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Author Topic: AN758 300W AMPLIFIER HELP  (Read 10776 times)
WB2WIK
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Posts: 21836




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« Reply #15 on: February 04, 2010, 10:31:54 AM »

Tom added a lot more, but I was going down the same path with the heat sink when I asked about it.

At these power levels, it's *very* critical.

I also asked what kind/size heat sink you were using, but you didn't answer that.
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N2XZS
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Posts: 8




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« Reply #16 on: March 18, 2010, 01:11:59 AM »

I Like to thank everyone for all the good tips.

I still have a problem. Tom I made sure of the heat sink issue was resolved. The size of the heat sink is 6”x10” the base of the sink is about ¼” thick.

Now here is what I found so far:
One day I decided to disconnect the Low pass filter just to see what happens. Guess what. No more problem or its not as bad. Now I can transmit for about a minute CW tone. It still gets hot but not as bad. it looks like its operating normal now.

So now what???

Here is the scenario of the LPF. I just had 2 pcs. Of coax connected to it and laying on the table (temporary connection) one coax went to the ant. And the other went to the PA. why would the LPF cause the transistors to heat up??

Also what I noticed is the input transformer T1 and the 4 resistors R1, R2, R3, R4 gets hot (see schematic here http://www.communication-concepts.com/appnotes/AN758300Sharp.pdf ).
Any idea on this?

Thanks
Robert
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W8JI
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Posts: 9748


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« Reply #17 on: March 18, 2010, 05:18:40 AM »

I Like to thank everyone for all the good tips.

I still have a problem. Tom I made sure of the heat sink issue was resolved. The size of the heat sink is 6”x10” the base of the sink is about ¼” thick.

Now here is what I found so far:
One day I decided to disconnect the Low pass filter just to see what happens. Guess what. No more problem or its not as bad. Now I can transmit for about a minute CW tone. It still gets hot but not as bad. it looks like its operating normal now.

So now what???

Here is the scenario of the LPF. I just had 2 pcs. Of coax connected to it and laying on the table (temporary connection) one coax went to the ant. And the other went to the PA. why would the LPF cause the transistors to heat up??

Also what I noticed is the input transformer T1 and the 4 resistors R1, R2, R3, R4 gets hot (see schematic here http://www.communication-concepts.com/appnotes/AN758300Sharp.pdf ).
Any idea on this?

Thanks
Robert

Robert,

First, these are not production ready kits. They are engineering or experimenter's kits, and some are unstable or have other problems. Because Helge got one to work in a lab, that does not someone else can build one and have it stable or have long component life. They are really just starter kits.

One thing I notice with the particular kit you bought is the feedback is through a transformer, and there is no shunt resistance swamping the input. I don't like either of those things, not in the least. I do not think the feedback system is well-engineered at all.

You could have a much broader bandwidth and much more phase stable feedback system if you simply series connected a very low reactance capacitor and a resistor from collector to base. I would also add resistor to ground (through a low reactance low inductance capacitor) swamping of the transistor bases.

There is also a lack of bypassing. Figure 3 shows no bypassing on the bias feed. The push-pull transformer T2 has independent bypasses and series chokes on its feed, something I would not do.

Assuming you have the filters right and the transistors mounted properly, it sounds to me like you have a stability problem. All of the above suggestions would contribute to stability, which can vary widely from unit to unit. This is espeically true if you add varying lead lengths through coaxial cables to a low pass filter and the basic "design" has some flaws.

I know people like those engineering prototype kits, but I also know that most of them I have seen are not production ready units. More often than not I have had to redo board layouts and ciruitry to make them stable and repeatable.

I would start with a some base to ground swamping resistance (perhaps 10 ohms or less) from each base to the groundplane through some multilayer .33 uF capacitors with short leads. I would remove the original feedback circuit and add a .33uF 250 volt multilayer capacitor or chip cap in series with a 100 to 470 ohm 2 watt metal composition resistor (like an OX or OY series resistor) from collector to base.

This will give you much wider bandwidth much more phase stable feedback than the original circuit. The swamping resistors to the groundplane on the base will reduce gain, but greatly increase stability and terminate the exciter better over the entire RF cycle.

I would also try a shorter coaxial line between the filter and output transformer, and I would add some extra bypass capacitance at the bias feed.

73 Tom
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EA2FL
Member

Posts: 4




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« Reply #18 on: May 08, 2010, 10:16:31 AM »

I built An758, but using bigger and newer transistors. I cut the bias voltage through antenna relay. Be careful to adjust bias current after heatsink is already hot (even if there is a small transistor bloted to heatsink, working as temperature tracking diode). If not doing so, idling current can increase too much when hot and thermal runaway can be produce.

I just wrote my tests here:

http://www.eham.net/ehamforum/smf/index.php/topic,68148.0.html
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