[Elecraft] K2/100 current consumption problem - update

[Geoffrey Mackenzie-Kennedy]

Fraser,

This is a very very long shot and probably daft! I see that the current
drawn on 160m tracks that drawn on 80m until the RF output passes 80w.going
up.From this I think that it is safe to say that the "bad" KPA100 is working
as it should to that power output level. Above that it is either starting to
flat-top for X reasons or its standing DC bias is changing perhaps towards
Class A or the driver has run out of steam.

Let me focus on bias change. Is it poissible that 160m RF above a certain
output power level mucks up the PA bias oscillator or that some circuit(s)
in the bias supply resonate at 160m thereby changing the DC bias to the PA.?
There is a lot of PA output RF around, but it might be worthwhile to measure
the DC bias voltage as you run up the PA power.

Capacitor tolerances alone would move such resonances around greatly.

Just a thought.

73,
Geoff.
GM4ESD

----- Original Message -----
From: "Fraser Robertson" <g4bjm at hotmail.com>
To: <elecraft at mailman.qth.net>; <support at elecraft.com>
Cc: <t.b.smith at open.ac.uk>
Sent: Thursday, March 03, 2005 2:21 PM
Subject: Re: [Elecraft] K2/100 current consumption problem - update


> So far I haven't found anything wrong as such on #3764:
>
> The LPF components appear fine (value and condition), and check out ok
with
> an MFJ analyser for SWR on receive.
>
> T1 and T2 and associated components appear ok.  One thing; C83 is 1000pF,
> not 1200pF as per circuit.  However I tried padding that with a 220pF SM
and
> it made no difference.  Looks like 1000pF was supplied with the kit.
>
> I've realigned the BPF on 160/80m, which was pretty much ok anyway.
>
> I've re-adjusted the bias, which was also ok.
>
> I've readjusted the pwr sense pot, again was ok.
>
> With the PA disabled the base K2 draws about 2.5A @ 10W op on all bands.
>
> With the PSU set to exactly 13.8V, and measuring the current (V across
> 10x0R1 6W in parallel, in series with V+) I get the following, into a 50R
> load.  Current on the PSU display reads similar.
>
> Set power W     Current A 160m     Current A 80m
> 50                    14                       14
> 60                    15                       15
> 70                    16.5                    16.5
> 80                    18                       18
> 90                     24                      19
> 100                   27                       20
> 110                   29                       21
>
> The actual power out is similar to the set power, to about 10%.    So
> topband behaves ok up to about 80W op.  This also happens from a cold
start,
> so I don't think it's thermal runaway.
>
> No instabilities are apparent on a scope or spec. an.
>
> Any suggestions appreciated.  Thanks / 73 Fraser G4BJM
>