Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > Early Color Television

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #31  
Old 01-19-2017, 01:50 PM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 795
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.
Reply With Quote
  #32  
Old 01-19-2017, 03:16 PM
Electronic M's Avatar
Electronic M Electronic M is offline
M is for Memory
 
Join Date: Jan 2011
Location: Pewaukee/Delafield Wi
Posts: 8,847
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.
__________________
Tom C.

What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4

Reading between the scan lines since the mid 2000's.
Reply With Quote
  #33  
Old 01-19-2017, 04:07 PM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 3,541
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?

jr
Reply With Quote
  #34  
Old 01-19-2017, 04:09 PM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 795
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.
Reply With Quote
  #35  
Old 01-19-2017, 04:23 PM
benman94's Avatar
benman94 benman94 is offline
Super Moderator
 
Join Date: Jun 2011
Location: Detroit, MI
Posts: 767
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....?
Reply With Quote
Audiokarma
  #36  
Old 01-19-2017, 04:41 PM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 795
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.
Reply With Quote
  #37  
Old 01-19-2017, 05:03 PM
Electronic M's Avatar
Electronic M Electronic M is offline
M is for Memory
 
Join Date: Jan 2011
Location: Pewaukee/Delafield Wi
Posts: 8,847
Quote:
Originally Posted by dtvmcdonald View Post
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.
__________________
Tom C.

What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4

Reading between the scan lines since the mid 2000's.
Reply With Quote
  #38  
Old 01-19-2017, 07:10 PM
old_coot88 old_coot88 is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 1,845
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.
Reply With Quote
  #39  
Old 01-20-2017, 12:59 AM
N2IXK's Avatar
N2IXK N2IXK is offline
Technohippie
 
Join Date: Apr 2011
Location: Sittin' on the "Group W" bench...
Posts: 737
Quote:
Originally Posted by Electronic M View Post
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....
Reply With Quote
  #40  
Old 02-02-2017, 10:04 AM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 795
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.
Reply With Quote
Audiokarma
  #41  
Old 02-02-2017, 10:58 AM
benman94's Avatar
benman94 benman94 is offline
Super Moderator
 
Join Date: Jun 2011
Location: Detroit, MI
Posts: 767
On my flawless 15GP22, the red screen did function just like the other two. Why wouldn't it?
Reply With Quote
  #42  
Old 02-02-2017, 12:24 PM
Pete Deksnis's Avatar
Pete Deksnis Pete Deksnis is offline
15GP22 demo @ ETF 2007
 
Join Date: Jan 2005
Location: Big Rapids, MI
Posts: 734
Quote:
Originally Posted by dtvmcdonald View Post
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
Reply With Quote
  #43  
Old 02-02-2017, 02:26 PM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 795
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.
Reply With Quote
  #44  
Old 02-02-2017, 04:10 PM
benman94's Avatar
benman94 benman94 is offline
Super Moderator
 
Join Date: Jun 2011
Location: Detroit, MI
Posts: 767
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...
Reply With Quote
  #45  
Old 02-02-2017, 06:31 PM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 795
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%).
Reply With Quote
Audiokarma
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 02:10 PM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
ęCopyright 2012 VideoKarma.org, All rights reserved.