I wouldnt put any cleaner on the contacts.
Use a Q tip to rub off any residue and that is sufficient. Many cleaners
leave a residue on the ceramic that will guarantee an arc.
Carl
KM1H
----- Original Message -----
From: "Adrian Rees" <rees.a@btconnect.com>
To: <amps@contesting.com>
Sent: Sunday, August 03, 2008 1:29 PM
Subject: Re: [Amps] Dentron MLA2500 SWR Question
> 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
>>> >> _______________________________________________
>>> >> Amps mailing list
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>>> http://lists.contesting.com/mailman/listinfo/amps
>>> >>
>>> >
>>> > _______________________________________________
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>>> >
>>>
>>>
>>>
>>>
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