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Zetagi B300P amp; First impressions and problems
Topic Started: Friday, 18. November 2011, 07:00 (5,093 Views)
P-and-Ham
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I have just bought (over the internet, from a reputable UK supplier) a Zetagi B300P linear amplifier for use on the 10m Ham band. I am driving it with the Chinese-made Anytone AT5555-N, and mainly using this combination (so far) on the New York repeater NQ2H on 29.620MHz and on the AM sub band 29.000 to 29.100MHz.

I'm having some problems with it which I know other people have probably had, so I would be glad to hear some comments and advice. I'm interested in your knowledge, by the way, not in whereabouts you operate in the radio spectrum, so long as I don't get your splatter. B)

Firstly, I had hoped for a bit more in the way of a manual. What you get with it is a slip of paper in four languages, giving very basic information, a circuit diagram you need a magnifying glass to see, and a diagram of what to plug where. What isn't explained anywhere is what precisely the function of the 6-position knob is. Without that knowledge I'm flying blind. Has anyone compiled a 'manual' of his or her own?

Secondly, and following on from that. I drive it with the rig's rf power backed off a long way and with the 6-position knob on '1'. I get a comfortable 50W of AM at that setting, and I use it to work across the Atlantic without a problem. However, I don't seem able to use settings 3 to 6 without the relays chattering. If I had that knowledge about the 6-way knob, of course, things would be different...

Thirdly, the 'pre-amp' switch attenuates the incoming signal, or rather it mutes the receiver entirely. This is laughable! I understand this is a common fault with this amp. It's annoying, but I suppose one can live without the pre-amp. Zetagis are cheap after all, and you get what you pay for.

Fourthly, it runs very hot. I have a small 12V fan which I am going to mount behind it in order to run air over the fins of the heat sink.

Lastly, I laughed when I pulled it out of the box and saw the power lead - all six inches of it!

Any comments? I have to decide whether the 'faults' mean I should return the amp, or whether I can work round them.
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MadMark1
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163 Charlie Tango 020
Hi Paul, The B-300 is a well known amp and as you know its also very common, Circuit diagrams and info on the amp is Freely available on the net.

Power wise the original B-300 had a pair of MRF455 transistors each was rated for 60 watts, so that a realistic output power of 120 watts although ive seen some do a bit more than this around the 140 watts .

The 6 way rotary switch is nothing more than a 'Resistor network' its a Antenuator and its function is to reduce the amount of power going into the amp, The power from your radio goes into the amp via the main TX-RX relay, It then enters the 6 way antenuator, On stage 1 it has maximum Antenuation, Stage 2 a little less antenuation, Stage 3 a little less again and so on, Stage 6 is basically straight throught with little or no anteneuation and on into the Power Transistors, On this stage youve got to be carefull of how much power you put in as the Transistors could be perminantly damaged or destroyed, I think its about 6 watts maximum for each MRF455 Transistor although it would be best to check on what transistors are actually in you amp and look up the input / output power.

As for the pre-amp section its nothing more than a wideband preamp, Its not tuned to the 11 mtr band at all and as such being a widedband untuned preamp its PRONE to picking up everything and this shows up on times as Radio stations and signals that arnt really there.

The SSB/AM switch should obviously be in the SSB position when on SSB as in this position there is a capacitor inserted inline with the relay This has the effect of 'Holding' the relay closed for a second or so before it goes back to RX this should stop the relay chattering.


Mark
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P-and-Ham
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Hi Mark

Thank you for all the info. Since I made my last posting about this amp I have been experimenting and also have had some advice from the guys at Nevada.

I'm not too bothered about using the pre-amp, to be honest, because I have discovered just how good the receive side of the AT-5555 is. I compared the received signal of a weak station on the AT-5555 with the same signal on my FT-847 using the same antenna; if anything the Anytone was a little bit better than the Yaesu!

The chattering turned out to be the fact that I wasn't using a hefty enough PSU. Nevada recommends one that draws 25-30 amps of current. I took it off my old Yaesu FP-12 and tried it with my Watson W-20AM which is rated 20 amps, and it worked fine. I restrict the input power to about 5W and was getting 100W of AM on setting 6 without any problem. It's nice to know the potential power is there but it is overkill for AM - I tend to use 40 or 50W at absolute maximum.

The amp runs very hot. I experimented with mounting a little fan behind it but it made too much racket.

Thanks again,

Paul
Edited by P-and-Ham, Saturday, 19. November 2011, 07:01.
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P-and-Ham
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... and by the way, Mark, I also have an old FL-2100Z in the shack which I managed to blow up...

*sigh
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MadMark1
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163 Charlie Tango 020
Thats a shame on the FL-2100-Z Paul, There a good amp, Are you going to get it fixed ?, Its worth doing..


Mark
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"It's not the class of license that the Amateur holds but the class of Amateur that holds the license"
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P-and-Ham
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madmark1
Saturday, 19. November 2011, 18:58
Thats a shame on the FL-2100-Z Paul, There a good amp, Are you going to get it fixed ?, Its worth doing..


Mark
Eventually yes, Mark. Meanwhile it's sitting in my shack as a kind of Da-da sculpture. B)

By the way, experiments with the Zetagi amp gave a comfortable 200W in SSB mode,
Edited by P-and-Ham, Tuesday, 22. November 2011, 17:27.
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