[SBE] RCA Dummy Load

Sat Apr 7 17:41:19 EDT 2018

Sorry I'm jumping in so late. I didn't have time to answer yesterday. If it's a 50's era load, it's more than likely that it was filled with Dowtherm A. Here's a link:

http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0030/0901b803800303cd.pdf

The way I know this is that many years ago I worked for a Chief Engineer who had been an RCA field service engineer in the 1950's, when RCA was selling TV transmitters faster than they could build them. He installed a lot of these loads.  At this particular station we had a number of the old loads (3-1/8" coaxial) left over from a retired low band TV transmitter.  Although the loads and associated meters were painted RCA brown, they were most certainly made by Bird.

I remember that distinctive smell when those loads would get warm.  According to the Dow MSDS sheet, the smell it's actually put in on purpose to warn you that Dowtherm A is not as benign as the normal mineral oil you could put in your Heathkit Cantenna.  However, from what I can see, it's not particularly dangerous, unless you drink it or have extensive skin exposure, or breathe it directly for a long time.  They probably were 51.5 ohms, almost everything was back then. So you'l end up with a match of  1.03/1 for a 50 ohm source.   As was pointed out, CH 13 (216 Mhz) is probably the highest frequency you could expect to be close to 51.5 ohms.  I know I've swept some old Bird oil filled loads with a network analyzer and the match isn't always so good when you get up 500 Mhz.

If the load is truly dry, you might get in touch with Bird about buying replacement oil.  If your power is 50 watts or less, mineral oil will probably work.

OWTHERM Heat Transfer Fluid A - Dow eLibrary<http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0030/0901b803800303cd.pdf>
msdssearch.dow.com
5 FLUID SELECTION CRITERIA Four important properties that help determine the viability of a heat transfer fluid in a particular applica-tion are stability, vapor pressure,

Jon Frank
WGBH TV and Radio
1 Guest Street
Boston, MA 02135
617-300-2597

[1504620836459_PastedImage]

________________________________
From: SBE <sbe-bounces at sbe.org> on behalf of DanRapak at verizon.net <DanRapak at verizon.net>
Sent: Thursday, April 5, 2018 10:46 PM
To: sbe at sbe.org
Subject: Re: [SBE] RCA Dummy Load

Thanks Jack. Much appreciated!

On 4/5/2018 10:26 PM, jack.davis wrote:
Darn spell check PCB

Sent from my Verizon, Samsung Galaxy smartphone by smoke signals

-------- Original message --------
From: "jack.davis" <jack.davis at engineer.com><mailto:jack.davis at engineer.com>
Date: 4/5/18 7:19 PM (GMT-08:00)
To: sbe member discussion mail list <sbe at sbe.org><mailto:sbe at sbe.org>
Subject: Re: [SBE] RCA Dummy Load

It will be either 50 or 51.5 ohms and should be good to at least channel 13.  The plug should be a normally closed thermal switch that opens on high temperature.

The oil could be something that smells like fly spray. Very obnoxious stuff!  Will dissolve vinyl floor tile (don't ask).  Could be COB.

Jack
K6YC

Sent from my Verizon, Samsung Galaxy smartphone by smoke signals

-------- Original message --------
From: DanRapak at verizon.net<mailto:DanRapak at verizon.net>
Date: 4/5/18 6:02 PM (GMT-08:00)
To: SBE Broadcast Group <sbe at sbe.org><mailto:sbe at sbe.org>
Subject: [SBE] RCA Dummy Load

All:

I'm looking for information on an old dummy load. I have an old RCA liquid cooled dummy load that's been collecting dust for several years. It is an RCA  P/N 216251. It's a nice unit with substantial heat fins. It's rated at 1 KW continuous and 1.5 KW for 30 minutes. I'm about to put it in service in the ham shack. However, I have no manual for it.

My first question is, does anyone know what the frequency range of this unit is?

Second, there is a two-pin, military style Amphenol connector on the the dummy load. What is the purpose of this connection?

Finally, what is the recommended transformer oil / coolant for this thing?

Any help would be appreciated.

Thanks in advance,
Dan Rapak

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