View Full Version : My CT-100 lost HV (from losing one B+ line).


dtvmcdonald
12-17-2016, 09:35 PM
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?

dtvmcdonald
12-18-2016, 09:07 AM
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?

Electronic M
12-18-2016, 11:55 AM
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.

jr_tech
12-18-2016, 12:29 PM
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

TV'S&MORE
12-18-2016, 01:19 PM
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.

stromberg6
12-18-2016, 04:54 PM
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 :tears:.
Kirk S. restored the set, and did a wonderful job :thmbsp: !
Kevin

dtvmcdonald
12-18-2016, 06:13 PM
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.

ohohyodafarted
12-19-2016, 12:50 AM
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.

dtvmcdonald
12-19-2016, 01:37 PM
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.

dtvmcdonald
01-12-2017, 02:56 PM
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?

jr_tech
01-12-2017, 03:49 PM
Perhaps isolate the different branches of the circuit with several smaller fuses....the fuse that blows might point you in the right direction. :scratch2:

jr

dtvmcdonald
01-12-2017, 05:01 PM
Perhaps isolate the different branches of the circuit with several smaller fuses....the fuse that blows might point you in the right direction. :scratch2:

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.

dtvmcdonald
01-14-2017, 02:01 PM
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?attachmentid=193458&stc=1&d=1484420473

Time will tell.

dtvmcdonald
01-15-2017, 10:32 AM
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).

dtvmcdonald
01-15-2017, 11:33 PM
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.

jr_tech
01-16-2017, 12:34 AM
Vibration or coincidence.... that is the question :scratch2:

Perhaps try tapping tubes that are connected to the +375V supply?

jr

Tim
01-16-2017, 12:44 AM
Can't help with your vexing fuse blowing problem but, as you noted, switches were a common item used on many controls. Posting a detailed photo of the switch along with any numbers and the EIA manufacturers code from the control there is an excellent chance a replacement switch can be located. I have also had success opening and cleaning out the old hardened lubricant and repaired switches on controls.

old_coot88
01-16-2017, 12:51 AM
You mentioned earlier that you replaced a 250 ma. fuse with a 200 ma one. Assuming this is a standard glass fuse, can you take a closeup pic of it showing clearly the opened element?

Just a W.A.G., but is there by chance a filter cap directly upstream of this fuse, which, if the cap were open, would pass a large ripple?

On rare occasions, I've seen a bad filter cap passing enuff ripple to smoke the resistor directly 'downstream' in the B+ line. But never seen a fuse blown from this condition before.

dtvmcdonald
01-16-2017, 10:53 AM
I have the latest blown one, which is a 250 mA. I'm out of 200s. I looked at it
and didn't see any remaining element at all. I'll look tonight.

It survived two turnons this morning.

Given that it has blown at a time of rapid change from black to
normal intensity, or rapid change in AGC, maybe we have a clue that
it is the AGC circuit. Other than the screen pots, that's the only thing
(its screen) connected directly to the 375 volt supply.
Of course, there is the possibility of arcing across at the tube socket.
Maybe I should put a 5K resistor in series.


But I see no trace.

Perhaps I should try a filter cap, say 10 uF at 450V
across the "far" side of the fuse.

Note that while all the 450V caps see 500V at a normal turnon
I tried a couple of ones of different capacitance from the
same batch on a tester at 550V and none showed any leakage at all.

But in any case, the fuse never blew except at normal voltage, 365 to 390 volts
depending on Variac setting (barring the event that caused the blow, of course).

old_coot88
01-16-2017, 11:12 AM
I would put a light bulb (maybe a #47) in series with the fuse, and mount the bulb where it's clearly visible when the set is operating. That way you'd have a visual on the current at any given instant. Maybe a video cam to record any sudden brightening while you're not looking.

If the fuse is going open without a current spike (no sudden brightening of the bulb), maybe it's just opening from 'running hot'.

jr_tech
01-16-2017, 12:57 PM
Do you have a dmm with the "peak hold" function?

jr

dtvmcdonald
01-16-2017, 02:26 PM
Yes I do have a DMM with peak hold, but the current setting seems to be kaput.
Maybe it blew an internal fuse.

