Featured
Latest
Can you fix a CRT w...
 
Share:
Notifications
Clear all

Forum 141

B&W TV Can you fix a CRT with an open circuit heater? Yes you can!

18 Posts
7 Users
15 Reactions
464 Views
irob2345
(@irob2345)
Posts: 575
Honorable Member Registered
Topic starter
 

My friend Pete acquired this mint condition (Oz) HMV with the PM series chassis from 1964.

After replacing the power cord and the doubler cans, we powered it up.

The CRT heater cycled on and off like one of those flasher Christmas tree bulbs.

What to do?

I had an idea. Let's hook the CRT heater up to a high voltage that was current limited to something near the 600mA the heater was rated at. That way, when the heater goes open, the arc from the high voltage will weld it back together (like happens with a sticking power relay)

The TV's transformer power supply was deemed capable of supplying 240 volts DC at 600mA for a short period so we used that, with a 120 watt tungsten lamp in series. This gave us just over 4 volts on the working heater.

Success! The lamp blinked off once and stayed on! Obviously the internal break had welded itself back together.

Reverting to normal connections we have a picture! Short lived, though.

 So we tried a couple of 300 watt lamps in series. These giant NOS lamps came from a 2nd hand store in Wodonga. They have a large ES base and used to be used in street lights in the 1950s. That gave us 840mA and 9 volts on the heater.

This came up after one blink. We left it on for half an hour and noticed the voltage across the heater dropped by about 0.2 volts during this time, hopefully as the weld consolidated. The C-core transformer got quite hot but it stood up to the 4X overload.

TV was run this time for a couple of hours while we fixed a few minor faults.

You can get some idea of the amazing cosmetic condition of this nearly 60 year old TV from the picture.

 

 

 
Posted : 28/10/2023 7:47 pm
helloekco, Nuvistor, Alex728 and 4 people reacted
slidertogrid
(@slidertogrid)
Posts: 1068
Honorable Member Registered
 

That was very fortunate ! Any chance of a picture of the set and the chassis? I have had a few tube heaters open over the years but I have only ever been able to get one to reweld. I suppose it depends on the size of the break...?

 
Posted : 28/10/2023 9:11 pm
Cathovisor
(@cathovisor)
Posts: 6449
Famed Member Registered
 

In the UK, there was a device made by Grunther in the 1950s called the "Heatamec" that did exactly this job - a friend has one. More better known was their "Beamec" CRT tester.

 
Posted : 28/10/2023 10:28 pm
irob2345
(@irob2345)
Posts: 575
Honorable Member Registered
Topic starter
 

"Any chance of a picture of the set and the chassis? "

TV is nearly 700km south! I'll ask Pete to send some pics, I only took a couple.

Schematic I can do!

Like many Oz TVs of the period, this one is a service tech's dream. Remove 4 screws, pressed metal back comes off, loosen the wing nut that holds the tuner bracket and the chassis folds down flat, giving you easy access to everything. Being a 1964 model there are no paper caps.

CRT is original Thomas and it must be one of the first shellbond (not laminated) 23" CRTs. The schematic has the CRT as a 23HP4 (laminated), this one is a 23ARP4. The data sheet for this tube does not show the bonded shell....

Gun faults were not unheard-of with Thomas tubes in the early 60s. Open circuit cathodes were more common. Tap the neck with a screwdriver handle and they'd come good.

 
Posted : 30/10/2023 8:35 am
Lloyd
(@lloyd)
Posts: 1940
Prominent Member Registered
 

I managed to weld the heater in a little 11” tube from a Philips T-vette, using a cold cathode fluorescent tube power supply and a 12V SLA battery, that was about 20 years ago now! The tube lasted a good few years, but died again a few years back. I have a replacement tube now, and the set is working (thanks Rich!) so maybe I should have another blast at that duff tube! I also have a dead tube in a Bush TV22, and an old scope tube that also has an o/c heater, both have had treatment with high voltage, but didn’t work.

Regards,

Lloyd

 
Posted : 30/10/2023 10:14 am
Nuvistor
(@nuvistor)
Posts: 4622
Famed Member Registered
 

Posted by: @irob2345

Gun faults were not unheard-of with Thomas tubes in the early 60s. Open circuit cathodes were more common. Tap the neck with a screwdriver handle and they'd come good.

Did they use imported electron guns from Mazda UK, that is exactly the same fault that plagued Mazda CRT’s. Tap on the neck and they would work for a while, usually very short while.

