The Junkyard Dawg crapout I caused

[KA1ZGC]

So I wandered out to the shack yesterday and decided I'd try and track down the HV fault in the Junkyard Dawg.

First, visual inspection. Whip out the big pistol-grip 5-bajillion-candlepower flashlight, open the back, look for anything foreign, any exposed HV paths, anything that might give a clue as to what the problem is.

Everything looks normal to me.

So I fire it up at half power and... no fault. 900V plate, 200mA cathode, 20mA screen. Perfect.

Huh. Okay, how about full power? 1650V, 340mA, 50mA. Perfect.

QTF?? This thing was broken, dammit! Ah, maybe it still is, the modulator idling current is only 20mA.

Okay, 3885's clear, throw out some test transmissions.

"Sounds distorted, like a dead mod tube" replies Harry, VE1CDT.

Yep, smells like one, too. Bend down to peek in the window, the color looks good on the front one, can't quite see the color of the back plate... wait a minute, I can't see the color of the back filament!

Stick my head back behind the transmitter, and what do I see? An 811 with a black filament and a white getter. The plate contact had broken right off the envelope.

How did this happen, you ask? Well, I'll tell you:

The 811s are taller than the modulator chassis. Any time I need to take the RF deck (which sits above the modulator) out, I first need to pull the 811s. There's only about 1-1/2" of space between the plate contact and the RF deck.

The rear socket requires quite a bit of force to extract a tube. No matter how careful I was, that back tube would frequently pop out of the socket rather suddenly, and the plate contact would impact the RF deck chassis rather sharply. The front socket never gave me any issues, it held on firm but let go gently.

So that's how I killed my transmitter: repeated blows to the head of a perfectly good RCA 811 finally broke its neck and gave the B+ a chance to check out a few scenic detours it hadn't taken before.

Oh, well... at least it wasn't another damn dial cord.

--Thom
Kilovolts Alter One Zorched Glass Constrictor

[WA1GFZ]

sounds like you need to stuff a rag over the tube to limit the upward travel.

[W1IA]

Duct tape
 

[KA1ZGC]

Yeah, a rag would probably do the trick.

I got thinking about it and I remembered that there was one time I forgot to pull the 811s before I pulled the RF deck. I had a pretty good hold of the back when I took out the last bottom rack screw, but it still dropped a bit when I was taking it out gave that plate cap a good thud.

That was several months ago, but I'll bet that's what started it down the path to The Dark Side.

I still need to shock-absorb the back tube when I take it out, though. It has been rapped a few times since it got sat on. It might have survived longer if I had.

The really embarrasing part is that I had the thing right in front of my face while I was visually inspecting the rig with the mains off and never even noticed the getter was white. Would have saved me some troubleshooting if I had seen that.

Oh, well... live and learn!

--Thom
Killer Agony One Zorched Groin Component

[W1RKW]

Thom,
I don't know if you can do this but what I did with my rack was put in slide rails using cheap perforated angle iron to capture the sides of each transmitter section. My modulator section is above the RF section like yours and because the mod xfmr is on the chassis and is heavy, I figured I'd best protect the RF section if I have to unbolt the mod section from the rack.  I eventually did that to the other sections in the rack, makes it easier for removal and installation.  Not sure if you'd be able to do that but something to consider.

[KA1ZGC]

Not very easily.

This is one of those 3' Gates cabinets. There's several inches between the side of the cabinet and the side of the chassis.

What I may be able to do is build some angle-iron rails onto the modulator chassis since the RF deck isn't all that heavy. I'll have to look, the floating chokes may get in the way.

If I did that, I would have to pull the RF deck to pull the modulator from that point on, since the rails would be higher than the front panel of the modulator. But it would save me having to pull the tubes and risk damaging them, or leave them in and risk damaging them!

--Thom
Killer Aircraft One Zeppelin Goes Crash

[WA1GFZ]

why not sub mount the sockets.?

[Mike/W8BAC]

What might work to free up the contacts in that tube socket is to use a small amount of silicone grease on the pins. The 4-400A's in my 20v3 are very tight as well and this works really well.

I use silicone grease on light bulb sockets (both 120 and 12 volt). In automotive bulb sockets as well as boat trailer bulb sockets I find no corrosion after years of use. On tube sockets I have no fear and a thin coat keeps things clean and easy to move. Hope this works for you.

Mike

[WA1GFZ]

I do the same thing Mike. keeps the water out.
deoxit might also work