[ARC5] "Curing Chirp in Command Transmitters"

Sat Oct 4 23:00:32 EDT 2014

I put several 3-4 and 7-9 mhz SCR-274N transmitter on the air back in 
the early 50's.  They were still available in original new condition
and fairly easy to modify and worked very well on CW with no chirping at 
all.  Generally the power supplies I built had one plate transformer 
that supplied B+ at around 650-750 volts DC to the 1625 Plate circuits 
and Around 250 - 300 volts to the oscillator tube and the 1625 screen 
grids.  This was regulated by means of gas filled regulator tubes, 
usually an 0D3 and an 0C3 in series.  The voltage should be 250 volts and
this is delivered to the supply line for the oscillator plate and the 
screen grids of the 1625 PA tubes.  The 1625s actually draw around 5 ma 
of screen current each and the 1626 draws around 20-25 ma.  Since the 
voltage supplied is regulated the VFO usually does not "chirp" and the 
keying is very clean.  I used a 50 watt voltage divider resistor from 
the B+ supply to ground and adjusted the slider on the resistor until 
current
supplied by the resistor tap and VR tubes in series from slider to 
ground was around 40 ma.  When the transmitter is keyed the VR tube glow 
will be less as will be the current they actually draw.  You want the 
maximum current drawn by the regulator string to be no more than 40 ma, 
maybe 45 ma in a pinch.  They act as ballasts to stabilize the screen 
and oscillator plate voltages instead of them wandering around from key 
up to key down conditions, which will make the transmitter possibly 
chirp!  Seems to me the voltage divider resistor was around 30000-50000
ohms at 25- 50 watts.  They may be hard to find now.  Ohmite made them.

I used them on 80 and 40 for years when I was going to grammar school 
and high school until I could afford a better transmitter. They served 
me well and will deliver around 75-90 watts of output depending on the 
voltages the supply was delivering or the condition of the tubes.

Hope this helps.  It's been many years since that happened.

73,

Sandy W5TVW
On 10/4/2014 8:41 PM, J Mcvey via ARC5 wrote:
> I am yet to get my transmitters up and running due to lack of time and materials. I am EXTREMELY curious if they will chirp or not.
> The only problem with any theory is the details. In this case, do we know how this gentleman was keying his transmitter? Was it modified or using the original equipment?
>
> I checked the transmitter diagram and amazingly, the only place they decoupled the LV line was with c61 because the the filament is part of the oscillator circuit for some reason ( anybody know?)and that is separated from the LV line via the series magic eye tube filament. No LV decoupling in the rack or MD7 either.
>
>
> On Saturday, October 4, 2014 8:42 PM, Leslie Smith <vk2bcu at operamail.com> wrote:
>   
>
>
>    Hello Dave,
>    If this theory is correct - that RF is entering the filament leads
>    from OUTSIDE the transmitter - then adding a balun externally  to the
>    filament lines should prove/disprove the theory.
>
>    The remedy (shielding the filament wires) cures the problem (according
>    to Mr. Cline) but surely the shielded wires dissipate RF energy in the
>    shielding.
>    My strategy would be to keep the RF "out" by some other means - see
>    the following:
>    Given that the filaments were originally wired in a series connection,
>    and Mr. Cline measured 12 volts (and 18 volts when transmitting), I
>    suspect the filaments had been re-wired.
>    If your theory is correct, then the RF power getting into the filament
>    lines is wasted power - about 2.7 watts per "tube".
>    (i.e. (18 x 0.45) - (12.6 x 0.45) per tube.
>    Power heating the filaments isn't reaching that magical substance
>    called "ether".
>
>    All this is speculation on my part - I don't have a set to make
>    measurements, but it's speculation informed by maths.
>
>    73 de Les Smith
>    vk2bcu at operamail.com
>
>
> On Sun, Oct 5, 2014, at 10:38, Dave Merrill wrote:
>> One of my 'finds' at Shelby last month was a 1947 edition of "Hints and
>> Kinks for the Radio Amateur."  In the 20 page "Converting War Surplus"
>> section, I found the following:
>>
>> <begin quotation>
>>
>> Curing Chirp in Command Transmitters
>> Alfred Scott Cline, W6LGU
>>
>> My BC-459-A chirped and from what I've heard on the air, most everybody
>> else's does too.  I tried various methods of keying, and extremes of
>> voltage stabilization, but the chirp persisted.
>>
>> Checking with a good v.t.v.m. showed 12.6 volts on the filaments with the
>> key up, but from 18 to 22 volts when the key was closed!  The added
>> voltage
>> was r.f.
>>
>> To remedy this situation, shielded filament wire was substituted in the
>> rig, with by-passes at each end of the wire.  Old microphone cable (with
>> high r.f. losses) seemed best.  A heavy copper strip was run across the
>> chassis and the "cold" ends of the 1625 filaments and the cathodes were
>> connected to it to get a good ground.  This change resulted in chirpless
>> keying for me and has done the same for all the others to whom I have
>> passes this hint.
>>
>> <end of quotation>
>>
>> Okay, I know Command Set keying chirp has been beat to death but perhaps
>> this is still worth discussing.
>>
>> W6LGU does not mention how he is keying the BC-459-A which might be
>> helpful.
>>
>> Even without this information, my suspicion is the unwanted r.f. is
>> coming
>> from unshielded filament leads OUTSIDE the transmitter.
>>
>> Thoughts?
>>
>> 73,
>> Dave
>> N9ZC
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