Carl beat me to it.
The swamping resistor is probably kaput, disconnected, or missing.
Don
N8DE
Quoting Carl <km1h@jeremy.mv.com>:
> 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@btconnect.com>
> To: "amps@contesting.com" <Amps@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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>>
>
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