Waking up a Bush TV96.

[linescan87]

Good afternoon,

I Put this set up on the bench this morning, replaced one of the fuses which appeared to have died of old age. Powered up and after 30 seconds heard a fairly strong line whistle and got the familiar cramped raster.

Not a bad start, I shall replace the boost cap tomorrow and start work around the frame stage. Off to work now.

Cheers,
John Joe.

[Heatercathodeshort]

Guaranteed good Mullard AW43-88 CRT with this model. The silicon diode HT rect mod looks a bit rough but is probably OK. Put the mains adjuster in the 240/250v position. Expect LOPT dampness problems and if you have line sync problems, before you start checking diodes etc, check the continuity of the primary of the discriminator transformer. J.

[mark pirate]

That looks a very promising start, probably fully working when retired.
Chassis shows no signs of bad storage & the CRT is nice & bright.
As John has said, change the mains tapping to 240v.
I would start with recapping the PSU & line stages first, you don't want to put any undue stress on the LOPT.

Mark

[Al (astral highway)]

Sounds like a promising start , John Joe! I wonder when it had last been used ?
One thing , I don’t see how a fuse can die of old age... it’s a piece of wire in a capsule. Do you mean the contacts were corroded or had it gone o/c

Anyway, I look forward to hearing more about your progress!

[linescan87]

Good morning,

I have lifted the set from it's cabinet. The first thing I did yesterday was move the tap to the 240/250v before powering up. I like to under-run all my set this way, usually aim for 6 volts across the crt. Mains here is around 235v.

I'll see how lopt develops, if it's bad I'll use the dc current trick to dry it out, if only I knew about that years ago when I did my Pye v510.

The chap I got it off said it was his father's and it had been working when put away in the early 70's.

Hi Al, when I said died of old age I meant that it hadn't been killed by a short, sometimes they just go open on there own.

Thanks for the interest lads.

John Joe.

[Heatercathodeshort]

The wire simply rots and corrodes with age. The end caps are rarely airtight. J.

[linescan87]

I have the set going now, all just down to leaky caps but sure enough as you said HKS the lopt is damp, the over wind is getting hot and the width is reducing after half an hour. I've started drying out the lopt with dc, got about 2.5 watts running through it and it's just warm to the touch.

I been thinking lately if there would be any merit in placing damp lopts in a vacuum to draw out the damp, perhaps using an old pressure cooker as vacuum chamber and a fridge compressor as the pump.

John Joe.

[space_charged]

I am drying out a LOPT at the moment, but just by gentle external heat and time.

I do also have vacuum equipment, and could put a LOPT under vacuum. Such evacuation is needed in refrigeration to make sure there is as little water (and oxygen) as possible in the system.

It IS true that a fridge compressor will pull a vacuum, but they are single stage pumps and can't pull a good enough vacuum to evacuate refrigeration systems. They might have some effect on a LOPT, particularly if you could pass a low current through the coil while under vacuum.

A hermetic fridge compressor if still in its steel "dome" is NOT suitable for pulling a vacuum because it pulls the air from the inside of the dome. If you leave such a compressor running for any length of time, the motor will overheat in the vacuum. It needs return gas (freon in the fridge context) to keep it cool.

[linescan87]

Originally Posted by space_charged

They might have some effect on a LOPT, particularly if you could pass a low current through the coil while under vacuum.

That's what I was thinking, the water molecules would evaporate more readily in a vacuum, it wouldn't need to be a hard vacuum even 15 inch's of mercury should make a difference. I think, don't really know. Could be interesting though.

John Joe.

[space_charged]

In point of fact, it is quite hard to remove water under vacuum. That is particularly true if the water is trapped such as will be the case between the fine windings of the overwind in particular. Warming by passing a current as well as vacuum could work, I think you'd need to get well below the boiling point of water.

As a wee rule of thumb (easy to remember) water will boil at 20C when the pressure is about 20 torr. That is 20/760 ie about 2% of atmospheric pressure.

The reason you'd probably need to be well below that is because as the pressure decreases the air becomes less and less like a fluid and resists being "sucked".

I'd love to know if there is an agreed "best" way to dry out a LOPT, overwind in particular.

[Heatercathodeshort]

Carry on passing the current through it for around 5 days. This is deep inset dampness and removal cannot be rushed. It has taken decades to be absorbed into the pitch and windings.

Do not poke, fiddle, unscrew or mess about with the LOPT. Refrain from even handling it if at all possible. [Please don't be tempted to join the polishing brigade] The above should sort it out. It did mine. John.

[linescan87]

Will do John. Don't worry, a little bit of dirt doesn't bother me. I'll leave it for the week.

John Joe.

[Heatercathodeshort]

Just a bit of encouragement! This is the TV97. It's the same as yours but has FM radio.

The LOPT on this one was very damp. Started off OK but after 30 mins the picture width reduced considerably, the EHT dropped to half and the overwind became too warm. If operated in this state for a long period the overwind would have broken down.

Picture was taken under difficult conditions..IE, too much junk in the way, but it gives some idea. They do produce a very good picture. Regards, John.

[Boom]

Originally Posted by linescan87

I been thinking lately if there would be any merit in placing damp lopts in a vacuum to draw out the damp, perhaps using an old pressure cooker as vacuum chamber and a fridge compressor as the pump.

Just pop the LOPT in the airing cupboard for a week.

[linescan87]

Thanks John, that is a good result. Thanks for the encouragement, I'll keep on keepin' on.

John Joe.

[space_charged]

Despite having vacuum equipment, I have always just gently warmed lopts and let time do the work. I'm not even sure about passing a current through its coils. Warm dry air for a long time has always worked for me.

HKS is right in saying with damp LOPTS we are dealing with adsorbed water. That isn't liquid water and it has a MUCH lower vapour pressure, ie harder to remove - even in vacuum.

Personally I don't run a set at all until the LOPT had been dried. I'm worried about damage to the insulation that could occur, even in a short time. I'm drying a set out right now, a Philips G23T210. It needs some HT caps and spares are on the way but I'm itching to power it up. Basic checks are OK, heater chain, HT line but I'll wait a few more days drying it out. I've been advised of possible tracking problems on the printed circuit board with tracks carrying boost HT (930V on this set). So my drying cycle is also aimed at the PCB. These also adsorb water and need long gentle drying.

[Hunts smoothing bomb]

I'm prepared to be shot down in flames here by somebody that actually knows but it would seem to me that rather than a vacuum, quite the opposite should work more effectively.


You will not be able to pull much moisture out of a device within a vacuum as the moisture that is released has to be absorbed by which ever medium is present, in this case the rather "thin" air surrounding the LOPT in the sealed evacuated container.


However if you were to pump air into the sealed environment via a fridge compressor and also pass a current through the winding then I would expect this to be much more effective, more moisture will be drawn out as there are a lot more air molecules to absorb it.


Cheers

[space_charged]

Firstly, just have to say that I still think the best way to dry a LOPT is gentle heat over a fairly long time, at least a week. That may be external or by heating the windings.

As to vacuum, I agree that of its self, a vacuum won't do much good. Actually what we want is "mass flow" ie air movement to shift the water. We must remove all water as gas that is in equilibrium with that absorbed on the coils and insulation.

So a chamber that could be evacuated and repeatedly cycled from atmospheric pressure then pumped down to a couple of Torr might shift the water faster than passive drying.

I still think its not worth the effort and also agree with HKS that if possible the LOPT not be
un-soldered/messed about/removed if at all possible.

[mark pirate]

Firstly, just have to say that I still think the best way to dry a LOPT is gentle heat over a fairly long time, at least a week.

I have had some success with this method, on the last TV22 LOPT I used a heat gun to remove most of the pitch, then a couple of hours in an oven to remove the remainder.

The LOPT was then coated with anti corona spray, it runs cool & has proved itself to be reliable in operation, no drop of EHT after several hours running.

I would also recommend using the method of passing a current through the winding, this worked a treat on my Pye LV51F, it would loose a couple of kv
in half an hour running.
After a couple of days on the bench PSU, it now holds up well, losing only around 500v after running all evening.

Mark

[space_charged]

Mark, when you used the current heating, did you heat only the overwind or did you do the same to the primary? The primary is much lower resistance so a different current (higher) would be needed to produce the same heating effect.