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.


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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.

  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