Interesting tube fault, any ideas? - UK Vintage Radio Repair and Restoration Discussion Forum

beery · 19th Jun 2007, 2:23 pm

Hi All,
Can anyone shed any light on the problem with a faulty tube I have?

I gained a scrap TV22 some years ago and the owner (Fernseh) told me the tube was dud.
Well I took the tube out and tried it in another set. The results were interesting...

The tube is a MW22-18. The base and the anode cap are loose.

As the set (Ferguson 951T) warmed up a picture appeared briefly for about 2 seconds or so and then disappeared leaving a blank unlit screen. When the picture was there it seemed to have good brightness and contrast.
If I let the set cool down for a few seconds and turned it on again the picture would appear another time.
If I disconnected the ground from the Aquadag and reconnected it the picture would appear for one second.
I measured the tube voltages and they all seemed normal. The brightness control effected only the grid voltage, and the contrast control affected only the cathode voltage.
I did not have my EHT meter with me, so I could not check that.

Any clues?

Cheers
Andy

Steve_P · 19th Jun 2007, 2:32 pm

Clean and resecure the base and if poss the anode cap.

Are you sure the set's OK? Measure the EHT with the cap on and off the tube base. All I can think of so far is that something is decking the EHT.

Cheers,

Steve P

beery · 19th Jun 2007, 4:58 pm

Hi Steve,
When the picture fades out it does not seem to balloon.
No blue flashes are observed internally.
The chassis was working perfectly, and tried with another two tubes which were fine.
I have since broken up the chassis for spares.
The base is bakelite, rather like you would get on an octal valve, so it is connected to the tube with long thin wires.
I was wondering if the fault was consistent with an o/c A1 connection??

Cheers
Andy

Focus Diode · 19th Jun 2007, 6:00 pm

I'm sure I read of a similar CRT fault in "Television" caused by one of the pins being bent thus not making contact with the holder, although I can't remember which pin was responsible. In this case the picture came on perfectly for a second, then "blur" away.

Brian R

Mikey405 · 19th Jun 2007, 7:06 pm

Hi Andy.

My Philips K4 had a similar problem where the picture would appear and then fade out shortly afterwards and then sometimes fade back in etc. It turned out to be the heater connection from where the wires came out of the tube to the tube pins themselves. Melting some solder down the pins remade the connection and the problem never happened again.

From Mike.

Heatercathodeshort · 19th Jun 2007, 9:34 pm

Hello Andy,
This sounds like an intermittent O/C cathode connection. It's rare with Mullard tubes, but very common with Mazda 'Gold Star' tubes from the mid-sixties.

If you tap the neck of the tube while switched on you might find that the picture appears for a moment. Switching the room lights out may produce a very dim but focused raster that is not affected by the brightness control.
It is unlikely to be the first anode pin, unless the fault is in the base wiring. The base can be removed, the wires extended with five amp fuse wire, [six inches] the pins cleaned of solder and rethreaded.

It's a nightmare job, but you have nothing to lose if the tube is 100% faulty.

Regards, John.

Duke_Nukem · 20th Jun 2007, 8:55 am

When testing the tube in my TV12, it behaved very oddly. As the heater warmed up the emission rose, then suddenly it dropped right back. It was quite repeatable. Sounds like the same fault ?

An intermittent o/c cathode certainly sounds very plausible; as soon as o/c occurred the cathode would go +ve until no more electrons could be accelerated. If a bit of capacitance remained between pin wire and cathode then I guess some sort of image would be produced, though I wouldn't expect it to be very good (major loss of low frequencies).

