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?

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?

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.

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

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

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.

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.

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.

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?

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.

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.

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).

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.

Vibration or coincidence.... that is the question :scratch2:

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

jr

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.

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.

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).

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'.

Do you have a dmm with the "peak hold" function?

jr

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.

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.

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.

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.

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.

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.

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.

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.)

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.

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.

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

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.

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....?

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.

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.

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.

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....

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.