get a meter, and we'll walk you through some basic B+ voltage checks.

[Jeffhs]

If the raster collapses to a dot (or a horizontal line, for that matter), don't leave the set on for long; the CRT phosphors will be burned, resulting in a permanent brown line across the center of the screen -- ruining the tube, of course. A horizontal line across the width of the CRT indicates the vertical sweep has failed, possibly due to a defective vertical oscillator and/or output tube (some sets use one tube for both functions), an open vertical output transformer, an open vertical sweep winding on the deflection yoke, or even an open vertical hold control.

In any TV with a flyback HV system (all sets made since the early 1950s until the solid-state scan-derived systems of the '80s-'90s), it is impossible for the raster to collapse to a dot, as this would mean both sweep systems have failed. The horizontal sweep must be operating in these sets in order to produce high voltage for the CRT's second anode; no horizontal sweep always means no HV. Very early TVs from the late 1940s, however, used an RF high voltage system; this system could generate the necessary CRT second-anode voltage regardless whether or not the horizontal sweep was operating. This type of sweep system could cause either a dot (if both sweeps were inoperative) or a vertical or horizontal line on the screen, the latter depending on which sweep system had quit. If the vertical osc/output circuit was dead, no vertical sweep (horizontal line across the width of the screen); if the horizontal sweep circuit was inoperable, no horizontal sweep, resulting in a vertical line extending from the top of the screen to the bottom. In either case, shut off the set immediately; do not use it again until the problem has been corrected.

[jbivy]

Well i bought this tv in poorly working condition. Brought it up with a variac and it semi worked. I had it on its side in stand id built and replaced every cap carefully. It worked beautifully for the ten minutes after that. The next day I turned it on to get a picture and sound for maybe ten seconds, then there was no sound and no picture, nothing but a horizontal line for perhaps three seconds then a dot in the center. This all happened while i was scrambling to turn it off.

Im sorry for such newb question, but we all have to start somewhere. Plus..the tv repair books ive been attempting to read just go over my head.

Though thinking about it..i didnt check the NOS tubes id bought to replace the ones that tested low.

[David Roper]

Check the fuse if the set has one.

[jr_tech]

It sounds to me as if it is loosing B+ voltage... as the B+ voltage drops the vertical oscillator quits, leaving a horizontal line... then the horizontal oscillator quits (which will indeed kill the high voltage) but a dot in the center may remain for several seconds until the high voltage filter capacitor is discharged, then fade out slowly. Does this describe what you are seeing?
If do not have a multimeter, get one... the $20 one linked above is a killer deal! With that you can monitor voltages in the set as the problem occurs.
If the set has a tube B+ rectifier like a 5U4, you might observe it as the problem occurs perhaps the filaments go dark or the plates turn cherry red or the whole tube glows purple. Might be a clue there!
jr

[jbivy]

Originally Posted by jr_tech

It sounds to me as if it is loosing B+ voltage... as the B+ voltage drops the vertical oscillator quits, leaving a horizontal line... then the horizontal oscillator quits (which will indeed kill the high voltage) but a dot in the center may remain for several seconds until the high voltage filter capacitor is discharged, then fade out slowly. Does this describe what you are seeing?
If do not have a multimeter, get one... the $20 one linked above is a killer deal! With that you can monitor voltages in the set as the problem occurs.
If the set has a tube B+ rectifier like a 5U4, you might observe it as the problem occurs perhaps the filaments go dark or the plates turn cherry red or the whole tube glows purple. Might be a clue there!
jr

That seems to be exactly what is happening to my set. So..where do you suggest i go from here?

[jr_tech]

Does the whole set go dead... are you loosing audio as well? It could be a problem in the entire power supply or perhaps only one section.
Can you monitor B+ voltages as the problem occurs?
Do the heaters of any of the tubes dim or go out when the problem occurs?
Again, some simple observations can narrow down the likely causes.
jr

[jbivy]

Originally Posted by jr_tech

Does the whole set go dead... are you loosing audio as well? It could be a problem in the entire power supply or perhaps only one section.
Can you monitor B+ voltages as the problem occurs?
Do the heaters of any of the tubes dim or go out when the problem occurs?
Again, some simple observations can narrow down the likely causes.
jr

the audio goes dead as well. I didnt notice, but i wasnt looking, to see if any of the tubes dimmed. How do i check the B+ ?

[DaveWM]

is it a series string set? (pull one tube out and they all go dark?)

[jr_tech]

Originally Posted by jbivy

How do i check the B+ ?

Do you have a schematic?
I would use a multimeter attached to the power supply output with alligator clips (so you don't have to hold them in place) positioned so that you can see the meter while the tv is operating... watch for changes in the B+ voltage as the set goes into failure mode.
If the set uses a 5U4 or similar rectifier tube to generate the B+ voltages, try to watch it as well. What is the model# of the set... perhaps I can look it up and point out specific points to monitor.
jr

[old_coot88]

Often some judicious prodding and tapping with an insulated tool will localize an intermittent.

[wa2ise]

Bad resistor that overheats? After you get a digital volt ohm meter, measure all the resistors. Be aware that in some cases, you will also measure what else is parallel with the resistor, but that will make for a lower reading. A too high reading will mean a bad resistor.