But I have something better! I have a Hall effect current probe
that measures below a mA and has a 50 MHZ frequency response.
I could feed this into a digital scope with glitch detect and store.

It does have the problem that the largest wire that it can take
is about 1mm dia and its **not insulated** :thumbsdn:. So I often must
install a loop to use it on.

I think its the AGC. I got a simulation to generate 40 mA spikes
lasting tens of milliseconds coming from the AGC tube screen during
simulated signal changes. That's not enough to blow the fuse, but
its just a simulation.

Perhaps this is supposed to be a slo-blow fuse? I lost the
original and as far as I can tell, the parts list does not say.

Electronic M
01-16-2017, 02:49 PM
I think its the AGC. I got a simulation to generate 40 mA spikes
lasting tens of milliseconds coming from the AGC tube screen during
simulated signal changes. That's not enough to blow the fuse, but
its just a simulation.


It is possible more than one stage on that rail is drawing more than it was designed to...If it is running a little heavy on current across the board, then it should be more susceptible to spikes.

old_coot88
01-16-2017, 02:58 PM
Dang, I was gonna suggest a slo-blo fuse, but then thought the better of it because it'd be 'masking the problem'. That's assuming a fast-blo is what's called for.

dtvmcdonald
01-18-2017, 07:51 PM
Bad thoughts. Could it be arcing in the CRT?

It blew tonight, first turn-on after attaching the cabinet back.
Just at the instant the HV was expected to come on there was a tiny click,
the fuse blew, and there was a very small flash of light on the screen
(in a dark room). Examination showed the fuse blown, but also ...
no light from the CRT back and no heat: no filament. There was nothing wrong
with the crt filament, it wasn't contacting. cleaning the pins and reseating the socket
and it works perfectly.

Is arcing in the CRT a known problem ... if it is that, will my 15GP22 soon die?
arcing to the red screen might blow the fuse, or less likely arcing to the blue
or green ones.

On the other hand, what about socket contact problems? Looking at the socket
contacts, they are not in great shape and need attention.

But how does one get the back off the socket? I'm afraid to turn it on again
until the socket is attended to.

This has me worried for my 15GP22.

Penthode
01-18-2017, 11:54 PM
Was the fuse replaced with a slow blow fuse? The slow blow will much better handle the surge and is recommended for most sets. Also reducing the rating below 0.25A especially if non-slow blow is asking for it to blow on switch on.

To better troubleshoot, I would suggest temporarily putting in a 0.5A, let is run and quickly investigate what is wrong. My guess is likely the slow blow 0.25A will fix it.

benman94
01-19-2017, 08:24 AM
Bad thoughts. Could it be arcing in the CRT? ....
This has me worried for my 15GP22.
It's a very real possibility, and I'd start looking for ways to rule it out.

My Great-Uncles Neil and Skip told me that the 15GP22 was a notoriously unreliable tube even when they were new. Many of them failed under warranty, costing RCA even more money, with arcing as a primary symptom. That's part (albeit a small part) of the reason that the 21AXP22 was rolled out in late 1954.

That said, I wouldn't worry too much. Tom's scenario sounds the most likely IMHO: all circuits on the +375 rail drawing excessive current with a tiny spike being the straw that breaks the camel's back.

dtvmcdonald
01-19-2017, 10:51 AM
It's a very real possibility, and I'd start looking for ways to rule it out.

My Great-Uncles Neil and Skip told me that the 15GP22 was a notoriously unreliable tube even when they were new. Many of them failed under warranty, costing RCA even more money, with arcing as a primary symptom. That's part (albeit a small part) of the reason that the 21AXP22 was rolled out in late 1954.

That said, I wouldn't worry too much. Tom's scenario sounds the most likely IMHO: all circuits on the +375 rail drawing excessive current with a tiny spike being the straw that breaks the camel's back.

Its clearly not a tiny spike on the 375 volt line. It normally runs in the
30-40 mA range, rock solid. I had a meter on the line for many turnons.

The solution is probably a big box of 250 fast blow mA fuses and don't use it
except for demos. A blown fuse is not a common happening.

