[ARC5] SCR-274N update

[Mike Everette]

Your "whine" may actually be something coming out of the side tone oscillator in the speech amp unit.

I forget whether the tone oscillator in the ART-13 is supposed to run whenever the transmitter is keyed.  I wouldn't think so; but maybe some problem in your transmitter or setup is holding it on.

In an ARC-5, I have seen the side tone oscillator's 1000 cycle output show up on the supply feeding the master oscillator plate.  This is in CW mode of course, and the ripple was low level but enough to be quite noticeable.  The quick test to see if that's the source of a ripple is to pull the 12J5 out of the modulator.  I forget whether this ripple showed up in voice mode, but don't think so because if I remember right the tone oscillator is shut off in voice; there's a contact on the mode switch in the control box which does this.

73

Mike
W4DSE



________________________________
 From: Christopher Bowne <aj1g at sbcglobal.net>
To: "arc5 at mailman.qth.net" <arc5 at mailman.qth.net> 
Sent: Saturday, October 5, 2013 5:47 PM
Subject: Re: [ARC5] SCR-274N update
 

I had a really rough CW note come out of nowhere one night on the Old Milrad CW net from my Command Set system's T-19/ARC-5.  I gave the DM-33 on the BC-453 a thump ( or as the Navy would say - mechanically agitated it -I have seen that phrase for real in a current operating manual where they meant give the valve a whack) and cleared it up immediately.  One of the HV end commutator brushes was sticking badly it its carrier.

All DC generator outputs will have some amount of "brush ripple" AC component.  If you are not getting anywhere by easter-egging dynamotors onto your receiver, start looking for open filter and bypass caps on the DC path to your various receiver stages, and or any cathode bypass caps. Or, simply add some capacity across the dyno HV output side and see if it helps. Should only need a few up to maybe 10 microfarads. for starters.

My ART-13 has an inherent amount of brush ripple (actually whine) in the audio on voice, which is not present in the CW  output waveform.  It appears to be at about the same frequency as the DY-17 fan vane frequency, the whine is still present with the transmitter on but the microphone audio input turned off, probably from some ripple on the LV B plus output getting into the speech amp stages.  I've never bothered to fix it, its become a hallmark of the transmitter's signal over the years.
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