[ARC5] Drift in ARC-5s - real-world measurements. - quite long.

[Bob Macklin]

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

These units were designed for aircraft to aircraft and aircraft to ground
operation using VOICE. Not CW. They were also only intended fro short range
communication.

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

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

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.

What hams did with them after WWII is another story.

Bob Macklin
K5MYJ
Kent (Seattle), Wa,
"Real Radios Glow in the Dark"

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at verizon.net>
To: <ARC5 at mailman.qth.net>
Sent: Monday, December 08, 2008 12:05 PM
Subject: [ARC5] Drift in ARC-5s - real-world measurements. - quite long.


> OK. I dug out the ER Magazine in which the drift of those
> transmitters was measured and discussed.
>
> The title of the series is "Secrets of the Dead...Command Sets,
> Part 2", and written by Mike Murphy WU2D, published in Electric
> Radio Magazine number 231 of August 2008 starting on page 20.
>
> 5 different "ARC-5" variants were tested.
>
> Power output, for all transmitters tested, was 15 watts into a 250
> watt 50 ohm dummy load, coupled by a PROPERLY
> CONFIGURED three-element "Tee" network.
>
> Measured drift on 80 (3600 Khz) and 20 (!) (14040 Khz) averaged
> less than 1 Hz per minute.
>
> The transmitters were the following:
>
> 1) BC-696 in complete original condition. This unit chirped badly.
> Oscillator and finals keyed during testing periods (to be described
> below). I.e., the oscillator was only "on" during the short periods
> when it was keyed during the tests. Otherwise it was off, although
> the filaments were lit and plate voltage was applied. This is the way
> they originally worked.
>
> 2) T19/ARC-5. Owned by Mark WA1QHQ, converted using the
> 1962 handbook method. 12 VAC filaments, all relays removed, key
> jack added. Only the finals are keyed. Oscillator runs continuously.
>
> 3) BC-458. This unit was SEVERELY damaged, then rebuilt as an
> 80 meter mobile transmitter. Only the finals are keyed. Oscillator
> runs continuously.
>
> 4) BC-696: This is unit #1 with a new 1626 and that rear-panel-
> mounted triple-can capacitor replaced. Chirp is now non-existent.
> Drift is only slightly less than before.
>
> 5) BC-457A: Chassis arrived completely stripped except for the
> VFO. Rebuilt as a fully functional 20 meter CW transmitter. 1626
> replaced with a 12SK7 operating at 7 Mhz. 12A6 buffer/doubler.
> Voltage regulated oscillator screens. Only finals and buffer keyed.
> Oscillator runs continuously.
>
> All power supplies (except in the case of # 5) were UN-regulated,
> although the power supplies were quite "robust".  I.e., were capable
> of considerable current output. Oscillator voltage is 250 VDC. Final
> voltage is 450 VDC.
>
> The first reading of frequency was taken at 6 minutes after turn-on,
> thereafter, every 15 minutes for just long enough to get a reading.
>
> Test equipment used was all modern.
>
> Total time for each test was 2 hours, or 6 readings, total.
>
> The LAST, or 7th, reading was an "Old Buzzard" test in which the
> key was held down for 6 minutes.
>
> The author states that the 1626 oscillators "...drifted mightly..." for
> the first 15 minutes.
>
> Personally, I find that his description of "...mightly..." would not be
> something I would agree with. The results for the first 15 minute
> test (AFTER the 6 minute "warm-up") are as follows:
>
> 1) -273 Hz.
>
> 2) -467 Hz.
>
> 3) -930 Hz
>
> 4) -507 Hz
>
> 5) -1030 Hz (on 20 meters, doubling from 7 Mhz, or -515 Hz at 7
> Mhz.)
>
> I am going to skip most of the rest of the "standard" tests, but will
> tell you that after the first 15 minutes, the drift slowed markedly.
>
> For instance, between test 2 and test 3 the drift was as follows:
>
> 1) - 56 Hz
>
> 2) + 34 Hz
>
> 3) + 66 Hz
>
> 4) + 12 Hz
>
> 5) - 30 Hz (!)
>
> Total drift for the entire 2 hour period, or 120 minutes, start to stop,
> was as follows:
>
> 1) - 542 Hz.
>
> 2) - 437 Hz.
>
> 3) - 1093 Hz
>
> 4) - 669 Hz
>
> 5) - 870 Hz
>
> Skipping to the "Old Buzzard" test, i.e. transmitters keyed on for 6
> minutes at full output of 15 watts the total drift measured DURING
> that 6 minute test was as follows:
>
> 1) - 519 Hz
>
> 2) + 68 Hz
>
> 3) - 10 Hz
>
> 4) - 463 Hz
>
> 5) - 340 Hz (again, on 20 meters).
>
> So, as some of us have repeatedly stated here, the "ARC-5"
> transmitters do NOT drift unusually if run at original specified
> ratings.
>
> In fact, at the time they were plentiful, they were FAR more stable
> than most of the VFOs sold commercially or built from handbook
> articles of the time.
>
> Even today, 65 years and more after their first use, they will give
> many more-modern VFOs a run for their money... if run according
> to original specifications.
>
> I rest my case.
>
> Ken Gordon W7EKB
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