But what tube CRT elements are arcing? The convergence electrode
is 1 nF fin series with 820K. The focus electrode is 7200 ohms in series with
the substantial inductance of the vertical convergence transformer and
the 10 nF to ground. That leaves the HV itself, which is about .002 to .004 nF
to ground, which is a lot of energy. But they would have to arc to the
screens.

The CTC2B has 100K resistors in series with the screens, filtered by
0.1 uF caps. 0.1 is 25 times .004, so even an instantaneous connection
would only put 800 volts on the cap.

Perhaps I should add that to each screen. But where? The caps are rather
large, and the resistor, during an arc, will have up to say 18 kV across it
(2 watt resistors, even modern ones, are plenty long enough to withstand
that though two 47K ones in series might be better). There's room in the
LV power supply cage, or underneath where it is in the 21CT55.

dieseljeep
01-19-2017, 11:10 AM
It's a very real possibility, and I'd start looking for ways to rule it out.

My Great-Uncles Neil and Skip told me that the 15GP22 was a notoriously unreliable tube even when they were new. Many of them failed under warranty, costing RCA even more money, with arcing as a primary symptom. That's part (albeit a small part) of the reason that the 21AXP22 was rolled out in late 1954.

That said, I wouldn't worry too much. Tom's scenario sounds the most likely IMHO: all circuits on the +375 rail drawing excessive current with a tiny spike being the straw that breaks the camel's back.

On a side note, regarding a set using a 15GP22. The set was stored in an attic for something like 25 or 30 years, that was freezing in the winter and extremely hot during the summer.
The CRT looked really good, after the set was restored. I guess, some things are meant to be, either pro or con.

benman94
01-19-2017, 11:35 AM
IIRC the HV would start arcing to the screens when the vacuum became softer. Still hard enough to produce an image with minor focus issues, but soft enough to allow an arc. Your tube may be going gassy, which wouldn't exactly be a surprise with a 15GP22. (Neil and Harry have been dead for years so I can't ask them directly anymore.)

dtvmcdonald
01-19-2017, 02:50 PM
My tube does not have "minor focus issues" or convergence issues. It has
absolutely perfect focus and convergence. It probably does have a minorly
weak red cathode. However I just looked at the data sheet for the 15GP22
which says that the red gun can need 4 times the current of the green one,
per spec, and given the voltage-current curves for similar tubes mine is
in that range.

I think I will install the 100K series resistors and use it only on special occasions.

Electronic M
01-19-2017, 04:16 PM
If it was not for the rarity and value of the set*, if I found myself in your shoes I'd be tempted, after all that fruitless troubleshooting, to simply bridge the fuse and see what if anything lets go...
*Then again with all the shot CRTs and some loose chassis out there it may not be the worst set to look for parts for...

Sometimes the best way to figure out an intermittent is to stress a set till the issue becomes a near constant. I had an issue with a '71 Zenith hybrid where a color was dropping out and others were shifting bias from it...It could hold it for 15 min straight, but if I changed channels, or whacked it right it'd be fine...I could never get a meter on it. If I had it set up to take measurements it would just sit there and work properly for hours...Maddening! I kept it in daily use along side a Silvertone roundy (I like redundancy when using tube TVs) until it got so bad it was harder to make it work right than wrong....Then I was able to track which CRT element was not being driven right then trace out it's driver stage. All that seemed like it could be wrong was a transistor and swapping it with another color of it's video out stage confirmed the issue.

jr_tech
01-19-2017, 05:07 PM
Did you replace the convergence transformer with one of the new ones from John Folsom? I understand that the original transformers can arc and take out the focus pot... but have not heard of one blowing a fuse, but possibly? :scratch2:

jr

dtvmcdonald
01-19-2017, 05:09 PM
The problem is unobtanium CRT and flyback.

I'm very very afraid of those. I did try it a very long time with the
yoke and CRT unplugged (disables horizontal output and HV) and it never failed. I could try your idea with the CRT only unplugged (both ends),
except I'm afraid for the flyback.

