Crackling Inside Beovision 7802

[Okto1984]

Hi, I'm new here and hope someone can help me with my TV set. Hopefully it's also vintage enough (I think it's from 1982)?

I have just received a beovision 7802 which was apparently in good working order when it left. Well it does have perfect picture and sound, but it's making a loud fizzing-hissing-crackling from inside (it's not from the speakers) whenever it's out of standby and displaying a picture.

It might have had a bad time in transport as the fold out tuning door was kind of bumped and needs some glue.

Anyway, I'm a competent DIYer, I can wire plugs, solder and have replaced parts like motors in things before, but I don't know a lot about TVs beyond the general theory. So, I'm not going to open it up just yet as I don't know what to look for, and I'm not using either it until I have a better idea what might be going on. Just in case it's a very bad thing that's happening.

[Nickthedentist]

He and welcome!

It could indeed be a very bad thing so be wary about switching it on again.

It's probably "arcing" that you're hearing - basically sparking when insulation's breaking down or when there's a bad connection, often a "dry" solder joint.

I would operate it briefly, in the dark and with the back off, and look for sparking, to help you track down where the noise is coming from. Obviously, don't touch anything while it's plugged in etc. etc.

Good luck in fixing it,

Nick.

[bobbyball]

There are a few things that can "fizz" on this series of models.

The high voltage for the tube is created within the large transformer assembly on the main power board (on the right, looking from the back). The lead from this comes out and takes a fairly tortuous route to the tube and anode cap, where a "bleed" resistor for the focus supply is fitted inline on the cable - looks like a small box. A wire from this then goes back to the focus pin on the tube base. The main EHT cable is screened, and can break down causing arcing, also if there is a lot of dirt around the final anode cap on the tube (at the top in the middle), there can be discharge in this area. The high voltage transformer can arc internally, making the noise you hear.

The high voltage is 25KV and is very dangerous: it can kill. Even when the set is switched off there will still be a residual charge in the capacitance of the tube. If the anode connector needs to be removed, the metal connector undeneath should immediately be touched to the graphite coating on the tube, and then, using a thick piece of wire, the anode connector on the tube be itself shorted to the graphite coating.

If at all in doubt, do not touch!

[dazzlevision]

Hello,

It may be as simple as condensation forming inside the set when brought from a cold environment into a warm one (i.e. whilst in a cold van or lorry and then into your house).

I would leave it in a warm room for a day or two, to dry out thoroughly and then try again. Be ready to turn off quickly if there is a sharp "crack" noise which indicate high voltage flashover.

Regards,

Dazzlevision

[Okto1984]

Thank you to everyone for your kind advice. I have good news too, I was too excited to try out the new TV so put it on again (before reading the cautions here), and after a little while the crackling got quieter and eventually just stopped. Only a faint humming now, which I believe is normal for a tube TV.

Originally Posted by dazzlevision

It may be as simple as condensation forming inside the set when brought from a cold environment into a warm one (i.e. whilst in a cold van or lorry and then into your house).

I suspect this was it in the end. A big heavy glass tube coming in to my relatively warm house would have gathered quite a bit I suspect. It did sound pretty scary for a while. I'll be more careful to let it warm up if I move it again. Hopefully the other possible causes like breaking down insulation aren't a problem with this TV, but is there any way to know?

As for the TV itself, I'm impressed. I know it's not SD resolution, but it really does look sharp, bright and has good colours. Excellent sound too. Power switch is a bit unreliable, just like a similar age beovision 9000 I have in the sitting room, but I can manage with the wall switch and leave the switch on the set alone. Only trouble I have is a small scratch on the screen, it's not too bad really, but perhaps there's some way to improve it?

Anyway, thank you all again for the advice. I wouldn't have suspected high voltage or humidity before.

[Nickthedentist]

Well done Dazzlevision for working it out, and glad it's working well now.

I had a Panasonic CRT set in an outhouse for a few years and brought it in recently as a stop-gap measure to replace a big set that had failed. I proudly switched on and there was a loud crack and the set went into standby, much to my embarrassment as I'd just announced that "these old tellies never go wrong". As with your set, it was fine when it had dried out and I tried it the next day - So your story should have set alarm bells ringing for me!

BTW if you're into your "retro" technology and have VHS videos as well, the same caveat applies but for a different reason: condensation on a cold video head drum or tape will cause the two to stick to one another, wrecking the tape and making a mess of the head drum, so beware.

The buzz you're hearing is probably the scan coils on the tube neck: the vertical deflection ones often make a quiet buzz, which is quite normal.