TTFN,
Jon

19th Jun 2007, 2:23 pm	#1
beery Heptode Join Date: Jun 2006 Location: Ware, Hertfordshire, UK. Posts: 988	Interesting tube fault, any ideas? Hi All, Can anyone shed any light on the problem with a faulty tube I have? I gained a scrap TV22 some years ago and the owner (Fernseh) told me the tube was dud. Well I took the tube out and tried it in another set. The results were interesting... The tube is a MW22-18. The base and the anode cap are loose. As the set (Ferguson 951T) warmed up a picture appeared briefly for about 2 seconds or so and then disappeared leaving a blank unlit screen. When the picture was there it seemed to have good brightness and contrast. If I let the set cool down for a few seconds and turned it on again the picture would appear another time. If I disconnected the ground from the Aquadag and reconnected it the picture would appear for one second. I measured the tube voltages and they all seemed normal. The brightness control effected only the grid voltage, and the contrast control affected only the cathode voltage. I did not have my EHT meter with me, so I could not check that. Any clues? Cheers Andy

19th Jun 2007, 2:32 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Interesting tube fault, any ideas? Clean and resecure the base and if poss the anode cap. Are you sure the set's OK? Measure the EHT with the cap on and off the tube base. All I can think of so far is that something is decking the EHT. Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

19th Jun 2007, 4:58 pm	#3
beery Heptode Join Date: Jun 2006 Location: Ware, Hertfordshire, UK. Posts: 988	Re: Interesting tube fault, any ideas? Hi Steve, When the picture fades out it does not seem to balloon. No blue flashes are observed internally. The chassis was working perfectly, and tried with another two tubes which were fine. I have since broken up the chassis for spares. The base is bakelite, rather like you would get on an octal valve, so it is connected to the tube with long thin wires. I was wondering if the fault was consistent with an o/c A1 connection?? Cheers Andy Last edited by Mike Phelan; 1st Apr 2008 at 5:00 pm. Reason: S&P

19th Jun 2007, 6:00 pm	#4
Focus Diode Octode Join Date: Jan 2006 Location: Borough of Gateshead, UK. Posts: 1,420	Re: Interesting tube fault, any ideas? I'm sure I read of a similar CRT fault in "Television" caused by one of the pins being bent thus not making contact with the holder, although I can't remember which pin was responsible. In this case the picture came on perfectly for a second, then "blur" away. Brian R

19th Jun 2007, 7:06 pm	#5
Mikey405 Octode Join Date: Jul 2006 Location: Solihull, West Midlands and Beaford, Devon Posts: 1,626	Re: Interesting tube fault, any ideas? Hi Andy. My Philips K4 had a similar problem where the picture would appear and then fade out shortly afterwards and then sometimes fade back in etc. It turned out to be the heater connection from where the wires came out of the tube to the tube pins themselves. Melting some solder down the pins remade the connection and the problem never happened again. From Mike. __________________ G7TRF

19th Jun 2007, 9:34 pm	#6
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Interesting tube fault, any ideas? Hello Andy, This sounds like an intermittent O/C cathode connection. It's rare with Mullard tubes, but very common with Mazda 'Gold Star' tubes from the mid-sixties. If you tap the neck of the tube while switched on you might find that the picture appears for a moment. Switching the room lights out may produce a very dim but focused raster that is not affected by the brightness control. It is unlikely to be the first anode pin, unless the fault is in the base wiring. The base can be removed, the wires extended with five amp fuse wire, [six inches] the pins cleaned of solder and rethreaded. It's a nightmare job, but you have nothing to lose if the tube is 100% faulty. Regards, John. Last edited by Mike Phelan; 1st Apr 2008 at 5:02 pm. Reason: S&P

20th Jun 2007, 8:55 am	#7
Duke_Nukem Octode Join Date: Jan 2003 Location: Birmingham, West Midlands, UK. Posts: 1,268	Re: Interesting tube fault, any ideas? When testing the tube in my TV12, it behaved very oddly. As the heater warmed up the emission rose, then suddenly it dropped right back. It was quite repeatable. Sounds like the same fault ? An intermittent o/c cathode certainly sounds very plausible; as soon as o/c occurred the cathode would go +ve until no more electrons could be accelerated. If a bit of capacitance remained between pin wire and cathode then I guess some sort of image would be produced, though I wouldn't expect it to be very good (major loss of low frequencies). TTFN, Jon