But currently, even with no other symptom whatsoever of a gassy CRT,
I'm suspecting a CRT arc.

benman94
01-19-2017, 05:23 PM
Hey I'm just passing along tips and hints from two of the guys that were in RCA's color products division back then... at this juncture there are no questions that aren't worth asking. You're still stumped.

How does the dag look on your 15GP22?

Are you 100% positive the dag has a good solid connection to ground?

Harry had a set (21 incher though) where the connection between the good looking dag and ground was intermittent. The arcing caused excessive HV current draw and had a tendency to pop fuses.

Is your CRT a rebuild? Harry said factory rebuilt 15Gs (ones that never made it through the initial processing) are more failure prone (gas or arcing) but he never explained why....?

dtvmcdonald
01-19-2017, 05:41 PM
I have no idea whatsoever about the dag. Its invisible, isn't it, under
the mumetal shield? I have never looked under it. I'm not about to touch the yoke ...
I'm not joking about convergence quality. It took two days
of tweeking to get the yoke tilted just perfectly.


I'm thinking ... carefully .. about installing the 100K resistors inline in
the three screen leads, covered with shrink wrap. In the 21CT55 they
are under the chassis, in later roundies, attached to the tube socket.

Electronic M
01-19-2017, 06:03 PM
I could try your idea with the CRT only unplugged (both ends),
except I'm afraid for the flyback.


There was a tube type (wish I could remember it's number...) that was essentially a 10BP4 gun and base firing into a metal anode that was sometimes used as an HV dummy load in early sets...The 6BK4 and the like were refinements on it IIRC...You could probably use one of those or a 6BK4 connected to the CRT base and HV circuits to sub for the load of the CRT.

old_coot88
01-19-2017, 08:10 PM
I once had a somewhat analogous problem with a CTC-36. Someone had spilled a sticky liquid in through the back. It caused a bunch of B+ arcing that had burned deeply into a pc board before the breaker tripped. Cleaned out all the char that was visible, but it still popped the breaker. Several more cleanings and it still popped.

Finally I jumpered the breaker to make the hidden leakage paths visible. They incandesced spectacularly, so I got them all finally.

The rectifier diodes survived the brutalization, but were replaced anyway.

N2IXK
01-20-2017, 01:59 AM
There was a tube type (wish I could remember it's number...) that was essentially a 10BP4 gun and base firing into a metal anode that was sometimes used as an HV dummy load in early sets...\

Sounds like the 5890, which was used as a shunt regulator in theater projection sets. kind of a grandfather to the 6BD4 or 6BK4 types. The RCA production documents describe it as a 5TP4 gun firing into a 2X2A anode assembly. 12 pin "duodecal" base as used on most early CRTs. I have a couple of them in my tube collection. Seriously obscure devices. Handy for load testing the HV on older B/W sets, or for testing a CRT tester....

dtvmcdonald
02-02-2017, 11:04 AM
I think my 15GP22 red gun is either bad or sick.

At low brightness, not all that much lower than I remember it when I got it,
maybe the same per old versus recent photographs made with my Canon
7D in "raw" mode and therefore with the raw data available to compare,
it still produces reasonable pictures with good color.

I do think that the 1.8K resistor in the CRT divider chain may be a bit high;
bridging the high end of it and the wiper of the brightness control with a
5K resistor makes the brightness control no need to be almost all the way up.

BUT ... the red screen control, while generating the correct voltage range,
has very little effect. Not NO effect, in fact it does seem that there is a bit
more red when it is all the way DOWN. But not the huge effect of the
green and blue screens. I find it hard to believe this is
normal. Is it possible that it is disconnected inside the tube? I measured
pin to pin capacitances on the tube plug and the red screen is in fact a little bit
higher than the blue and green ones, a fraction of a pF. I tried resoldering the
tube pin with no change; it did have only a little solder on it.

If you've adjusted a CT-100, and remember playing with the red screen,
did it work like the other two? Until recently I had never turned it
down below 80% up because I was always told to to set it
almost all the way up.

benman94
02-02-2017, 11:58 AM
On my flawless 15GP22, the red screen did function just like the other two. Why wouldn't it?

