Need advice on making CRT tester lead, shorted tube?

[crtgamer]

Hi
I have a sony bvm crt monitor (BVM-14E5E), that has developed an intermittent red burst
fault.

Tapping tube seems to have no effect at all,
swapping video amps(internal leads) and the problem was still on red,
possible thermal/voltage sensitive as it does not usually appear straight away,
however turning monitor off and on can make it disappear (temporarily).
As it is booting up? the screen is still briefly red then goes to normal, leading me to believe that the beam feedback correction circuit is hiding a cathode to heater short, therefore the only possible to diagnose using crt tester

I have a muter bmr 95, but unfortunately not the "881" lead required so i need to make my own.I have obtained 9-pin sockets that will fit the tester side, but as i am unfamiliar with crt sockets i have no idea what will socket will fit the tube(M34LHF23X), what it might be called or where i might get one.

If anybody is able to give me advice on obtaining suitable crt socket or other hints on diagnosing or fixing problem will be most welcome
Thanks

[G6ONEDave]

It will help if you post a picture of the Sony crt socket that's fitted to the monitor showing front and rear, to help identify it. Not all members will have access to a service manual to look at printed images.

If after trying a check for shorts, clean and balance with your Muter and the crt still has the problem, you might get an improvement by fitting a heater isolation transformer to supply the crt heaters, thus isolating them from the rest of the monitors circuitry.
Dave

[crtgamer]

Hi, have attached several photos of the socket.

If fitting an isolation transformer i assume 240v-6.3v(correct me if i am wrong), what minimum amps/va would be safe?

[G6ONEDave]

The heater supply voltage will need to be confirmed but most likely will be 6.3v. The transformer amp rating will depend on what the crt actually requires. The service manual may give a crt rating, otherwise check the online specification data for the actual crt. Remember that htere are 3 heaters due to the crt being colour. 240v primary input will be fine and the secondary will be properly isolated if it's a crt transformer, as this is part of the design for this type of tx. These crt transformers often have a boost tapping at 10% or 20% above the nominal 6.3v, which is useful if the crt is losing emission and can't be revived any other way. Of course there is a finite length of time that the boost will work for as it over runs the crt heaters.

I have found a used crt connector socket but I think it's for the smaller screen size. It has all the holes including the twin seperated section but only has a ring 8 pins to the pcb print. Also the black wire contact near pin 12 is not part of the plastic mould in the one I have, there only being a red and then a white in place of the other black on yours.

I would put a request under the wants section for a Sony crt socket and include the picture, so that other members can see what they are looking for. You will probably get offered a used one but that won't matter for your rejuvenator. You could also include the crt type number.

Hope that helps in some way.
Dave

[crtgamer]

After quite a few hours I eventually worked out all of the wiring and built a suitable universal cable, which I fully tested on a junk 14" TV.

When I tested my BVM tube, after about 15 mins it did indeed measure a short between red cathode and filament, unfortunately the repair function did not remove it (in fact looking at the tube neck it did not appear to do anything, maybe fault in tester, so will have to open and have a look), note unlike most testers this one can repair filament to heater shorts.

I have looked at online repair guides which have said I don't necessary need to repair short, and after isolating filament or removing reference voltage from it (usually earth) can fix the problem.

Has anybody here ever removed that reference voltage, able to give advice?

[beery]

Hi crtgamer,
On the tube socket PCB, unsolder the grounded ends of R16 and C5. Do this by lifting the component legs off the PCB so that they stick up in the air.
Solder the now free ends of R16 and C5 together and also solder a wire to this point. The other end of the wire should be connected to the Red Cathode (pin 12 / TP2). This wire must curve around the CRT socket, not simply going across it and it must take the longer route so as to avoid going near the focus pins.

Hmm, that's some bodge I've described there! How well it will work will depend on the design of the flyback transformer.
If the transformer has a low capacitance between the heater winding and chassis then you will be ok. If the capacitance is too high then the bandwidth (horizontal resolution) of the red gun will be compromised when the heater/cathode short is present. You could check this out with a test card that has a frequency grating on it.

Let us know if this works!

Cheers
Andy

[beery]

Hi again,
Just another quick note of explanation.
There is an argument for simply leaving the free ends of R16 and C5 disconnected, but what you must not be tempted to do is to simply link the heater to the cathode.
This is because the heater is a piece of filament wire which has been coiled like a very thin spring, coated with ceramic insulation and folded into an 'M' shape. This 'M' shape is then pushed into the cylinder that forms the cathode. When heater cathode breakdown occurs, it is usually where the filament is folded. So if then if someone then adds an external connection between one end of the heater and the cathode then there would now be part of the filament shorted out - very nasty.

