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
  #16  
Old 01-15-2017, 11:34 PM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 3,541
Vibration or coincidence.... that is the question

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

jr
Reply With Quote
  #17  
Old 01-15-2017, 11:44 PM
Tim's Avatar
Tim Tim is offline
VideoKarma Member
 
Join Date: Mar 2004
Location: New Jersey
Posts: 476
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.
__________________
Tim
Reply With Quote
  #18  
Old 01-15-2017, 11:51 PM
old_coot88 old_coot88 is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 1,845
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.

Last edited by old_coot88; 01-16-2017 at 12:10 AM.
Reply With Quote
  #19  
Old 01-16-2017, 09:53 AM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 795
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).

Last edited by dtvmcdonald; 01-16-2017 at 10:05 AM.
Reply With Quote
  #20  
Old 01-16-2017, 10:12 AM
old_coot88 old_coot88 is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 1,845
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'.
Reply With Quote
Audiokarma
  #21  
Old 01-16-2017, 11:57 AM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 3,541
Do you have a dmm with the "peak hold" function?

jr
Reply With Quote
  #22  
Old 01-16-2017, 01:26 PM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 795
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** . 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.
Reply With Quote
  #23  
Old 01-16-2017, 01:49 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 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.
__________________
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
  #24  
Old 01-16-2017, 01:58 PM
old_coot88 old_coot88 is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 1,845
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.
Reply With Quote
  #25  
Old 01-18-2017, 06:51 PM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 795
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.

Last edited by dtvmcdonald; 01-18-2017 at 07:05 PM.
Reply With Quote
Audiokarma
  #26  
Old 01-18-2017, 10:54 PM
Penthode's Avatar
Penthode Penthode is offline
VideoKarma Member
 
Join Date: Jun 2009
Location: Syracuse, NY
Posts: 658
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.
Reply With Quote
  #27  
Old 01-19-2017, 07:24 AM
benman94's Avatar
benman94 benman94 is offline
Super Moderator
 
Join Date: Jun 2011
Location: Detroit, MI
Posts: 767
Quote:
Originally Posted by dtvmcdonald View Post
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.
Reply With Quote
  #28  
Old 01-19-2017, 09:51 AM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 795
Quote:
Originally Posted by benman94 View Post
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.
Reply With Quote
  #29  
Old 01-19-2017, 10:10 AM
dieseljeep dieseljeep is offline
VideoKarma Member
 
Join Date: Jul 2010
Posts: 5,729
Quote:
Originally Posted by benman94 View Post
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.
Reply With Quote
  #30  
Old 01-19-2017, 10:35 AM
benman94's Avatar
benman94 benman94 is offline
Super Moderator
 
Join Date: Jun 2011
Location: Detroit, MI
Posts: 767
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.)
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 03:47 PM.



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