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  #1  
Old 12-17-2016, 09:35 PM
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dtvmcdonald dtvmcdonald is offline
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My CT-100 lost HV (from losing one B+ line).

My previously 100% reliable ct-100 broke today. I turned it on
for a football game and after running about 30 seconds,
with sound but no picture (due to, as it turns out, to the video
cable being disconnected from the modulator) I turned it toan
unused channel and saw the usual snow at the usual brightness.

It then emitted a "snap" and the picture (and raster) was gone.

A quick check showed no HV at the connector that goes to
the CRT anode. More checks showed no horizontal drive.
(The horizontal output tube is fixed bias so it can't red-plate.)
There is B+ on the plate of the horiz output tube (removed until I get drive.)
The fuse in that system is OK.

Checks on the horiz osc with both voltmeter and scope
showed NO B+ on either side. Vertical system works OK.

There is B+ on the plate of the horiz output tube (removed until I get drive.)
The fuse in that system is OK.

There is no B+ on the plates of the vertical convergence amp!

This would all explained IF ... and here I'm using the Sam's schematic
because the RCA ones have the vertical centering control in the wrong place
for my set ... IF Sam's have a 2.5 amp fuse-+-+++ M3 in wrong line
OR if some circuits including the horiz oscillator are put in that line rather than the high power horizontal
one a 5 bolts more positive (which feeds the and it were blown. Well, its blown. After lots of looking yes, both Sam's and RCA have th horiz osc
in the wrong line.

I checked various things running at either of those two voltages (except
the parts clearly in the higher voltage line,
incluing horizonal output.

I then replaced the fuse and, with the CRT and hiorizontal
output unplugged, turned it on. It ran for 2 minutes
whgen I shut it off, no problems, and the horiz oscillator
woked properly.

As far as I can tell the only things running off this line
are the horizontal oscillator and the CRT screen controls.
The vertical convergence amplifier is said to, but that's
not the case in my set, it runs off the fused B+ in the horizontal area.

I did note that the purity control that is in series with this fuse
is open at the high end, but it is run wwell into the working range.

Could the problem be arcing in that purity control or the
three CRT screen pots? What do you CT-100 gurus recommend
I do before turning it on the the horizontal working and/or the CRT connected?
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Old 12-18-2016, 09:07 AM
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So far I haveunable to find a problem
that would blow the fuse. I replaced it (1/4 amp)
with a 200 mA one.

I turned it on adding one plug at a time, and it just worked.

I did find a problem that was affecting the focus. The
convergence electrode pin in the socket was a bit corroded.
I scraped it with a tinned resistor lead of the same size
as a pin, cleaned the pin itself, and applied DeOxit to the pin
very carefully with a Q-Tip. The focus is more stable.

BUT .. if CT-100 gurus read this ... is there
some specific part I should check for arcing?
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Old 12-18-2016, 11:55 AM
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It could be the fuse just naturally reached the end of it's life (from decades of starting spikes), and that the arcing you heard was B+ jumping the break till it opened too wide to arc across.
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Old 12-18-2016, 12:29 PM
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Although I have not observed the problem on a CT-100, I would check for "tin whiskers" (dendritic growth? )on pots connected to that B+ line.

jr
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Old 12-18-2016, 01:19 PM
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I the past 2 months I have seen where there was evidence of arcing and corrosion on the 15gp22 focus pin and socket of 2 different CT-100 I think this is probably common.
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Old 12-18-2016, 04:54 PM
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Quote:
Originally Posted by TV'S&MORE View Post
I the past 2 months I have seen where there was evidence of arcing and corrosion on the 15gp22 focus pin and socket of 2 different CT-100 I think this is probably common.
When I had my CT-100 there was evidence of arcing on the focus and convergence pins and socket connections. I cleaned them carefully and restored normal operation, at least until the CRT sucked air .
Kirk S. restored the set, and did a wonderful job !
Kevin
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Old 12-18-2016, 06:13 PM
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On mine the focus pin was fine, it was just the convergence one.