The original point of R16 is to prevent the heater from floating to a high voltage. Similarly C5 is there to shunt to ground any flyback pulses that have been capacitivly coupled to it via the flyback transformer. Such pulses if not dealt with via C5 would be capacitivly coupled to the cathode and cause vertical lines on the left hand edge of the picture.

That's rather a lot of waffle from me for now.

Cheers
Andy

[beery]

Originally Posted by beery

The original point of R16 is to prevent the heater from floating to a high voltage. Similarly C5 is there to shunt to ground any flyback pulses that have been capacitivly coupled to it via the flyback transformer. Such pulses if not dealt with via C5 would be capacitivly coupled to the cathode and cause vertical lines on the left hand edge of the picture.

Actually by my own logic there, it would be best with C5 completely disconnected then I think. Still connect R16 to the red cathode though.

Cheers
Andy

[crtgamer]

Wow That was a little more of an answer than i was expecting!

First things first,the circuit, as i understand it R16 is the reference voltage, It prevents the voltage going up, and is 220k is to prevent it simply taking the voltage all the way to 0V, and c5 is to bypass any high frequency interference that could lead to potential difference between filament and cathode thus influencing the beam(similar to how cathode resistor bypass cap is used on amplifier valves)

What are the benefits of this method compared to using isolation transformer,are there any risks that letting the voltage float could result to too higher voltage/damage?

Could you explain a little more why i should connect R16 to RK, as i can not see what the purpose of doing so is, and simply lifting the component would be easier and maybe safer?

Thanks

[murphyv310]

Hi.
I worked for Sony in the 90s and this issue was seen on occasions. Isolating the heaters (only done for confirmation of the fault) does work but you get smearing on the red gun, and on monitors it looks pretty awful. Whether there is a way round it is hard to say. I had a Sony monitor to look at in 2019 and it had the same issue and it was eventually scrapped, its also usually the red gun that gets these H to K shorts.

[crtgamer]

Ok, I will try isolating by lifting the 2 components.

This is intermittent temperature sensitive if i get lucky with thermal cycles might get better over time,
If not, well its all ready broken so i guess nothing to lose.

[beery]

Hi crtgamer,
The idea of linking R16 to the red cathode was simply to ensure that the heater supply is not floating during the times when the fault is not present. In theory, leaving it floating means a small risk of compromising the heater-cathode insulation of the other guns, but this is highley unlikely, so by simply disconnecting R16 and C5 it will probably be fine.

About using this method vs a heater isolation transformer. The idea of simply removing the reference for the heater supply is certainly the easiest option.
As I said, it might compromise the bandwidth of the red drive when the short is present due to the capacitance of the heater winding of the flyback transformer to other windings or the transformer's frame.
As I mentioned earlier a black and white high frequency vertical grating will show if that is a problem.
If of course the bandwidth is limited, then a heater isolation transformer would be the only solution.

The downsides to using a CRT heater isolation transformer...
1 where to put it inside the monitor?
2 where to obtain one from?
A modern day 6v split bobbin transformer may or may not have sufficiently low capacitance to do the job. A genuine heater transformer designed for the job is hard to find today and most would only supply enough current for the filament of a black and white CRT.

I hope that makes your options a little clearer.

Anyway, do keep us posted on how we'll it works with the heater reference components removed.

Cheers
Andy

[crtgamer]

Hi, have done a couple of things since last post,

First i connected to my tester so i could see the short and then began tapping the the tube with a stick, idea being if it was debris it might come out, If isolation failure make a solid connection rather than intermitant, in the end created a solid short, i see this as a benefit as reduces chance of flicker, it is shorted when booting allowing the beam feedback correction to work without have to turn off the on later, and no need to wire R16 to Rk.

I then lifted the components legs as you said.

Result where
I could not see any visible flicker/ripple, A little smear but only happened with bright traffic light red and even not by that much.

Other, than that, there is a faint line(1-2mm wide) from top to bottom of screen about 1 cm from right edge, red gun reduction, does not move left to right, when horizontal phase is adjusted(therefore it is possible to under scan it out),I would think this is a signal refection(kind of distortion you get at edge of screen if you forget to put on an 75ohm terminal plug) all most certainly the video amp not liking the load given, have no idea if there is any way to adjust impedance value inside monitor, maybe a custom value terminator plug might reduce it?(at least for RGB)

Anyhow as that line is near the edge and is a reduction in brightness really not that objectionable, especially when the rest of the screen looks fantastic.