Hi all
First of all thank you all for the very helpful replies, on and off list.
I have the amp on the bench at the moment, the swamping resistor has been
replaced, and bot the input and output sockets, together with the ALC and
Relay Sockets have been replaced.
I noticed a HV cable running from a door knob capacitor was so tight it had
forced a gap in the tank coil windings. I have replaced the HV cable with a
length of wire that isn't anywhere near the tank coil. I also had a length
of HV shrouding, and shrouded the cable. The reason this was so tight, was
that at some point the Aluminium bracket supporting the door knob capacitors
had broken off (over time due to vibration from the fan, perhaps) and had
been repaired in such a way as to shorthen the HV wire.
I havn't re-assembled the amp yet, as want to blow all the dust from the
inside of the amp, and put some switch cleaner on the switch contacts.
Once re-assembled, I'll run it up and see what happens.
73
Adrian
-----Original Message-----
From: Carl [mailto:km1h@jeremy.mv.com]
Sent: 01 August 2008 23:08
To: Joe Subich, W4TV; 'Adrian Rees'; amps@contesting.com
Subject: Re: [Amps] Dentron MLA2500 SWR Question
Dentron used nothing but quality Centralab and HEC ceramic caps for
padders in the MLA-2500 and all their other KW+ amps..
The same brands used by Alpha.
Carl
KM1H
----- Original Message -----
From: "Joe Subich, W4TV" <w4tv@subich.com>
To: "'Adrian Rees'" <rees.a@btconnect.com>; <amps@contesting.com>
Sent: Friday, August 01, 2008 6:26 PM
Subject: Re: [Amps] Dentron MLA2500 SWR Question
>
>> I will investigate the bandswitch tomorrow, as that sort of makes
>> sense.
>
> While you're in the area of the bandswitch also look at the padding
> capacitors ("doorknobs") since they are notorious for thermal drift
> with high current. A bad padder would also be consistent with high
> drift on one or two bands depending on the particular switching
> arrangement and how much of the total capacitance was being provided
> by the padder.
>
> Dentron was not known for the highest quality components
> and might well have substituted capacitors with marginal current
> capacity if they could get new ones at a lower cost.
>
> 73,
>
> ... Joe, W4TV
>
>
>
>
>> -----Original Message-----
>> From: amps-bounces@contesting.com
>> [mailto:amps-bounces@contesting.com] On Behalf Of Adrian Rees
>> Sent: Friday, August 01, 2008 5:49 PM
>> Cc: 'amps@contesting.com'
>> Subject: Re: [Amps] Dentron MLA2500 SWR Question
>>
>>
>> There is a 200 Ohm Swamping Resistor in place.
>>
>> I replaced the original (which had been removed by the previous
>> owner) with a 200 Ohm non inductive resistor made up of a few
>> resistors in Series / Parallel arrangement to achieve the 200 Ohms at
>> 50 Watts.The resistor was also replaced to lengthen the life of the
>> 8875's as well as to be clean on air.
>>
>> I will investigate the bandswitch tomorrow, as that sort of makes
>> sense.
>>
>> 73 for now
>>
>> Adrian
>>
>> -----Original Message-----
>> From: amps-bounces@contesting.com
>> [mailto:amps-bounces@contesting.com] On Behalf Of Alex Eban
>> Sent: 01 August 2008 18:42
>> To: 'Carl'; n8de@thepoint.net
>> Cc: 'amps@contesting.com'
>> Subject: Re: [Amps] Dentron MLA2500 SWR Question
>>
>>
>> Hi Adrian:
>> It's not hysteresis!
>> What happens is more akin with motor boating: the power cutback isn't
>> instantaneous, so that it takes a little time for it to activate and
>> then to release when it detects the lowering of the SWR. You need the
>> input resistor in order to equalize the input impedance of the tube
>> over its operating cycle, don't go without it: maybe a little more
>> power but also a lot more distortion.
>> Alex 4Z5KS
>>
>> -----Original Message-----
>> From: amps-bounces@contesting.com
>> [mailto:amps-bounces@contesting.com] On Behalf Of Carl
>> Sent: Friday, August 01, 2008 7:15 PM
>> To: n8de@thepoint.net
>> Cc: amps@contesting.com
>> Subject: Re: [Amps] Dentron MLA2500 SWR Question
>>
>> Another option is to build a broadband input assembly patterned after
>> the 2 hole 8874 Alphas.
>>
>> Carl
>> KM1H
>>
>>
>> ----- Original Message -----
>> From: <n8de@thepoint.net>
>> To: "Carl" <km1h@jeremy.mv.com>
>> Cc: "Adrian Rees" <rees.a@btconnect.com>; "amps@contesting.com"
>> <Amps@contesting.com>
>> Sent: Friday, August 01, 2008 12:22 PM
>> Subject: Re: [Amps] Dentron MLA2500 SWR Question
>>
>>
>> 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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