[Amps] Dentron MLA2500 SWR Question

[Carl]

Is the input swamping resistor connected? Many clip it out so they can 
get another 200 Watts and fry the tubes faster.

Also check that the bandswitch doesnt have a fried contact.

I have a MLA-2500 here as backup and the TS-940 drives it with no 
problem on any band.

Carl
KM1H
----- Original Message ----- 
From: "Adrian Rees" <rees.a at btconnect.com>
To: "amps at contesting.com" <Amps at contesting.com>
Sent: Friday, August 01, 2008 4:56 AM
Subject: [Amps] Dentron MLA2500 SWR Question


> Hi all
>
> I have a Dentron MLA2500 linear, which mostly works fine, and I'm 
> getting lots of RF out.
>
> However, on 3.5MHz (no other band)  it is difficult to tune and was 
> wondering if someone could explain to me what is going on, and how I 
> may cure the the problem ? So here goes, remember this is just on 
> 3.5MHz (80 M)
>
> Prime mover is IC740
>
> I tune the Dentron with about 10w from the IC740 into a Dummy load. 
> When I have peaked the linear, the SWR seen by the IC740 is around 
> 1.8:1, as I increase the drive, the SWR seen by the IC740 goes up to 
> as much as 5:1 or more, thereby reducing drive power. This slow 
> cycling between lots of drive, but high SWR, then the IC740 reducing 
> drive (to protect the finals) reduces the SWR, the power drops, and 
> the whole thing starts again.
>
> I have got a small ATU installed between the Linear and the Radio, but 
> this is not really curing the problem. My other HF radio (756ProII) 
> with its Auto ATU is a little better, but again it cycles.
>
> Both Prime movers get to about 25 - 30 watts of drive and the cycling 
> starts. (Which means I only get about 250 - 300 watts out).
>
> I know the Dentron doesn't have a tuned input, hence the small ATU 
> between it and the Prime mover.
>
> I guess the amp is going into some form of hysteresis ? Perhaps I am 
> wrong, but some insight into whats happening would be good, and any 
> suggestions as to how to fix this annoying action would be great.
>
> Thanks
>
>
> Adrian MW1LCR
> Wales
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