I tried to get the back off the socket but failed. Soes anybody know
ho to do that? It looks like the three pins are those spiral
push-in pins. I could not garb them.
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Old 12-19-2016, 12:50 AM
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I have had similar B+ problems in sets where the insulation is starting to break down inside the candohm resistor riveted to the back of the chassis. Typically the insulation will break down and a spark will jump between ground and the resistance element inside the candohm. It will blow the fuse if there is one. Sometimes you can put a new fuse in and the set will operate again until another spark jumps and blows the fuse again.

If you get lucky a carbon trail will form inside the candohm and cause a constant arcing failure, and than you can narrow it down to a defective candohm. I guess you also might be able to disconnect the leads and measure the resistance between ground and the candohm terminals. I would think a good candohm would have megohmss of resistance from the lugs to ground. Perhaps someone else here might be able to give us a better idea of how much isolation resistance you would find in a good candohm versus a bad one.

And like someone else here said, you could also have tin wiskers in a control pot. But when I have had a pot arc, the arc usually wiped out the pot.

Keep that 200ma fuse in the set and if it blows again, perhaps it will help reveal which part is arcing. I do not believe the arc/snap sound you heard would come from a fuse. When you hear a loud snap/crack sound and the B+ goes out, you likely have a component that arced to ground. I have even had one occasion where, there was a lead on a B+ filter cap terminal that was too close to ground and arced over. Replacing the fuse and watching under the chassis in the dark for the arc to happen again revealed where the arc occurred.
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Old 12-19-2016, 01:37 PM
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dtvmcdonald dtvmcdonald is offline
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Quote:
Originally Posted by ohohyodafarted View Post
I have had similar B+ problems in sets where the insulation is starting to break down inside the candohm resistor riveted to the back of the chassis.
One side of that Candohm is at -20 volts to ground, the other
at -10 volts to ground. The only fuse it could blow is the one in the
B+ line to the horizontal output transformer, by reducing (to -10 volts)
the fixed grid bias on the horizontal output tube ... which will even then
get bias from the drive waveform. If it were to open, it would just
just ruin the vertical centering and slightly raise (to a higher negative
voltage) that fixed grid bias.
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Old 01-12-2017, 02:56 PM
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dtvmcdonald dtvmcdonald is offline
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It did it again.

As Murphy's law says, my CT-100 really is sick.

The fuse M3 blew again today. I had run the set for two hours, no problem.
After an hour off I turned it on again. It behaved exactly like the last time
the fuse blew: I turned it on, after 15 seconds sound appeared.
After about 60 seconds in this case snow appeared (the modulator was off).
Almost exactly 1 second after snow the fuse blew. Is roughly 75 seconds
total normal warm-up time? It has always been exactly the same. If your
CT-100 takes less than a minute to get a raster, maybe there really is
something wrong with mine, likely in the horizontal.

The first time there was a "snap" when it blew, today, no pop at all.

This is the 375 volt line. Pete Deksnis described, as far as
I can tell, in a recent article all the places it is used, except for the three CRT
screen pots and the vertical osc and output (but maybe the output only).

This is going to be a bitch to fix its intermittent. What do you folks suggest?
The only really likely suspect is a short to ground in one
of the screen pots. Do you agree? Another possibility is a short of pins
3-4 of the ballast ... but this connects the AC from the power transformer
to the 375 volt line directly after the fuse ... I would have expected magic
smoke the first time it happened as it stayed on for maybe 30 seconds.
There was none.

A short to ground at the bottom of the three screen pots would only
generate about 80 mA, not enough to blow the fuse.

A dead short to ground at the screen of the AGC keyer would do it ... but
is not that very unlikely?

The nominal draw of the CRT divider chain, which is on this line, is only
about 30 mA, which I did later measure with a Simpson meter. I did not wait
long enough for a picture to appear as there was no fuse.

