[Amps] ML!-2500 Filter Cap Replacement

[Peter Chadwick]

Tom said:
>Carbon resistors should never be used for 
critical voltage equalizing or voltage dropping 
applications, because carbon resistors age down in value 
with heat and time<

I've never found one that aged down in value, always the ones I've checked have gone up in resistance. Even when they've never been installed, and that includes brand new military ones in NATO sealed packages.
It's quite amazing how well some of the old gear still works with resistors that were 20% tolerance to start with and in many cases, are now 50% tolerance.
But I agree whole heartedly about not using them for voltage equalising or dropping.
73
Peter G3RZP




========================================
Message Received: Feb 19 2007, 10:32 AM
From: "Tom W8JI" 
To: "amps" , "Kim Elmore" 
Cc: 
Subject: Re: [Amps] ML!-2500 Filter Cap Replacement


>I have my Dad's MLA-2500 in my shack for what I suspect is 
>a filter
> capacitor replacement job. It hadn't been operated in a 
> long time and
> Dad wanted me to check everything out (he's had health 
> problems). I
> was tuning it up and all seemed well until I heard a soft 
> pfft,
> smelled a Bad Smell, and heard the PS hum loudly, clearly 
> drawing
> lots of current.

There are a few component selection errors in Dentrons that 
should always be taken care of immediately.

The major error is they used carbon resistors across the 
electrolytics. Carbon resistors should never be used for 
critical voltage equalizing or voltage dropping 
applications, because carbon resistors age down in value 
with heat and time. That causes a destructive cycle where 
the resistor decreases in value until something fails, 
usually an electrolytic or the resistor itself.

Get rid of ANY carbon resistor you see across any 
electrolytic in any amp, and replace them with metal based 
resistors.

Also they used low voltage rated resistors across diodes. 
That's another component selection issue. You can just 
remove them and leave only the caps.


Many Dentron amps using sensitive control grid tubes also 
don't have grid protection. Some, very stupidly, don't even 
have a grid current metering system. You might want to be 
sure the MLA has a grid current overload system installed.

73 Tom 


_______________________________________________
Amps mailing list
Amps at contesting.com
http://lists.contesting.com/mailman/listinfo/amps