[Heathkit] DX-40 Keying trouble

[Carl]

Edit: Change the second "distributor"  to  "individual who returned the 
warranty card"

Carl


----- Original Message ----- 
From: "Carl" <km1h at jeremy.mv.com>
To: <W9RAN at oneradio.net>; <heathkit at mailman.qth.net>
Sent: Monday, August 30, 2010 7:52 PM
Subject: Re: [Heathkit] DX-40 Keying trouble


>
> ----- 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
>