Adventures in High Voltage Breakdown

Todd, KA1KAQ

Administrator
Member

Offline

Offline

Posts: 4312

AMbassador

Adventures in High Voltage Breakdown

« on: October 02, 2010, 01:16:44 PM »

Thought it might be a good idea to follow up the nekkid pics of the transmitter in the QSO section with the reason and findings for the stripped down look.

As some are well aware (mainly those whom I've been pestering for answers through email), I've had some HV issues for a while with the KW-1. In all honesty, I've had a number of issues over the past few years, from bias problems to blown mod iron. Most of them can be traced back to my own actions, unfortunately. Pinching a Sprague Atom into a clip that it was too big for caused the bias problem, for example. And who can I blame for blowing the modulation transformer since I was at the mic?

The HV has been another issue, though. It's bedeviled me in a number of ways off and on since getting the old girl back on the air in 2006. They probably existed back in 89 or 90 when I first fired it up, but since it wasn't on the air for a long period, the problems never surfaced.

One of the first issues that many will be familiar with (since most of us have been through it)was the blowing of line fuses. On this transmitter, it takes two 15A screw in types like the fuse boxes of yesteryear. Two things I did to combat this: replaced the 872 MV rectifiers with 4B32 Xenon and bought a large stock of 15A fuses in bulk, 100 or so, on ebay. This worked great for a while. No more flashovers, don't think I used more than half a dozen fuses in 2-3 years.

But it would still toss fuses now and then, usually after or even during a long, buzzarding transmission. Then last year things went bad fast just before Shelby when Steve was talking with Getty N2GBF on 40m. A large purple *ZORCH* appeared in the tank area followed less than an hour later by blowing fuses. This time it blew the fuses as soon as you keyed the transmitter. A long, slow process of elimination took place with help from WA2PJP, W3GMS, W3JN, WB3HUZ, K4OZY and probably others I've missed. JP/OZY even brought a HV supply over to test components in the HV supply for breakdown and we found none, yet the problem persisted. After a steep learning curve (for me, that is) and a lot of reassurance from others since HV scares the HELL out of me, I was able to finally isolate the problem or so I thought. The multi-tap choke in the splatter filter had apparently shorted to ground and was only showing up under load. The problem is, I had bypassed this filter networks YEARS ago upon the advice of Uncle Ed 'PUN and Jay Bromley, now W5JAY. Or so I thought, at least.

When JP was walking through the components with me testing them for leakage to ground, I told him that choke was bypassed, no need to check it. He said "you do realize it still has high voltage on it though, right?". No, I did not. Seems I'd only bypassed one side, moving a HV lead directly to the lead traveling up to the RF deck. Ooops. So at some point when Steve was talking, most like just by random luck, this choke shorted to ground (apparently). After doing a bit more cypherin' since Collins neglected to label the myriad of HV lines collected in the supply, I was able to fully bypass this network and bring the beast back to life.

For about a month. Last week while signing off with the afternoon group on 3705...*poof*. Tossed the two line fuses. Installed two more, said two words, *poof* - two more. I figured it was related to the problem I was having trying to load up in the 75m ghetto portion of the band, karma catching up with me since I'd bragged ho well the rig loaded to the aerial down on 80.

After whining to some of my usual sources like a 5 yr old whose fingers had been slammed in a door and getting some very basic advice (no, not GTFA), the tear down began in search of the problem.

And I found a few, at least one of which was clearly a culprit that I had been waned about repeatedly by WA2PJP, the resident KW-1 expert. I'll list them here and provide a few photos, since the main culprit is just too incredible to believe, at least for my limited mental faculties.

1.) First thing I found was a star washer, sitting atop of the door knob caps in the plate blocking circuit between te 4-250s. There it was, perched between the top metal strap and the center post of the doorknob. "WOW" thinks me, that could cause a short circuit.....if it wasn't already tied together mechanically and electrically. Yep, the post was attached to the plate, nothing to short out. And sadly, this is yet another Bigelow-induced faux gremlin, since I had to replace one of those caps back in 2006. Apparently that top washer wasn't on the screw when I tightened it.

2.) Next I found a place where a piece of buss wire in the tank circuit passes through a hole maybe 1/2" in diameter, connecting bottom of the chassis stuff to top of the chassis stuff (hope that's not too technical). In this case, the wire was not bent, but was still much closer to one side of the BARE metal chassis than the other, and it looked to me (imaginary or otherwise) like there were arc tracks on that very edge! I carefully persuaded the wore back towards the center, knowing in my heart that Art Collins would approve of my moving an original piece of wire for the safety of the rig. Mhm. I then checked another near the back of the deck and did the same, just to feel better.

