[Heathkit] DX-40 Keying trouble

[Carl]

----- Original Message ----- 
From: "Robert Nickels" <ranickel at comcast.net>
To: <heathkit at mailman.qth.net>
Sent: Monday, August 30, 2010 5:59 PM
Subject: Re: [Heathkit] DX-40 Keying trouble


>  On 8/30/2010 3:51 PM, Glen Zook wrote:
>> It showed less than 0.2 ohms to ground from the B+ line
> Some build runs of the National HRO-500 are notorious for disk ceramic
> failures, and the maximum DC voltage in this receiver  is 12 volts!
> The root cause of the problem is silver migration, which remains an
> issue even today for manufacturers of multilayer ceramic capacitors,
> membrane switches, and other devices.  Thankfully, with proper material
> selection and process control we don't have to deal with this problem as
> often as others.
>
> My experience confirms that even a .1uf bypass on a 10 volt line can
> develop an sub-ohmic short - after all it is silver!
>
> 73, Bob W9RAN


Only the first run, Series 75 of the HRO-500, had the black Cornell Dublier 
.1uF 50V discs installed. I happened to be the person to discover the 
problem with receivers returned under warranty. A quick response by National 
resulted in a bulletin to every distributor, contract buyer and distributor 
to return the radio for repair at no charge. Every 75 series set that came 
to National even after the warranty got a free repair. Hundreds of 
capacitors were mailed to US and foreign government and other owners.

I doubt many more are still undiscovered after about 45 years. If so they 
have been stored almost that long.

CDE admitted they tried a new process and I seriously doubt if any silver 
was involved.

Carl
KM1H
National Radio 1963-69
Service Tech, Service Manager and other hats were worn