Continental 816-R2 air chimney screws

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kcbooboo
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Continental 816-R2 air chimney screws

Post by kcbooboo » Tue Jul 30, 2019 9:38 am

When replacing the 4CX15,000 PA, one must remove the split air chimney by removing four #4-40 nylon screws that hold the chimney to the tube socket plastic baseplate. These screws always seem to be breaking off, sometimes leaving a little bit in the threaded hole. Fortunately these usually come right out when grabbed by a pair of pliers, but if they break off flush with the baseplate, they have to be drilled out and the hole tapped to clear the remaining nylon. There has to be a better way.

Has anyone tried other plastic screws besides nylon? I did some research and found polycarbonate screws as well as something called PEEK, that's supposed to be better for higher temps. Also, it might be better to re-drill and re-tap the existing holes for #6-32 screws, which would be stronger and might not break as often. Whatever is used, it can't be RF-conductive, which probably eliminates any metal such as stainless, brass, or aluminum.

I've ordered several different types from Digikey, including some thumb-screw styles, and will be trying those out. Two of the four screws are hard to get to and impossible to see (behind the tube) and there may not be much room for even a Greenie screwdriver, hence the thumb-screw style. I don't know how much air pressure is involved and if it's just too much for #4-40 screws to handle, as the air chimney is funneling all of the air coming up through the socket so it runs through the tube's plate fins.

Bob M.

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Re: Continental 816-R2 air chimney screws

Post by radio_guru » Tue Jul 30, 2019 12:46 pm

My problem was the fiberglass deck they attached stripped out. And threading fiberglass is a !@#$%^&*() headache. Plus the #6 thread pattern would not allow sufficient thread engagement to permit and maintain torque. Also, the slot for the chimney machine machine screw doesn't accommodate #6 as well as #4. Never mind a #6 (or #8) thumb sized head would nest tightly against the chimney wall. So going to #6 was out.

As for alternatives...your options are limited. You could try a poly-carbonate and see what happens. BE uses poly-carbonate extensively in their FM tube TX's. So it's properties should be OK. But it's a more brittle material and would likely snap more readily than the somewhat pliable nylon.

What I did was simply keep a stock of them handy as well as a spare mounting plate and associated nylon parts...which is an equally bad pain to change. But when you need to do it, it needs to be done then and there...

Check Fastenal as an alternate source for thumb machine screws... Likely a lot less than CEC.... (watch the wrap)

https://www.fastenal.com/product/fasten ... lyId=22521

Now if you have a spare plate which has already seen it's threads stripped, you could try and drill out larger hole and use a press-in captive nut which offers more threaded engagement. I was just about to do that experiment when we sold the station. My only concern would be seeing if the nut could be non-conductive as that area is sensitive to even the smallest added capacitance. Or epoxy a nylon nut in place of the threaded hole.

Good luck....

RG.

kcbooboo
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Re: Continental 816-R2 air chimney screws

Post by kcbooboo » Tue Jul 30, 2019 3:11 pm

Thanks. As I recall, the white plastic plate that the chimney screws are threaded into looks to be about 3/8 inch thick, at least at the edges of that plate, which is probably 10x10 inches, maybe bigger. It may be milled thinner where the chimney screws are threaded into it. I don't know whether the tube socket is mounted to it but I suspect it is. I also don't know what holds that piece of plastic in place. I didn't recognize what type of plastic it was; it looks like it could be teflon or nylon; I don't think it's fiberglass; at least not like the type they made the chimney out of. That should be plenty thick for a 1/2 inch #4-40 screw to secure itself into. This is a rather old R2C, definitely CEC (not Collins). It still has Collins-style rotary knob momentary switches for PA Tune and Load and real rotary switches for the multimeter and Remote/Local selectors.

According to the Eimac datasheet, the slots in the chimney are 0.140 inches across, which should clear a #6 screw, but just barely. I don't know why they couldn't be made wider. I would think it's relatively easy to tap that plastic, even after drilling out the remnants of #4-40 nylon screws.

I purchased my plastic hardware from Digikey. I try not to buy from CEC unless it's really a special part that only they can supply.

Bob M.

ncradioeng
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Re: Continental 816-R2 air chimney screws

Post by ncradioeng » Tue Jul 30, 2019 3:18 pm

I'm using the nylon thumbscrews (I think I got them from Lowes in the specialty hardware section). So far they seem to be working. Beats fiddling with a screwdriver since you have to install the rear ones by feel.

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Re: Continental 816-R2 air chimney screws

Post by radio_guru » Tue Jul 30, 2019 4:21 pm

My 816-R2A and forward had that deck plate as 1/8" (or maybe thinner) fiberglass. As I recall, none of the socket is attached to this plate. It's strictly an insulator and mounting deck for the chimney. Your's maybe an even earlier model as the one's I had (1984 vintage) and used a different material. A call to CEC would reveal that...

RG

kcbooboo
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Re: Continental 816-R2 air chimney screws

Post by kcbooboo » Wed Jul 31, 2019 5:43 am

I don't even recall seeing screws that hold that white plastic plate to the aluminum RF cavity. They could have used nylon screws from underneath. But it definitely IS thicker than 1/8. I'll have to bring a camera with me next time.

I did order some #4-40 screws with larger heads that still have an opening for a screwdriver (they called them thumbscrews) so we'll have plenty to try. Even by feel, they're not that easy to install. Right now we're using half a dozen ty-wraps at the top of the chimney to hold it together. The blocking capacitor holds it down, and the remaining screw stubs keep it from moving around.

Bob M.

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