Sunday, 17 October 2010

A Learning curve


A few weeks back I picked up this linear Amplifier on Ebay. Now in the 25 years or so that I have been licenced I have never run more than 100 watts or so maximum RF power and usually less. So why a linear? Well my original idea was to be able to use it on data modes, under running it so that perhaps with 10 or so watts from the rig I would get 50 or so from the amp. Of course I would use it on SSB from time to time. This is a valve linear but the prospect of tuning it up wasn't a problem as I am used to that with my TS830 and other rigs I have used in the past.


When the unit arrived here it all looked OK (and I should point out that I believe it was most likely working properly before it was sent but I don't think a journey through the postal system is good for equipment like this!)  so eager to test it out I connected up to my rig and a dummy load and switched on. The next thing I heard was a "pop" sound and I was sitting in darkness, no computer radio or anything- the linear had tripped the household RCD circuit. After a reset and replacing the internal fuse in the linear I tried again. I managed to get around 150 watts into the dummy load before the internal fuse went again, no surprise this time as I needed a 2.0A fuse but had only 315MA available. A few days later I got hold of a 2a fuse. Ran the linear up and before I could apply any RF to it "BANG!!" a capacitor on the output stage blew. I have an amateur friend locally who is skilled at electronics generally and he had a look at the circuit diagram. The blown capacitor was 3600pf, voltage rating unknown.


 He worked out that be using four lower rated capacitors 2 pairs in parallell, connected in series that should be ok. well I connected the new capacitors and much to my surprise everything was working well.


 So with a certain sense of satisfaction I had repaired the linear. All was well until today when I find that there is no current on the anodes of the valves according to the front panel meter and I can get no RF out of the linear. What has blown now? another capacitor? I cannot see anything obvious so I will be looking for advice from my G3 friend again.
You most likely will not recognise this model of linear from the pictures. I think it was manufactured in the 1980s/90s. It is an "SRW kilowatt loudenboomer" which actually produces a maximum of 400 watts out. If I ever get it working I will write a little more about it as currently there is very little information on the net.

Given my very limited electronics skills  this is a real learning curve for me, I am very fortunate that I have an experienced and skilled  radio amateur living nearby who is very helpful. I know that many people think that people with a limited technical knowledge/background should not be in amateur radio I of course would disagree! I try to use these things as learning points although I know I am never going to be skilled in the electronics field although I do enjoy learning.

3 comments:

  1. This comment has been removed by the author.

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  2. Hi Kevin, You should do what all other ebay idiots do and cause a right stink, ask for your money back and then keep the amp...

    Those little blue mica caps I suspect are 1 KV rated, look like the one's used in bridge rectifiers across the diodes, well in TV's anyway, so now I take it you arn't getting any sort of spark of the top of the finals ?

    Not really up on valves, but I suppose it must need some sort of drive/bias to work, check for any open circuit resistors nearby, they'll probably be largish ceramic jobs.

    73 Paul.

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  3. Hi guys

    Thanks for the comments.I am looking at the possibility of a blown diode in the circuit somewhere but no luck as yet.Ian, yes I have seen G3TPW's webpage, good to know he is still active, it may be worth dropping him an email if I can't get anywhere. Paul the blue caps you saw in the pic were the replacements I installed which were working OK initially. If it was those which had failed again I think I would have some current on the anodes still but no output.
    I will let you know how I get on with this.

    73 Kevin

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