Pete Deksnis
02-02-2017, 01:24 PM
I think my 15GP22 red gun is either bad or sick.

Sounds as though the 2.7K 1/2-watt in the red-gun cathode circuit should be checked.

Perhaps RCA put it there because the degeneration it provides smooths out the red phosphor. Haven't seen it on any vintage set other than the CT-100.

Pete

dtvmcdonald
02-02-2017, 03:26 PM
Benman: why not? because the red gun is sick? I checked again and
the voltage is getting to the tube pin, and the pin capacitance
to other things is the same as the blue and green guns. I was thinking that
maybe it was not connect to the actual electrode, but changing it does
have a small effect. IF in fact there was an internal arc it could
have damaged either the cathode or the connection to the red screen.
But if it were not connected, why would it work at all?

However, given that the blue and green screens run almost all the way down,
and do have only a modest range where they have a big effect, it might be that
lowering the red one further would cause a drop in the red beam current.
The 4K resistor tests OK in circuit, cold .. maybe its changing when hot.
So the next step is checking those two resistors under load by
unplugging the CRT socket to gain access and removing the
horizontal output tube to remove HV for safety, and letting it cook.
McVoy said my CRT test 90% on all guns.

Phil: the resistor measures OK and when running it has the expected
voltage (about 1 volt) across it when the set is showing a
bright screen.

benman94
02-02-2017, 05:10 PM
If I were you, my next step would be to acquire a Sencore CR-70 or CR-7000 and check the emissions on the red gun. You have put some time on the 15G, probably not a lot though, and cathode material is finite.
Given that the beam current on the red gun is so much greater than on the other two, I'd want to rule out poor emissions first. The drop off in emissions can be rather fast in a tube that is driven hard. I think the red gun of a 15G would qualify...

dtvmcdonald
02-02-2017, 07:31 PM
benman, I suspect you are right, although there WAS those two
arcing events that I can't prove were not in the tube.
I surely have less than 200 hours on it probably more like 100.
And it was never run for more than a few seconds at a time
with the red gun drawing grid current more than 10% of the time,
as when actually watching I always set it well below the "knee" where
the color rendition gets poor.

I suspect the arcng events.

Its not the resistor chain: rock stable for an hour.

I also vaguely remember asking McVoy about the red screen.
I seem to remember "just run it all the way up and don't worry".

Also ... my camera shows that the brightness limit for true
color accuracy has not dropped more than 1/2 f-stop (i.e. to 70%).

benman94
02-02-2017, 08:09 PM
Check emissions to ensure it isn't substantially lower than the green gun. If emissions looks fine, then I'm positive you have damage from arcing. Unfortunately, either way the long term prognosis isn't good. I strongly recommend you really use the hell out of that set while you can. No sense in "saving" cathode material in a dying tube.

benman94
02-02-2017, 08:16 PM
I really do hope it's not the tube, but if everything tied to the various elements of the red gun is fine, then that only leaves the CRT.

dtvmcdonald
02-03-2017, 09:19 AM
Check emissions to ensure it isn't substantially lower than the green gun. If emissions looks fine, then I'm positive you have damage from arcing. Unfortunately, either way the long term prognosis isn't good. I strongly recommend you really use the hell out of that set while you can. No sense in "saving" cathode material in a dying tube.

I have no way to measure the green gun without cutting wires. But I
measured the cathode resistor of the red gun and got 600 microamps ..
which isn't great but that was average with a mostly white screen.

I'm actually beginning to think I'm being paranoid. I will know for sure
after the next ETF convention when I will take several pictures of all their
15GP22s under the exact same conditions as mine ... same camera
settings, same picture of Dorothy.

benman94
02-03-2017, 09:44 AM
Just keep in mind, a 15G can go at any time. Ed Reitan lost at least one in a restored set, and both my Uncles lost a couple, seemingly at random....
I'd still want to know what the three guns look like relative to one another on a tester. If Steve said all three guns tested ~90%, and then you find the red gun much weaker relative to green and blue, then I think paranoia is out.

etype2
02-03-2017, 09:45 AM
How about posting a photo of Dorothy on your CT 100?

dtvmcdonald
02-03-2017, 10:23 AM
I've already long ago posted a picture of Dorothy that look just fine.
She looks just as good now.

