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Counting elephants

Posted by gm4uls on December 14, 2011

… or how to blow up a Yaesu FL-2100Z linear amplifier.

I don’t have much luck with linear amplifiers. As I write this my new Zetagi B300P is back with the dealer, one of the output transistors having failed. The pre-amp never worked anyway (which was not a problem because I didn’t need it). Prior to failure the Zetagi had worked very well. It was capable of delivering more than 100W on AM, by which I mean it would comfortably deliver the UK maximum allowed of 26dBW and a little to spare to allow for loss in the coax (ahem). I had also just tested it on SSB, driving it with my FT-847, ‘whistling it up’ to 300W pep and then having s short QSO. It drew a steady 20amps during AM and SSB operation, well within the capabilities of the psu. Then suddenly it was delivering half power and drawing half current. Conclusion – one transistor not working. The dealer suggested sending it back, which I promptly did.

Apparently the figures I quoted for the linear are pretty good, which means they are as quoted by the manufacturer but as seldom achieved in the real world. Zetagi are not known for modest or even realistic claims, so to have a unit which actually delivered 100W on AM and 300W on SSB was, however briefly, good!

Which brings me to the other linear amplifier currently sitting inert in my shack – the Yaesu FL-2100Z that inherited from my late father. This linear is meant to compliment my FT-101ZD, and when I retired in 2010 I decided to connect it to the rig, turn on the power, and tune it up. It hadn’t been switched on for many years, since the days when my dad ran a net on 80m, and seeing as he died in 2004 that represents a long time sitting idle.

The manual for the FL-2100Z is fairly straightforward, except in one respect. It tells you to close the PTT on your transmitter but never mentions what mode the transmitter is supposed to be in. So I approached this by process of elimination. In CW mode all that happens is that the receiver is muted; nothing is transmitted unless you hit the morse key. In SSB nothing is transmitted unless you talk or whistle into the mic. There is an AM/FM setting, and in the case of this 101ZD there is an FM module fitted; there is no reason why I couldn’t transmit FM into a dummy load on any band, but somehow it doesn’t seem… I don’t know… appropriate on 40m (where the rig was tuned). That leaves the ‘tune’ mode – fair enough, it seems logical to use that.

It quickly became apparent that trying to fiddle with the plate and loading knobs with one hand whilst holding down the PTT with another was going to be frustrating, particularly as the manual laid great stress on not ‘keying down’ for more than ten seconds. I bought a foot-switch, which left me perfectly free to use both hands to twiddle the plate and loading knobs. Now then, you have to keep your eyes on the meters on the front panel of the FL-2100Z, so there was no question of watching the second hand of a clock while I tuned. So how did I measure ten seconds?

I counted elephants.

Yep, you heard me. I counted elephants. It so happens that if you say aloud, “One elephant, two elephants, three elephants…” each elephant will take as near to a second as makes no difference. So I counted elephants and made sure I had taken my foot off the foot-switch before I had fully counted the tenth. The process was still very slow – ten seconds is not long enough to adjust the readings on the meters all that much – but at least I was inching towards being able to run 400W.

Suddenly… BANG! There was a flash from the linear and everything went dark. My heart sank. Nothing short of one of the big valves blowing could have made such a cannonade. Maybe I had accidentally counted eleven elephants, or one of the elephants had been extra large or extra slow, I don’t know. I pulled the linear out of its place and opened up the cover. Both valves were cloudy and, when I took them out, they rattled like a tin can full of tacks.

It would have been simple to buy a couple of replacements and plug them in, but for two factors. Firstly I had no idea if anything else had blown or, if not blown, had gone somehow out-of-tune. Secondly I could see that in its thirty-year life it had never been opened and there was so much dust and grime in the insides that it needed a thorough overhaul… by someone who knows more than I do and isn’t ‘all thumbs’.

So it has been sitting there since 2010, looking nice in shack shots but not doing much else. Anyone out there who is currently sniggering and thinking “What a steaming great wally”, and who can see where I went wrong, please tell me. Meanwhile I shall wait until I have a little more cash before boxing the linear up and sending it away. Carriage alone will cost an arm and a leg because it’s reeeeaaally heavy!

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3 Responses to “Counting elephants”

  1. Hans said

    What you experienced is a high voltage flash-over. The 572B tubes used in this amp are ‘famous’ for doing this when they reach the end of their service life, or when the amp hasn’t been used for a while. Here’s a quote from KK5DR:

    “The cause for the HV flash-over is usually barium contamination of the atmosphere in the tube making the vacuum conductive at all plate voltage levels. The oxide coating of the cathode will have been reduced to the point where areas will super-heat, causing ionization and then HV flash-over and or sparking to the plate. This is not a reversible condition and indicates end-of-life for the tube.”

    Reading material: http://www.kk5dr.com/Tubes.html

    Another website, in Dutch, is maintained by PA0FRI: http://pa0fri.home.xs4all.nl/Lineairs/FL2100Z/fl2100z.htm

    He describes a few mods for the 2100Z, and this problem is also discussed. Maybe Google Translate might be of some help.

    Hans

  2. Hans said

    To my surprise I found out that the 572B tubes are still being produced. In China, just like my 6164B’s. Not really cheap though, a matched pair will set you back $85. This includes shipping and a 1-year replacement warranty though.

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