3.) Here's the biggie, since Joe/PJP had asked me about this several times, warning me that he had the same symptoms from it. On this transmitter as with many others, there are feed-thru points for the HV which utilize those nifty little Millen 2 pc porcelain insulators. These are the kind that have a small inside with a hole through the center for a long, threaded stud used to collect both pieces tightly together against gaskets on the chassis. A few months back when Joe had asked me if I'd checked these, I checked the one on the back of the RF deck and it was as clean as a whistle, like it came from the factory. I um....didn't check the one up front that goes to the neutralizing cap because it involved removing the top front panel (2 seconds) and then the screen in front of the PA tubes (5 minutes) and possibly having to get beneath the chassis to have access to the stud which involved removing one of the back covers (22 screws, think Johnson Flashbox). So using my superior powers of extrapolation, I decided if the back insulator was clean, that front one should be as well, by golly.

And I was right. It was, when I finally checked it Thursday. Only one problem: I missed a third insulator of the same type connected to the plate choke and carrying 2500 volts. Huh! As I loosened this one, it had a grinding sand-inside feeling to it. When I removed it, I could see why. A zorch! A tiny, black zorch on the outside/top piece. So I spun the lower piece around a bit, and sure enough - there was another one, a mating zorch, if you will. At this point I was really annoyed with myself for being so negligent in checking these insulators previously (even though I didn't remember this one in the first place). And it's a difficult size insulator to find, too. But as luck would have it, and I do mean Luck, when going through some tubes two weeks ago in the other garage I came across a small box of porcelain insulators, and there was a NOS spare. Pure Luck.

After pulling the bottom piece out and inspecting it, I was amazed to see a small pin hole running somewhat diagonally through the inner sleeve where the HV had basically bored a hole through. And sure enough, there on the edge of the chassis was a little black mark. HV short to ground, apparently not always continuous, but always there. Or not? Even the upper gasket had a wee burn mark along its edge.

4.) In the process of removing this insulator I also discovered what is possibly a bad doorknob cap beneath the deck, used as HV Line Bypass, connected to the insulator stud with a piece of buss wire. Noticing they were a little loose, I started to tighten them by hand only to notice a lot of lateral movement once they were tight. Like one of the plates was loose. Turns out it was the top one, and as luck would have it - I had a spare (Thanks Dave Schneider!) purchased at the same time as the other I'd replaced in 2006.

5.) I'd also removed the top loading air-variable to de-burrrrrr any plates with zorches, the original suspect in the blown fuse scenario. After some cleaning and polishing, it went back in Thursday night and by 1:45 I'd finally gotten the last tiny screw and nut securing the side straps from the tank circuit tightened down. The toughest part of the whole ordeal, I did swear a lot.

So to make a long-winded story even longer, I got the rig cleaned up and reassembled by 4PM yesterday and had it on the air for testing. Other than blowing two fuses by having the rig waaaay out of resonance (in the TUNE position, thankfully) while playing with it, things seemed good. So I got on 3710 and 3725 last night for some good ragchewing and aside from a couple *snik* sounds (should've checked those arc gaps while I had the cover off), no blown fuses! But much to my surprise, while talking with Tim VE6PG and Al VE3AJM, they both related similar stories - Tim, from a DX-100 he had with the same issue on the same insulator, and Al from a common friend Donovan VE1BD and his Marconi 501 with the same issue. Though I'd heard of these insulators breaking down on other KW-1s (it happened to 'PJP with identical symptoms), I'd never heard of it happening to a DX-100.

So the 'wild, unbelievable' fault that I found really isn't as odd or rare as I first thought. But it is pretty damned interesting, if you ask me. For HV to bore a hole through porcelain seeking a path to ground - and succeed - is just amazing. Makes you wonder why bother using an insulator if it's just going to cause problems?

Anyhow, for those who are still awake or reading at this point, here are a few shots of my findings. First the wandering washer atop the doorknob cap, then the wire through chassis, after being moved.

The moral of the story is: don't be a dunderhead. Check *everything*. Don't assume, because when you assume, your transmitter breaks.

While not R@RE, they aren't really plentiful either. The 30K-5 uses 6 of them across the back for feedlines and it took me years to find enough. These are the same except smaller.

KW-1cap4.JPG (304.69 KB, 1600x1200 - viewed 525 times.) KW-1tank2.JPG (302.17 KB, 1200x1600 - viewed 465 times.)
	Logged

known as The Voice of Vermont in a previous life

Todd, KA1KAQ

Administrator
Member

Offline

Offline

Posts: 4312

AMbassador