That's not the question, which is "how bright is she now compared to then?"

I just realized I have the old .cr2 file ... I'll compare to a new
one adjusted to look the same!

dtvmcdonald
02-03-2017, 07:16 PM
Answer: a factor of five. The red gun max is down by five.

I am now convinced that an internal arc did it, at the time the
first fuse blew.

So I will just rarely demonstrate my set in a dark room, and
await and pray for rebuilding.

JB5pro
02-03-2017, 08:53 PM
I am always amazed at all you felas do for those very first color sets. I just hope to see one operate in person one day. It must be wild to see one perform as new.

dtvmcdonald
02-03-2017, 09:33 PM
Mine did. Well, I have no idea about "like new".
"Perfect" was more like it. Not perfect on color bars (ringing) but on
ordinary test static pictures, yes, especially near perfect color.

My avatar is a very easy scene, however.

old_tv_nut
02-03-2017, 09:35 PM
I am always amazed at all you felas do for those very first color sets. I just hope to see one operate in person one day. It must be wild to see one perform as new.

No chance of getting to Hilliard?

benman94
02-03-2017, 10:33 PM
Don't hold your breath on 15G rebuilding. Even of it does become possible, I doubt the yield of usable tubes is going to be terribly high. Subjecting an under vacuum tube to the rebuilding process would be an enormous risk. That said, buying a "good" 15G is a massive risk anyway...

Enjoy that 15G while it's still possible to do so. If the arcing is a gas issue, and I strongly suspect it is, you're saving cathode material in order to light up a purple neck. If you run the S.O.B into the ground, at least you'll have enjoyed the set more. Either way, the outcome is the same: you sit with a dark set and await the rather unlikely rebuild process.

Electronic M
02-04-2017, 12:18 AM
Running a gassy tube also may slow the advance of the gas issue by exciting the gas into moving more and thus being more likely to reach and be absorbed by the gether.

dtvmcdonald
02-04-2017, 09:01 AM
The tube shows no sign of gas. No blue glow and no problem with focus
whatsoever, sometimes I see the scan lines. Nor do the blue and green guns
show gas signs.

I'm in no hurry to get it rebuilt. I'll just watch the super bowl on my
55 inch TV. I just wanted to own such a set .. and I do note that it does
display pictures quite nicely, just dim (with blue and green throttled
back to match red).

benman94
02-04-2017, 10:44 AM
Going gassy is a process, not a single event. It's possible to have a tube that's going gassy with no blue neck, no focus problems, etc. Just periodic arcing as the first symptom. Remember, every gassy 15G out there (and there are a lot of them) was good at some point.

I slowly lost a 16AP4 to a leak. The first symptom was (probably, looking back on it) arcing. It became more frequent, then it got very dim and lost the ability to focus. Now that said, it's been a SLOW process. A few years minimum...

JB5pro
02-04-2017, 10:52 AM
No chance of getting to Hilliard?

Thanks for reminding me.

dtvmcdonald
02-07-2017, 09:07 AM
Yesterday I thought of a different test for internal wire breaks or shorts
in a CRT. Its best if the neck is clear of yokes/purity coils, but can be done in place.

I hooked up a 1 MHz signal generator, 25 volts, to each pin of the CRT in turn.
There was an aluminum foil shield, grounded to the purity coil, taped to
the bakelite tube base.

I used a high impedance scope probe and moved it around,
observing the pickup.

I expected modestly good symmetry about the three guns if
all elements were as expected, and a smaller pickup near an element
that had, for example, a bad pin connection. All was as expected if
good connections.

I then found some #6 wire and cut off a 1 inch piece of that,
connected it to the generator, and taped it to the outside of the
tube base, lying parallel to the suspect red screen pin. The pickup on my scope
was, again as expected, still there but down by a factor of
four to six.

So its official, my tube has poor red emission.

