Sunday, 5 April 2015

Tea .. and then the linear blew up.

I have had a very frustrating few days.


Friday was a nice start with morning tea with Doug GM6ZFI and Derrick GM4CXP. Alongside the usual banter, Doug offered to come round and try to solve my height access problem with the 6m beam. Seemingly it may involve moving the top bracket downwards, which looks good to me.

If "morning tea" gives you the impression of Darjeeling, cake, and buns, it was in the cafe of Pearson's Garden Centre and Coal Suppliers in Duns. I had a diet coke and a bacon roll. Not grand, but very nice all the same.

It is nice to have "tea" with a few ham colleagues and swap stories and gossip. I used to go to a radio club regularly, but they were very stuck in the mud and didn't really like my ideas, and who can blame them for that? So I left. I still go to the (much better) Cockenzie and Port Seton club near Edinburgh when I can, but it is a long drive.

Then I was back home and I was running the Flex 1500 on WSPR. I find the 5 watts is a bit low on 6m, and I was running about 20 watts using the RM VLA150 linear amplifier. Although this claims to be able to run well over 100 watts, I find it struggles a bit to top 90 watts with the 5 watt drive available from the Flex.

Anyway, it has a nice big fan on top, and with a capable heatsink it is no danger of overheating...

I noticed a strange "hot" sort of smell lately, which is never a good sign. I had it before when the power distribution system for the 12 volt accessories went off beam and some supply wires heated up. I guessed it was this again and hunted about for a hot DC supply wire (later I ordered a new, multi-fused, 12v distribution system as the current one relies on one fuse).

This smell was like hot heat-transfer compound, as used in a PC processor to provide a path for the heat from the chip to its heatsink and fan. You know, if you don't have enough compound and the system works hard. You get the smell of the heat transfer compound which is a telling you something is wrong.

Hot electrics is never something I like. While I was hunting about with the fans and things, the smell changed to a nasty acrid smell of something transistorised having given up the ghost.

It turns out that my RM VLA-150 has gone almost silent key. It was not working under any strain. I have an idea that one of the power transistors has over heated and blown. It could not have been overdriven as the Flex cannot manage more than 5 watts whatever you do. Maybe that "hot" smell indicated that there has been a problem brewing for quite a while. Anyway, curtains for now and I need to find out what has gone wrong. I think I will be needed a new PA transistor.

I say "almost" gone silent key. It still produces about 30 watts for a 5 watt drive, so perhaps one device has gone. More heat transfer compound next time. Could it be that RM amplifiers are not brilliantly made and not properly quality checked? Surely not! I see that the VLA-150 is not available any more.

I might say that 6m amplifiers have not been a great success for me. I have a 6m conversion of a Bremi CB valve amp which overheated and melted the anode choke. Now the VLA-150 has gone too. I have lashed up a (not linear!) amp to get me by ~ 20 watts and always use a low pass filter! I could get more power out of this using it in Class C mode, but I might want to try it on SSB so it needs to be more or less linear.

Is 6 metres doomed from here? Why have I had repeated problems with the Bremi, the RM amp, the Flex, the beam ... ???

There is something wrong with things on the Flex front too which I will detail later. Software seems to be the problem, so I should be able to fix it here myself.

The blown PA and the software issues have taken up about 75% of the weekend. Grrr.

73

Jim
GM4FVM

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