[Amps] MLA2500 issue

[Clark Turner]

Another long shot:  I had a MLA2500 that would also lose power when 
warm. After wasting hours trying to diagnose the amp, I unhooked it and 
realized the problem. The factory antenna output jack was worn out and 
would not make contact to the center pin. For lack of a better term the 
connector was "whored" out. I attempted to bend the fingers back in and 
that immediately snapped one off. I replaced the connector.

C


On 11/15/2016 4:20 PM, Alek Petkovic wrote:
> If there's no smoke, it could be the input capacitor/s to the cathode, 
> going bad as the temperature rises. That happens quite a lot on the 
> older Yaesu amplifiers except they usually fail completely open circuit.
>
> I notice in the 2500, there are 2 capacitors in parallel. A long shot 
> would be that one of the capacitors goes open and then the other 
> succumbs to the extra strain.
>
> Just my shot in the dark.
>
> 73 and good luck.
> Alek. VK6APK.
>
> On 15/11/2016 8:34 PM, Jim Thomson wrote:
>> Date: Mon, 14 Nov 2016 17:07:15 -0400
>> From: "Mike Smith VE9AA"<ve9aa at nbnet.nb.ca>
>> To:<amps at contesting.com>
>> Subject: [Amps] MLA2500 issue
>>
>> Hi All,
>> I'm no amp expert, but have owned an MLA2500 for close to 3 decades.  
>> I've
>> repaired it a few times (all new caps, timer tube replaced, PS board
>> replaced one time). Might have put in a new tube once.
>>
>> I use it for contesting. I've used it on DXpeditions back in the day, so
>> it's sees LOTS of use.
>> When I am on CW, I always operate it in the lower plate voltage mode.
>> Recently in a contest I started experiencing sudden/rapid power 
>> output loss,
>> from its usual 750w CW to perhaps 300w for some minutes, then briefly 
>> 100w,
>> then within several minutes, no output at all. If I shut it off and 
>> restart
>> it 20-30 minutes later from cold(warm?) I usually sees perhaps 
>> 300-400w out
>> for a couple minutes, dropping to 100w within a minute,then 0 (or
>> essentially 0) watts out.
>> It's just getting worse.
>> When I took the cover off, it appears one of the 8874's is much 
>> darker than
>> the other. This might be nothing, as I have had various 8874's from
>> different sources over the years, but thought I'd mention it in case it
>> means something
>>
>> Looked around and measured continuity with the ohmmeter here and 
>> there. Not
>> seeing anything obvious.Is it a bad tube? Where would I start 
>> troubleshooting?
>>
>>
>>
>> Thanks
>>
>>
>>
>> Mike VE9AA
>>
>> ## Can u run the amp with one tube removed ?   Its probably one or 
>> both tubes.
>> Run it with one tube at a time.  You can also swap positions in case 
>> its a socket issue.
>> Do you have any spare tubes ?   Does it do this on all bands or just 
>> one band ?
>>
>> Jim   VE7RF
>>
>>
>>
>>
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>