 

Frank

 
Posted : 30/10/2023 11:20 am
Cathovisor
(@cathovisor)
Posts: 6449
Famed Member Registered
 

@nuvistor Mazda were plagued by cathode connections going o/c right back to the AC/TP triode-pentode frequency changer of 1934. A friend once tried three NOS ones in succession to get my Murphy A24 to work and the late Chas Miller told of a set with a length of dowel sticking out the back so the AC/TP could be given a tap when needed.

How Brimsdown ever survived is beyond me.

 
Posted : 30/10/2023 12:17 pm
Nuvistor reacted
irob2345
(@irob2345)
Posts: 575
Honorable Member Registered
Topic starter
 

Mazda built CRTs in Oz in the late 50s, early 60s. They were only used in AEI Ekco TVs and most needed to be replaced after about 12 months use.

They also made "Galamatic" washing machines during this period.

AEI shut the whole operation down in 1962, another victim of the 1961 Credit Squeeze, and stuck with irons, electric jugs and toasters. And rebadged AWA radios.

Pretty sure Thomas was building CRT guns at the Riverwood, Sydney plant. If not, they would have imported them from Thomas in the US.

Pete sent me pics of the HMV. I'll get them off the phone and post them here.

 
Posted : 31/10/2023 8:29 am
irob2345
(@irob2345)
Posts: 575
Honorable Member Registered
Topic starter
 

As promised, here are some pictures of the HMV and its CRT rescue.

CRT label
Before flip down

That flip-down chassis arrangement became very popular in the '60s.

The PM chassis was the cheaper of the two versions HMV made at the time. Even so, it's a pretty solid performer. Lacks stabilised line OP, which you would expect to see in a design at this level.

Oz TVs of this period often had to work in marginal reception areas and handle Band 1 VHF which was plagued with noise from power line insulators. They also needed to be able to receive weak, distant signals in the presence of adjacent channel strong signals.

That shellbond CRT is obviously an early example, note the 23ARP4 label (not a shellbond according to the datasheet) but the letters SB have been hand written. Most PM chassis had 23HP4 laminated CRTs.

 
Posted : 01/11/2023 8:43 am
Nuvistor
(@nuvistor)
Posts: 4622
Famed Member Registered
 

@irob2345 The UK had bonded faceplates on some CRT’s others Fenbridge guards, a separate thin plastic sheet stretched over the faceplate and had to be reused on replacement CRT’s. I disliked the Fenbridge, easily damaged and could be difficult to reuse.

Can you explain the difference between the laminated and shellbond? I can probably guess but knowing me I would probably be wrong.

That TV has restored very well. 👍 

Thanks

Frank

 
Posted : 01/11/2023 11:27 am
irob2345
(@irob2345)
Posts: 575
Honorable Member Registered
Topic starter
 

I remember that plastic coating, didn't know it had a name! Didn't know you could replace it. Saw it on a British TV brought over by an immigrant family in the '60s. Had to change the standard selector switch to lock it to 625 lines while using the VHF tuner. Also had to re-tune the sound from 6MHz down to 5.5. Oh, and replace a dead damper - the series heater version of the 6AL3. It was hard to find here at the time.

TV was fairly new but the Mazda CRT was already inverting its whites. I was not impressed.

A laminated CRT had a separate piece of glass bonded to the CRT faceplate. We only ever saw 23 inch CRTs that were laminated.

A shellbond CRT is a standard tube that's been glued with epoxy into a pressed sheet metal shell with integral mounting holes. This holds the face glass in compression in the event of an implosion, preventing injury to viewers.

There was a similar technique called "rimbanding" where the mounting brackets were attached with a tensioned steel band. It was found that this worked just as well and was cheaper to implement.

I wonder how many CRTs imploded while the bands were being tensioned?

I'm afraid that TV's CRT looks better in the pictures than it does in real life, it looks like you'd expect a high hours tube to look.

 

 
Posted : 02/11/2023 7:44 am
Nuvistor
(@nuvistor)
Posts: 4622
Famed Member Registered
 

@irob2345 Thanks for the information, I can’t remember glass being bonded but I thought the ones in the UK were some form of plastic bonded onto the glass faceplate with lugs to mount the CRT.The ShellBond, yes we called them Rimband or the Mullard trade name was Panorama. On the whole Mullard mono CRT’s were much more reliable that the Mazda ones in my experience, can’t comment about colour ones, I think I saw very few Mazda colour CRT’s, either Mullard or Japanese ones.

 

Frank

 
Posted : 02/11/2023 8:45 am
irob2345
(@irob2345)
Posts: 575
Honorable Member Registered
Topic starter
 

We saw a few Mazda colour CRTs here in '74. Initially in 4KAs (Thorn 4000 chassis made by AWA) and then in the UK 3504s that AWA-Thorn imported because they couldn't make enough 4KAs (that kept working!) to keep up with the demand. The Mazda tubes just weren't competitive and they wore out quickly. Better colour than the RCAs that replaced them though.

