Ranch55 Well-Known Member
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Jan 17, 2016
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Ranch55 said:
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I have another Texas Star DX 500V amplifier in from a friend to take a look at. He purchased it new about 2 months ago.
It is experiencing high VSWR's on any antenna, but almost flat on a dummy load. This information is according to an experienced tech who has already had it.
He asked me to take a look at it.
My initial suspicion is the DEI transistors that it came with, based on the information he provided.
I won't have an opportunity to look at it for awhile yet.
Keep you posted.
Click to expand...
Okay, made it around to taking care of this amplifier.
First thing, I connected it to transceiver to observe the performance.
1) .... High VSWR reading in transmit , 2.5 plus ....
2) .... One board getting significantly hotter than the other board. Indication of unbalance in output ....
3) .... Output combiner resistor getting extremely hot, also indicating unbalance in boards output .....
So, pulled the power transistors and pulled the boards.
1) .... The transistors were not matched as tested ....
2) .... Found a glob drip of solder under the input splitter. Appeared that it was touching the heat sink.
3) .... Found some kind of foreign conductive material under one of the collectors on the other board, and it was also touching the heat sink.
Took care of the two above issues and reinstalled the boards.
Replaced the output combiner resistor that was getting hot.
Installed 4 new RF Parts branded HG transistors ...... RFP 2SC2879 .......
Did not need to do any re-tuning as it was good.
The previous tech who had worked on it had already completed a very good power wire upgrade and the tuning capacitors he had installed were spot on.
14.0 vdc on the power supply ......
Dummy load VSWR was at 1.01 ~ 1.1 .........
On the antenna, VSWR was at 1.25 ~ 1.35 ......
16 watts (PEP) in produced 330 ~ 350 watts out .......
50 watts (PEP) in produced 450 ~ 475 watts out ......
All four transistors performing well and within 3 to 4 degrees of each other.
Output combiner resistor not getting hot any more, just warm .....
The customer is extremely happy now. He only pushes the amp with 16 ~ 20 watts PEP .......
His power supply is a 70a Astron set at 13.8vdc .......
I told him that it should last him years running it like that.
#29 Ranch55,
Jan 17, 2020