[Amps] MLA2500 issue

[Alek Petkovic]

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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