Mullard CRTs were Philips here of course and definitely the best after Toshiba and Hitachi tubes.

 
Posted : 03/11/2023 8:42 am
slidertogrid
(@slidertogrid)
Posts: 1068
Honorable Member Registered
 

It sounds as if the CRT you know as ShellBond was what we had as 11W (or 11X if colour) that is direct vision but not 'push through'. later Rimband CRTs were 120W or 120X.

A 59-11W, Tube A59-11W; Röhre A 59-11W ID3506, Television pi | Radiomuseum.org

 
Posted : 03/11/2023 10:11 am
jcdaze
(@jcdaze)
Posts: 160
Reputable Member Registered
 

I remember been shown how to try and fix an open circuit crt heater in the days of valve line output stages. It would sometimes work and there was nothing to lose by trying it. The method was to unplug the crt base and link the heater pins in order to keep the heater chain intact. Then, on the crt itself, one of the heater pins was linked to the chassis. Also another lead connected to the top cap of the line ouput valve and the other end connected to a screwdriver with a well insulated handle. The television was then turned on and when line output was heard working we would dab the screw driver to the crt heater pin that wasn't shorted to chassis and with a bit of luck there would be an arc inside the crt neck and, hopefully, the open circuit heater would have welded itself together and save a crt change. Job done!

 
Posted : 08/11/2023 11:54 am
unit no 16 reacted
Nuvistor
(@nuvistor)
Posts: 4622
Famed Member Registered
 

I must have been lucky, I don’t remember ever having an o/c heater in a CRT, other faults, o/c cathode, low emission being about the only reasons.

Perhaps time has faded my memory, quite likely.

Frank

 
Posted : 08/11/2023 1:55 pm
slidertogrid
(@slidertogrid)
Posts: 1068
Honorable Member Registered
 

I had a few back when I was a lad playing with old scrap sets I had found. An odd one in colour sets where one heater had gone out. Nothing could be done there. More recently though I had one in a dual standard Murphy black and white set, it wouldn't respond to attempts to weld it, but luckily I had a second hand tube to replace it with. 

I used the arcing EHT method to reactivate a thin neck mono tube in a scrap Thorn 1591 a couple of years ago. Just to see if it would work, I strapped the heaters together to prevent blowing them O/C earthed the cathode and then sprayed the EHT around the pins... It worked quite well. The tube was very very flat and it came up to be quite good.  The set was a complete scrapper I saved from a skip with a destroyed cabinet so I don't know how long it would have lasted because after a short reprieve in the shed I saved the chassis and tuner and binned the rest.

I Found the write up I did at the time, I can't believe it was three years ago! 

Rough Alba - UK Vintage Radio Repair and Restoration Discussion Forum (vintage-radio.net)

 

 
Posted : 08/11/2023 2:36 pm
unit no 16 reacted
Captain Peacock
(@captain-peacock)
Posts: 29
Trusted Member Registered
 

I too, was shown and used a similar method which worked almost every time: however, I used to 'brush' the top cap of the boost rectifier (PY800 in mono sets) instead of the line output valve (PL36 or later PL500/504) top cap. You knew when the O/C heater had welded itself as the intensity of the spark from the valve top cap suddenly became much more fierce! I only gave a handful of CRTs this treatment as an O/C heater was a rare fault - particularly with Mullard CRTs. The more common heater failure was it going short circuit rather than open circuit. I think all those CRTs I gave this treatment to, lasted at least a year in the customer's house. Most of the CRTs I gave this treatment to were the 70/90 degree MW/AW43-69/80 series, along with a few narrow-neck 110 degree specimens (AW 47/59-91 etc.).

This procedure could not be done on a colour CRT as there were a) three heaters in parallel and b) many colour TV sets (e.g. the Thorn 3000/3500) used to apply a DC 'standoff' voltage of around 90V to the heaters to help prevent heater-cathode shorts and such treatment often resulted in ruined picture quality due to a high or medium resistance heater-cathode short.

I remember at Radio Rentals, after a few months of their introduction, we had terrible trouble with the Baird 8750s (Korting Transmare 53633 chassis(?) with a A56-120X) as in their 'wisdom', the designers had earthed one end of the heaters, creating a lovely opening for the 200+V modulating voltage on the cathodes to find its way to earth - and destroy the (initially yellow label Sylvania) CRT in the process. We had to urgently modify all those sets by removing the earth and taking a DC feed from the main HT onto the heaters via a suitable potential divider network. That killed the problem stone dead!

 
Posted : 18/11/2023 12:21 pm
crustytv and Cathovisor reacted
Share: