flyback cooling

[DaveWM]

My recently replaced flyback in a CTC-12 gets pretty warm, like 150f after about 45 min running, it looked like a NOS fly, dripped out quite a bit of wax. I have read in old service manuals that it is not uncommon, and indeed most of the sets I find that have OEM flys do indeed have a moderate to large amount of wax loss. I have checked out the HV and it all seems well (correct HOT cathode current and screen voltages, regulation current, HV setting).

I was considering installaing a small muffin fan on set. I would do this by fabricating a new top for the HV cage, with a small (1" to 2") fan blowing down and a vent hole next to it to allow the air to escape. would add a thin non conductive divider to act as a plenum down to the fly to make sure the air path is not short circuited. I have seen many sets that just have the HV cage removed, perhaps to aid cooling?

I realize the down side is the large volume of air will result in a large volume of dust, if that is the case maybe a airfilter on the intake side. I don't think I will be using it frequently enough to clog the filter, I would just like to watch a 2 hour movie and not be fretting of a too hot flyback.

oh and ROLL TIDE...

[DaveWM]

some more ideas, thin cardboard formed duct (like a tube from a paper towel roll), down the side of the cage, open up to a shroud around the fly. Fan mounted inside the tube. Reason for the fan in tube is to try and better muffle the sound of it running which has been my main reason for not using them in the past. Two exit holes on the top part of the cage that swings over the top. I suppose to be safe I should put a metal cap over the hole (spaced to allow air to escape) as an X-ray stopper although I highly doubt X-rays would be a problem.

I have discussed the issue of Flyback heat with another member of this forum, unsure of exactly what is causing the heat, since a lot of it seems to be in the HV tire, also since I am sure the insulation on the wire is fine at these temps, there may not be a problem even with massive wax loss other than the mess and perhaps the loosening of the mounting of the fly tire itself to the frame.

I am guessing here that the HV at say 25kv and 1-2ma avg about 35watts some of that will go to heat in the tire...

[marty59]

With the 6BK4 and the focus rectifier inside a small enclosed cage doesn't help the situation either...

[DaveWM]

Yea the shunt tube does make some heat as well, all inside a sealed up metal box, and ALL of its shunting goes to making heat, as well as the filament.

[andy]

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[N2IXK]

My biggest worry about forced-air cooling an HV cage would be dust. If you ever looked inside a computer case that has been run for a few years, there is an amazing amount of dust that coats the internals, even when operated in a "clean" environment.

At least in a PC, the dust just impedes proper heat dissipation. Inside an HV cage, it will cause leakage currents and eventual arcing, with a possible fire hazard.

If you want to go this route, I would suggest using the fan to pressurize the cage (rather than sucking warm air out), and fit a proper dust filter on the inlet side of the fan. Then, of course, you need to remember to change or clean the filter occasionally....

[DaveWM]

Originally Posted by andy

Most RCA chassis have a high line voltage tap on the power transformer primary. This can help a lot with flyback temperature. Also, make sure your HV system is operating properly (check HOT current and shunt regulator current). The shunt regulator should see about 1mA on a black screen.

I am on the 128v tap now, that reduced the HOT current some. Its at about 198ma shunt it about .8-1ma, HV is about 23kv.

control grid of the HO tube is -47 with with a vtvm, screen is 140v grid drive is 200v PP with the correct form, replaced the .01 coupling cap, adj the eff coil for max dip.

Diff 6DQ5's were used there where minor changes in both current and screen voltage, I settled on the one with the lowest cahode current and screen voltage.

[ctc17]

How about a small liquid cooling system with a heat exchanger on the back with a fan? You could use plastic tubing that wouldnt conduct the hv.

Having a fan blowing directly on the flyback would blow stray electrons throughout the room and cause x-rays to collect on the nails in the walls.

Or how about this....stick one of those slant flys in there that work in just about everything and see if it gets as hot.

[bgadow]

Originally Posted by ctc17

Having a fan blowing directly on the flyback would blow stray electrons throughout the room and cause x-rays to collect on the nails in the walls.

This might be good information for the investigation into why certain 60s GE color sets would explode, along with the nails being sucked out of the wall. (see the old, old thread) Must've been real good airflow in those sets, good updraft, maybe? Low-budget particle accelerator. The guys out there working on clandestine fusion machines, or whatever they are, might be very interested in this. I am mostly joking

[Findm-Keepm]

Dave,

Before doing any mods, take a look at RCA's patent for the CTC12 HV cage - there are MANY safety concerns in the design of the enclosure. The US patent is 3273021.

Aside from what has already been proposed, I'd use a solid state HV rectifier, and make sure the regulator was a 6BK4C, not a 6BK4 or 6BK4A. The 6BK4 was the worst of the regulator tube designs - RCA pulled all of them when they brought out the 6BK4A (actually a Sylvania design, improved upon by RCA). The 6BK4C/6EL4A employs all of the improvements. It seems RCA, in an effort to improve the 6BK4's reliabilty, punched holes in the anode to help in outgassing of the tube at manufacture. The holes allowed the escape of X-radiation, so they inserted some baffles to mitigate the X-radiation (6BK4A) and later added both the denser (leaded) glass envelope and improved anode to further deal with the X-radiation threat that exists when the regulator tube is operated even marginally out of it's normal parameters (25KV, 1 to 1.3mA).

Hope this helps in some way,

[wkand]

DaveM said: "will be interesting to see it spike when I turn off the set (and remove the power from the fan)."

Now that you are designing cooling systems, you need a 555 timer, a relay, thermistor, and a constant 12v power source to keep the fan running after the set is turned off until the fly cools. I'm kidding of course!

[andy]

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[DaveWM]

interesting I ran it with the back of the cage off for nearly 1 hr and it was just warm. Oh well maybe that is the answer for now. I will try it again and see how it goes.

[DaveWM]

perhaps it just needs some natural convective cooling a good sized opening on the bottom and the top right over the shunt with some spaced off caps I just hate the idea of cutting up the cage.

Duh make one, I have some scrap metal and its a very simple part to make.

[jr_tech]

Well, there are 3 different ways to transfer heat; radiation, conduction and convection. Why not use all three?
1. Paint the inside of the box black to absorb the heat *radiated* from the tubes/flyback.
2. The heat would be *conducted* through the metal to the outside surface.
3. Mount cooling fins on the outside of the box so that natural or fan-induced *convection* can carry the heat away.

jr