[Drake] Re: TR7 PA problem part3

[ik2bcp - Guido Tedeschi]

Sorry for the delay, my ADSL was down yesterday evening...

Hi Hank,
    thanks for you message.
Yes, I checked the bias and it is correct, 0.69V and the quiescent current
for both transistor is 290mA
If it is possible, it would be very useful for me to have a gain curve of a
correctly working PA (predriver+driver+final) from 1.8 to 28MHz, to check if
my PA is fine or not...
Ciao, 73 and thanks
Guido, ik2bcp


----- Original Message ----- 
From: "HENRY PFIZENMAYER" <pfizenmayer at worldnet.att.net>
To: <drake at mailman.qth.net>
Sent: Monday, February 07, 2005 3:11 AM
Subject: [Drake] Re: TR7 PA problem part3


> Guido - If we go back to the beginning - you said you were getting
> something
> like 150 watts out at 1.8 mhz - its hard to believe that the output
> transistors could be bad and do that (assuming a 2SC2879 is a bipolar and
> a
> reasonable replacement for a MRF421). I will email the device guy who
> designed the MRF421 devices and see what he says - but I don't remember
> ever
> seeing devices that would play at 1.8 and die at 30 (assuming they were
> designed for that freq of operation). Incidentally the data book of MRF421
> shows a gain drop in common emiitter from 22 db at 1.5 mhz to 12 db at 30
> mhz . Some of that gain variation will be taken out with the collector -
> base feedback network L2308-C2308-R2310 and its equivalent on the other
> side.
>
> By the way - you have been very thorough - but did you check the fixed
> bias
> on the output transistors ? Should be something like 0.65 volts transmit
> with no rf (key up) - also what is the quiescent current in the output
> stage
> ?
>
> Data sheet numbers are all given for 150 ma quiescent in each transistor -
> or 300 ma total.
>
> Rgds-- Hank K7HP
>
>
>
>> Message: 3
>> Date: Sat, 5 Feb 2005 16:26:49 +0100
>> From: "ik2bcp - Guido Tedeschi" <guido.ted at tin.it>
>> Subject: [Drake] TR7 PA problem: part 3
>> To: <drake at mailman.qth.net>
>> Message-ID: <002001c50b97$2459f690$0301a8c0 at MAIN>
>> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
>> reply-type=original
>>
>> Maybe, the problem seems to be in the finals, already substitued because
> it
>> is a 2sc2879 pair.
>> Disconnecting the bases of both final transitors and measuring the output
> of
>> drivers, across T2303 secondary, R2317 and C2306, the signal is regular
> with
>> about the same amplitude across the HF frequency range, when loaded with
>> a
>> 2,3OHM load to roughly emulate the transistors bases load.
>> When I reconnect the 2sc2879 bases, the driver frequency response returns
>> wrong with 12dB difference between 1.8 and 18MHz.
>> I don't know what happened in the finals, it seems like the bases have
>> increased their capacity to very high values.
>> With the bases not connected, the measurements with a multimeter of the
>> junctions gives as result two healty transitors...
>> I'm now in a "cul de sac", before changing the PA transistors, has anyone
> an
>> explanation of what happened?
>> Thanks and 73
>> Ciao
>> Guido, ik2bcp
>
>
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