Also .. playing with the blue and green screen controls
shows that the higher they are, the lower the tube gamma.
And I had noticed that over time my sets gamma seemed to increase ...
the blue and green screens had been turned down as the red
got weaker. Thus I think it was not necessarily a one-off disaster, possibly.

benman94
02-07-2017, 11:56 AM
Very sorry to hear that Doug. Just an idea here; maybe try splicing a small brightener transformer into the 15G filament line and only use the set on very special occasions?
CRTs that are rarely used seem to hold up pretty well on a brightener. I have a *dead* 10BP4 on a brightener in a set I use a couple of times a year. It still produces a nice bright image with the elevated filament voltage. You could probably recover decent looking emissions at ~7 VAC on the filaments. At this point you haven't got much to loose.

Electronic M
02-07-2017, 12:03 PM
I'd recommend putting a CRT tester on it to confirm, before trying a brightener.

There ought to be someone near you with a CRT tester and a universal adapter.

benman94
02-07-2017, 05:16 PM
ElectronicM,
He's already confirmed that the red gun is weak. His method of finding the low emission is just more sophisticated than the lowly emission testers we rely upon. I'd be more inclined to trust the results of Doug's test than those of a Sencore for instance.

The red gun is weak. Period.

Electronic M
02-07-2017, 05:41 PM
How does injecting a signal then capacitively picking it up with a scope constitute a form of emissions test? It seems more like a way of confirming none of the internal connections to the gun elements are not open. Okay so the red screen is not open, and the gun bias is correct, so by process of elimination the gun must be weak...In his shoes I'd still get a CRT tester and make sure that all the data that fed into that process of elimination is indeed leading to the right conclusion.

benman94
02-07-2017, 06:35 PM
It's a relative emissions test. He's comparing the red gun to the green and blue guns. In a color tube the absolute ability of a gun to emit electrons is meaningless. All three guns must be emitting electrons, and they must have abilities that are fairly close to one another, otherwise correct tracking becomes an impossibility. Just the fact that the red gun is weaker by some factor of four to six is enough: the gun is weak.

Now if all Doug cared about was say producing a uniform red raster, then an absolute test of emissions would be in order. A red gun four to six times weaker than the other two should still produce a uniform field of red. That said, Doug isn't likely to be watching any programming like that, almost all content is a mixture of all three primaries, and thus the relative strength of the three guns becomes the limit on usability.

dtvmcdonald
09-28-2017, 04:38 PM
Its official, my 15GP22 is bad because of gas :tears:

I left it unused for a couple of months, and when I turned on
last night, it arced badly with full HV, maybe twice a second for the
three seconds I left it on. With the socket removed there was no arcing.
With the HV removed there was a tiny ticking sound. Trying it a third time
with both connected I saw an arc near the base, inside it, and immediately
shut it off.

I will now wait the final results of Yurkon's work, unless a working one,
or even another set, shows up for sale.

Electronic M
09-28-2017, 05:03 PM
I seem to recall someone having luck on a mildly gassy tube running the heater alone for a few days(weeks?). IIRC the heat helped the getter action reduce the gas to a usable amount. It may not work, but at this point there is not much to loose in trying it.

miniman82
09-28-2017, 05:34 PM
Tried that with bens AXP which still has perfect getters, ran it for more than a week still has purple neck.

Eric H
09-28-2017, 06:14 PM
Did you ever get the CRT socket apart?

Just asking because at one time back in the late 70's I had a Solid State Philco 19" that I bought refurbished from a TV shop, it had a new CRT installed.

It would be playing fine and all of a sudden let out a loud HV snap that would me me jump out of my chair. It would play fine for hours or days but eventually do it again.

After a half dozen or so of these event the picture started looking lousy and a check showed the emissions had dropped into the bad range.

The shop replaced the tube under warranty but it soon started doing it all over again.
I sold it cheap to another TV repair guy and he figured out it was a bad CRT socket, tracked from the Focus pin to the other pins and would intermittently arc and damage the CRT.

dtvmcdonald
09-28-2017, 08:59 PM
Yes, Eric, I was worried about that. That's whY I let it sit quite a while and then
carefully turned it on, looking for gas. It was gas.

BUT ... I most certainly will open up the socket
and check it if I get a good tube.