[ARC5] Drift in ARC-5s - and other matters.

[Kenneth G. Gordon]

On 8 Dec 2008 at 13:10, Bob Macklin wrote:

> I have  not looked at an ARC-5/SCR-274 rig in over 40 years but I do
> remember some about them.

You might want refresh your memory by reading the manual here:

<http://www.mines.uidaho.edu/~glowbugs/PDF%20files/ARC5/ARC
-5-man.PDF>

> These units were designed for aircraft to aircraft and aircraft to
> ground operation using VOICE. Not CW.

Sorry. That is wrong. They were used very heavily on CW. The 
main use of voice with them was in fighter aircraft before the AAF 
switched to VHF.

> They were also only intended fro
> short range communication.

Again, not necessarily so.

> I seem to remember these units having provision for MCW operation.

No. None for transmitting.

> Also remember that these units had receivers with pretty broad IFs.

True to a certain extent, but that depended on WHICH receiver: the 
LF versions, covering 190 to 550 Khz were VERY narrow. In fact, 
at their narrowest, too narrow for voice. Ever heard of the "Q-5er"?

The IF frequencies depended on the band. As I remember it they 
went like this:

190-550 Khz, IF was 85 Khz. 
(Max BW at 60 db down was 4.5 Khz, min BW was 1.1 Khz at the 
3db point)
550 - 1500, IF was 239 Khz (Max 8 Khz, min 2.1 Khz
1.5 to 3.0 Mhz, IF was 705 Khz (Max 8 Khz, min 3.2 Khz)
3.0 to 6.0 Mhz, IF was 1415 Khz, (Max 26 Khz, min 7.3 Khz)
6.0 - 9.1 Mhz, IF was 2830 Khz. (Max 56 Khz, min 13 Khz)

The reason for the wider IF at the higher frequencies was to enable 
reliable communications with OTHER rigs which were far less 
stable.

Also, ARC chose the IF frequencies to follow the usual convention 
at the time of 5% of the lowest RF frequency in order to minimize 
images.

> And in the early days it was transmit or receive. Not QSK. When these
> units were in the transmit mode the oscillator was keyed continuously.

No. Not for CW. Only for voice. The keying relays (selector relays) 
keyed BOTH the oscillator and finals. If one adjusted the contact 
spacing, one could easily achieve a differential keying mode: i.e., 
the oscillator would start first, followed by the final, then the final 
would be un-keyed first, followed by the oscillator.

I am not sure about this, but I believe the antenna relay was ALSO 
keyed, which would, all thiings being equal (which they certainly 
were not) result in QSK keying...slowly.

> What hams did with them after WWII is another story.

Boy! You sure have THAT right! :-)

As far as I and many others are concerned, they essentially ruined 
most of them, especially the BC band versions.

I, myself, was guilty of that. I totally ruined, among others, one that 
covered 160 meters. Those are very rare now.

Ken Gordon W7EKB