[ARC5] T-19 first run issues.

[Brian Clarke]

Not good ideas, Mike,

 

The EMR environment inside aircraft pre-WWII and during WWII was nothing like the EMR environment today, and certainly different from the EMR environment in your radio shack today.

 

Pure aluminium is a good conductor of electricity. But as soon as it is exposed to air it oxidizes. Aluminium oxide is a very good insulator. So, relying on physical contact between a piece of Command equipment and its rack to provide grounding is not a good idea. I suspect you will find that if you go back through the original design discussions that led to the specification issued for contracted manufacture, you will find that aluminium was chosen because it was light, easily pressed or drawn, and cheap.

 

In refurbishing Command equipment, after cleaning the surface with an abrasive cleaner, I use a zinc-rich grease, as used for connecting aluminium house wiring conduit, for every ground connection to the chassis. That includes under and around the threaded portion of every potted capacitor whose capacitance and ESR are acceptable. Corrosion cells are also set up between other dissimilar metals – the tuning capacitors and the chasses. The problem is that potted capacitor cases are brass and the tuning capacitors are usually tin-plated brass. The electrolytic cells created between aluminium, copper, iron, tin and zinc are excellent for corrosion. Cadmium-plated and chrome-plated brass and steel screws also form lovely electrolytic cells with aluminium. Just look up the half-cell Voltages in any good electro-chemistry textbook.

 

For low-impedance electrical and RF grounding, you are better off to connect your equipment via short, thin copper straps to a 1” x 1/8” copper strap, and then to connect that to an array of copper-jacketed ground spikes, using a copper-rich grease between all copper-to-copper connections. Do not connect to your AC mains ground; this can be at some variable AC Voltage away from that of your ground spike array. The ground loop between your AC mains ground and any other grounding will surely provide ample opportunity for hum. On late-WWII Command receivers, you will find thin plated strap used between the chassis and the dynamotor. You will also find similar thin straps used between the racks and radio shelving. Have a good think about why that was done.

 

73 de Brian, VK2GCE  

 

On Tuesday, 21 April 2020 6:22 AM, Mike said:

 

Amen.  It's one of the reasons I use aluminum liberally for the support racks in my rig.  Reproduce the environment that they had in the aircraft, and you'll head off a lot of strange problems you sometimes get with grounding/bonding/shielding inadequacies.

https://aafradio.org/flightdeck/layout.htm has some construction thoughts on the bottom half of the page.

- Mike  KC4TOS

On 4/20/2020 12:22 PM, Mark K3MSB wrote:

Ground all your "boxes" together directly;  don't rely on the connectors.   

 

Mark K3MSB

 

On Sun, Apr 19, 2020 at 1:26 PM J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:

Just got this ARC-5 system wired up and going. Even the rack had hacked/dry wires that needed rewiring.

The T-19 was recapped, cleaned, etc.  

 

I have this strange problem with a hum in the carrier.  It sounds like a rig with a bad filter cap. I rechecked my wiring, etc, but everything is OK.

The only other "symptom" is low output, about 2  Amps in CW mode . ( It's not the meter because I use the the same one with the SCR274 which will go 3+ amps). I'm using an A-27 dummy load

 

I'm thinking that it may be an alignment issue where the finals are off-resonance and drawing too much current compared to the carrier amplitude and inducing a ripple modulation? 

It's real subtle when looking at it on a scope, but obvious on a radio.

The Dynamotor is almost ripple free , putting out around 590V under load. 

 

Any thoughts?

 

 

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