[Test-Equipment] Millen GDO question

[Fuqua, Bill L]

Not that this has much to do with this issue but some components values change over 50 or more years.

From: test-equipment-bounces at mailman.qth.net [test-equipment-bounces at mailman.qth.net] On Behalf Of Richard Knoppow [1oldlens1 at ix.netcom.com]
Sent: Sunday, July 31, 2011 10:38 AM
To: Mike Manes; Discussion of Electronic Test Equipment
Subject: Re: [Test-Equipment] Millen GDO question

Carbon composition resistors are very bad. In our stock room we had some carbon comp resistors that
were not only out of tolerance but were up to the next 10% value and they were only 10 years old!
The upper values were the worst. The 1 Meg and above were way off and had increased in value 50% or more
in some cases. Under use they can change value even more. 
  Much of the stuff made in WWII or soon after may have resistors may be way off value. 
I wonder if some of the resistors that were treated or coated retained their value better than the non-coated ones.
It seems to me that the oil component of the carbon comp dries up, you can see if apply several time its rated
power. It will sweat or bubble oil onto the surface. And after the test its value will be somewhat greater.
We replace all our carbon comp resistors with carbon films. The carbon comps had several advantages but
we just could not let them die while sitting in the stock room. Now they are very hard to get. 
The carbon film resistors can burn out without any outward sign of damage and if the epoxy coating is damaged
they can easily short out. 
   We got some Tektronix 2200 series scopes that started having focus problems not long after receiving them
Discovered a long voltage divider string on the HV supply had carbon comp resistors.  They possibly used them 
because they could be put next to the circuit board without worry of arcing. But, they increased in value and 
the scopes failed. We replaced them with carbon film resistors and placed them about a 1/4 inch above the PCB.
Since these were in instructional labs we could not wait forTektronix to repair them under warranty.

73
Bill wa4lav


----- Original Message -----
From: "Mike Manes" <mrmanes at gmail.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>; "Discussion
of Electronic Test Equipment"
<test-equipment at mailman.qth.net>
Sent: Saturday, July 30, 2011 8:37 PM
Subject: Re: [Test-Equipment] Millen GDO question


> Hi Richard,
>
> That's pretty characteristic of every GDO I've used,
> including the
> Millen, a Knightkit and a really nice piece of homebrew
> work. I
> think it has to do with the low VHF Q of the air variable
> cap that's
> necessary to get a decent L/C ratio and tuning range on
> the HF
> bands.  I've seen GDOs that are specifically designed for
> VHF and
> UHF, and tho I haven't used one, I'd expect them to
> overcome that
> problem.
>
> 73 de Mike W5VSI
>
   Thanks Mike, that's what I wanted to know. I think mine
is working as it should. The low limit of the coil is given
as 140mhz. The dial is calibrated down to 125mhz. The grid
current shows some indication down to maybe 130mhz. So, it
covers the range specified, just not the complete range
shown on the dial. The next lowest range goes up high enough
to provide some overlap.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com

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