[Drake] Re: TR7 PA problem part3

[HENRY PFIZENMAYER]

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