Nick.

[Okto1984]

It just sounds like a normal TV humming. Nothing worrying. I can also see some kind of glowing inside through the vent, but my other set glows too so I assume it's normal.

I wonder if it is possible to damage a TV by turning it on with condensation inside though?

As for VCRs, that's useful to know, but I don't actually have one any more. I've found something else which I think is a better way to use an old TV. I'm using this little box from a Chinese company called viewHD that turns HDMI output in to component video and audio. It keeps everything in sync with the audio and has a sharp smooth picture. So, now I can attach it to a blu ray player with smart TV features like BBC iplayer and 4OD. Another good thing is that BBC iplayer and most all the smart TV content is full screen on the 4:3 set, so no letter box. Not sure why that is, it's also full screen on my widescreen laptop, but it just seems to know and fits the screen size without being distorted.

[Nickthedentist]

Originally Posted by Okto1984

I can also see some kind of glowing inside through the vent, but my other set glows too so I assume it's normal.

That's the tube heaters. Perfectly normal

[hamid_1]

An orange glow in the back of the TV, directly behind the centre of the screen, is normal. That's the CRT heater.

If you see flashes of white or blue / purple light, often accompanied by a crackling sound, that's called arcing - it's definitely bad. Sometimes you can't see anything with the back cover in place, but you can hear sparking and sense a distinctive smell. In either case, you should switch off the TV straightaway.

To explain things a little, CRT TV sets generate high voltages, typically 25,000 volts for a colour TV. Normally this extra high tension (EHT) voltage is safely contained inside the set, but sometimes it can escape and take a short cut. When that happens, that's called arcing. Arcing can be caused by dampness, buildup of dirt or breakdown of insulation.

Originally Posted by Okto1984

I wonder if it is possible to damage a TV by turning it on with condensation inside though?

Possibly. If there's dampness or condensation inside, arcing can occur. Extra high voltage can take a short cut across the damp parts. Sometimes it can go through other components along the way. Transistors and integrated circuits (chips) are easily damaged by high voltage arcing. For example an arc could damage the field output chip, resulting in a bright horizontal line across the screen. If the TV was left switched on in this condition, the picture tube could be damaged too.

Another danger is that if the TV is left switched on and the arcing becomes worse, the sparks could start a fire. The risk is small, but since your TV has had a history of arcing, I would advise you not to leave it switched on when left unattended.

[Welsh Anorak]

Hi
Glad it's going now - these sets like to be kept warm, and it's always a good idea to make sure it's used frequently. A hair dryer is useful if necessary. If it's quiet now then I'd leave well alone.
On your other points, the switch is notoriously unreliable, and even more notoriously difficult to change, so again it's best to leave well alone. Don't be tempted to try and improve the scratch - rubbing compound appears to work until you turn the TV on when you get to see all sorts of rainbow effects that can't be removed!
Just enjoy your 'new' TV.
Glyn

[Peter.N.]

This was a very common problem when I was working, I would deliver a set that had been working perfectly in the workshop but cracked and banged at the customers, sometimes it would cause damage so I would have to return it to the workshop. If you remove the eht connector from the tube, just touch the connection with a wire connected to the chassis to make sure its discharged, otherwise it can bite, then thoroughly clean all the glass surrounding the connecter with methelated spirit on a soft cloth then dry. This will help avoid future problems.

Peter

[Marconi_MPT4]

Originally Posted by Peter.N.

...then thoroughly clean all the glass surrounding the connecter with methelated spirit on a soft cloth then dry.

Even when new, fizzing and crackling around the anode connector was a very common problem with this chassis. Although a good clean with methylated spirit cures most as described in previous post, on some it was necessary to also clean the surrounding insulation of the connector itself due to conductive tracks burnt into the material. This is not a problem that can be left to fix itself, as once tracking has started it will deteriorate.

Some that had been left to fizz away eventually developed conductive surface tracks in the CRT glass itself which were difficult to remove, but an abrasive metal polish usually fixes the less severe cases. I recall that only one tube had to be replaced due to a crack in the glass spreading out from the anode cavity.

The EHT cables themselves were prone to fail as was the focus voltage series resistor. At the transformer end, a similar crackling and flash over can occur if operated in a damp or humid atmosphere. Although the EHT cable makes a connection at about 2 inches or so inside the EHT transformer, there have been many cases where tracking occurs up through to the top by the cable clamp and sparks all over the place! Both the transformer and EHT lead have to be replaced. I am guessing this may not be the experience of others but could have something to do with being on the South coast, all that salt air!