Also of course, it could simply be a loose wire or blob of solder somewhere
shorting the line to ground. Unless I'm missing someplace, every other
place the schematic, corrected as describe by Deksnis, has a 5K or more
decoupling resistor. A short to ground through 5k will smoke the resistor but
not blow the fuse.

In you folks' experience, is this a known problem?

A first proposal would be to take out and examine (take apart and clean)
all three screen pots and move the lead from pin 4 of the ballast to
a new terminal strip, install a new fuse, and try some more. Is that
reasonable?

Last edited by dtvmcdonald; 01-12-2017 at 08:15 PM.
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Old 01-12-2017, 03:49 PM
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Perhaps isolate the different branches of the circuit with several smaller fuses....the fuse that blows might point you in the right direction.

jr
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  #12  
Old 01-12-2017, 05:01 PM
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dtvmcdonald dtvmcdonald is offline
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Quote:
Originally Posted by jr_tech View Post
Perhaps isolate the different branches of the circuit with several smaller fuses....the fuse that blows might point you in the right direction.

jr
That's a great idea. However, it would require great thought to
prevent circuit damage. I don't think a fuse that blows in series with
the CRT divide chain would hurt anything. It would put essentially zero volts
on all the CRT screens, and the 100K resistors in the DC restorer circuit
would cause the CRT 1st grid currents to rapidly go to 200 uA by clamping,
which is harmless. I had to so a simulation of this to convince myself.
This is harmless even with the screens being the same voltage as the cathodes,
as the cathode-grid voltage would be only a few volts.

A fuse blowing on the vertical system would be a problem, as it would
generate a single horizontal line at "usual" brightness. I suppose
I could run it a very low brightness. The horizontal oscillator fuse blowing
will warm up the horizontal output tube and transformer
with a bit of DC, but not a huge amount, since the horizontal output tube is
fixed bias at about 30 volts in absence of drive.

I could try it every day on the bench with the CRT and yoke (hence, B+
for the horizontal output) disconnected. That means I would have to move
the two boatanchors there now to their final stations ... they are far heavier than
a CT-100 chassis.
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Old 01-14-2017, 02:01 PM
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dtvmcdonald dtvmcdonald is offline
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I may have found it. I certainly have found a sickening sight. The red and green
screen pots were pristine.

The blue one looke like the object on the right.

The whole inside was filled with rusty goo.
I have cleaned this up and the resistor element seems perfect.
The baclk of the control is, as is visible, pitted.
The gooey part of the slider can't arc to the back as its grounded too. But
the slider or element could arc to the case or the slider shaft.
I am adding a bit of shrink wrap on the shaft and a mylar
sheet to insulatre the back.

http://videokarma.org/attachment.php...1&d=1484420473

Time will tell.
Attached Images
File Type: jpg BadBlueScreenPot_small.jpg (128.8 KB, 79 views)
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  #14  
Old 01-15-2017, 10:32 AM
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dtvmcdonald dtvmcdonald is offline
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I found out why the power switch is always on: its jumpered like that.

Removing the jumper, the resistance is a constant 1.4 megohms.

Thus, I need a new switch, which is part of a triple control: volume control,
brightness control, and switch.

These no longer exist ... but I strongly suspect that old RCA volume control+
switch units exist that have a compatible switch. What to do?

(Other than the current dedicated Variac, which admittedly is comforting for
a nice controlled start).
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  #15  
Old 01-15-2017, 11:33 PM
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dtvmcdonald dtvmcdonald is offline
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Its still sick. I put the chassis back in the cabinet and readjusted
the necessary controls. It was on say 7 hours today, on and off maybe 20 times.
I was getting ready to call it quits and go have a nice dinner. I decided
to switch channels one last time. As I was flipping through them, at the exact instant
it flipped, the fuse blew with a very small pop.

Channel flipping is not going to cause an arc in the screen controls, the vibration
is surely too small.

Other than adding more fuses in different lines, I'm out of ideas.
The set was working normally until it blew.

Help.
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