[Elecraft] KPA100 R 19 R20 question

[Geoffrey Mackenzie-Kennedy]

Werner,

The resistors might have different characteristics even though their ohmic 
value is the same and they might look to be identical. Some years ago it was 
reported on the List that the KPA100's output was low on some bands, and the 
cause was believed to be that the manufacturer of R19 and R20 had switched 
to supplying resistors that exhibited significant inductance. I believe that 
a mod kit was subsequently released which included 'non-inductive' versions 
of R19 and R20. It is possible that you have some of both types in your 
collection.

Unless you have the test equipment which is capable of measuring the 
inductance of a small coil (nH) shunted by a 3.6 ohm resistor, you could 
measure the series resonant frequency of  each 'resistor' and a capacitor, 
always using the same capacitor, my guess is that a 3300 pF capacitor would 
be a suitable value for HF resonance. Obviously those resistors which are 
'inductive' would resonate at a lower frequency than those resistors which 
are supposedly not. A crude measurement but adequate for the purpose I would 
think.

It is possible to deal with the resistors already installed without removing 
them, but very much easier to remove them - but ONLY if you are having power 
output problems with your KPA100 on some bands!! The truth of the matter is 
that all of the six 2w resistors are involved in the feedback networks and 
are suspects in any power 'droop' problem, and you have to isolate each part 
when doing on-board measurements. My KPA100 purchased in 2003 came with 
inductive resistors, but the gods were kind and its 'power droop' occurs 
outside of the ham bands.

73,
Geoff
GM4ESD













----- Original Message ----- 
From: "Dr. Werner Furlan" <furlan at gmx.net>
To: <elecraft at mailman.qth.net>
Sent: Friday, November 23, 2007 5:58 AM
Subject: [Elecraft] KPA100 R 19 R20 question


> while I am waiting for a replacement of J4 from Scott, I am looking at the
> parts left after finishing the KPA100. I can see 2 pairs of R19 R20 
> resistors
> in the box. They look identical to those on the board. I had the KPA100
> upgrade kit, so 1 pair is clear, there must have been another pair packed 
> to
> my KPA100 in double quantity. The reason was probably a replacement for
> problems with the formal type of resistor.
> Now my question: when sorting the parts, I was not aware of this possible
> problem and maybe mixed the different parts, so I am not sure if I built 
> in the
> correct resistor. From looking at them I cannot tell any difference. They 
> look
> exactly the same. Can I find a difference whith any measurement to 
> identify
> if I have by accident built in the wrong pair?
>
>
> 73! de Werner OE9FWV
>
>
> -- 
> Whenever you find that you are on the side of the majority, it is time to
> reform.
> -- Mark Twain
>
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