On the subject of EHT leads, it is quite common to find the metal connector has detached from the cable, thus leaving it inside the transformer turning a simple repair into a major job. Unless you need to detach at the transformer end, DON'T.

Cheers
Rich

[Maarten]

Originally Posted by Peter.N.

If you remove the eht connector from the tube, just touch the connection with a wire connected to the chassis to make sure its discharged

Never discharge the tube to the chassis on any TV set! This could cause damage to - amongst others - the beam current limiter circuitry.

Always use a wire attached to the aquadag (the outer carbon screening of the tube).

I agree on the cleaning with spirit (Isopropanol is also suitable). Take care not to touch the aquadag with it as it will dissolve and may even cause flashovers if rubbed out onto the insulated area around the connector.

[Peter.N.]

Another senior moment, you would obviously discharge the CRT to its coating - you would have a job reaching the chassis with a screwdriver. I did this for about 50 years without blowing anything up.

MPT4

Yes, I forgot about the tracking, have on many occasions had to scrape the carbon from the inside of the cavity connector with a Stanley knife.

Peter

[Nuvistor]

I would be interested to hear if any others members have heard of this method of discharging the final anode.
I was taught that just using a s/c could damage the final anode connector to DAG inside the CRT and it was better to discharge through a HV/high value resistor.

On the damage to logic circuits etc, this is a special probe made for the job of protecting the logic circuits if the discharge probe was used incorrectly.

http://www.biwa.ne.jp/~shamada/fullm...l#DischargeCRT

see section 4 Discharge Tool

I have not worked on Apple monitors, my training was many, many years ago.

[McMurdo]

I was taught to use an HV probe but I just use a long handled screwdriver with a croc clip lead linking it to the dag braid, warning anyone of a nervous disposition to expect a snap.
Sometimes of course the dag braid isn't connected directly to chassis so flashing it to chassis could possibly cause semiconductor damage...I might have done it once and regretted it, I dont honestly remember

[Okto1984]

This is all interesting, but I'm a little confused about some points.

The main thing is that I was still experiencing some fizzing, although much less dramatic, so I followed the advice to discharged the tube, remove the cap and cleaned up the area with meths. Reassembled it, and the fizzing is completely gone, in fact it's silent now.

So, that seemed to work great. However, I believe the stuff I cleaned off (thinking it 30+years of grime) was the aquadag. I cleaned up an area about 4" by 5" with the tube cap in the middle to make it perfectly shiny glass. It seems fine, better than before as there's no fizzling, but is this clean patch without coating a bad thing? Do I need to paint some new aquadag on? The rest of the tube is still coated.

[AC/HL]

The Aquadag doesn't extend to the connector, there is a patch of clear glass around it.

[chip-chop]

This takes me back, having replaced very many CRT's on the bench back in my old Comet days, (some 30 odd years ago!)

I quite often used to find that after simply shorting out the EHT charge on the CRT with a screwdriver or wire, it would often re-attain a charge albeit a lesser one but still enough to give a nasty surprise when you're holding the tube after removing it, I've always assumed this was due to some kind of piezo effect occurring due to the rapid discharge.

I took to using a high wattage 4.7 MΩ resistor to discharge it more gently, I also supplemented this by spraying a light coat of Foam Cleaner between the Dag and anode cap to provide a conductive path.

Care should be used with the foam cleaner though, I tried using as the main discharge route (without the resistor) and found that it's flammable

[Marconi_MPT4]

Originally Posted by Nuvistor

I would be interested to hear if any others members have heard of this method of discharging the final anode. I was taught that just using a s/c could damage the final anode connector to DAG inside the CRT and it was better to discharge through a HV/high value resistor.

I recall the lecturers used to strongly advise using a high voltage, high value resistor to discharge a CRT and leave it connected while working on the set. With glass as dielectric a high dV/dt during discharge nearly always results in the anode reacquiring previous voltage. In general a slow discharge generally overcomes this effect.

Regarding the B&O 33xx series TV, all owners of these sets should check aquadag earthing strap security and that a modification has been done. Early sets used plastic clips on either side of the tube to anchor the earthing strap and spring. Under tension these clips break allowing the strap to droop and touch the EHT/power board, that in the process of not blowing up the power supply could make the isolated chassis live! Modified sets should have two metal hooks either end of the strap attached to the CRT frame. I would have expected most to have been modded by now as any set supplied and serviced by a B&O dealer would have been scheduled for a check up.

Cheers
Rich