[ARC5] Chirp in Command Set transmitters.

[David Stinson]

Jim Mandaville wrote:

> 2.  I don't think I've (knowingly) ever heard an ARC-5 CW note 
> What do they sound like, say on 80 m?
> I think I remember reading somewhere that they don't always chirp...

I have a first W.E. run contract 1570-NY-41 BC-459,
operation as originally designed, with no chirp.
It is also the most frequency stable of my 459s.
Some later sets have a slight chirp.

Chirp in the ATA/274N/ARC-5 transmitters can have several causes:

1.  If keying with the selector relay (the one on the side wall
    and the best way to do it), be sure the oscillator set of 
    contacts makes before the PA set of contacts. 
    You can gently bend them to accomplish this.

2.  Leakage in the 3 x .05 bathtub cap on the back skirt.
    This will pull the oscillator, change bias on the PA, etc.
    The leakage in the caps is not constant.  
    It is proportional to time, the voltage across the cap 
    and the increase in internal temperature as heat is dissipated 
    in the leakage.

3.  Oxidation on the loading coil/link rotor contacts.
    Heavy oxidation between the contacts on these components 
    heats up when you key the rig.  This causes a slight change
    in antenna loading over the fraction of a second of the keying 
    pulse.  Any change in antenna loading in an MOPA rig will 
    show up as an impedance change reflected all the way back 
    to the oscillator, causing chirp.  
    I'll post a procedure to clean this up.
    It's an afternoon's work, but the results are worth the effort.

4.  Grid emission or other damage in the 1625s.  
    Some hams ran these radios with plate and screen 
    voltages in excess of specs, then mistuned them 
    by trying to run them directly into 50-ohm antennas,
    which they are *not* designed to feed 
    (more info on this if interested).  
    If you have any doubt, put NOS 1625s in the rig.

5.  Poor power supply design.  
    The supply needs to be able to deliver B+ voltage
    at 300 mills without going "wobbly."  Pick a voltage
    somewhere between 400 and 550 volts, then build 
    a supply that can "keep it up."
    Same with the filament voltage;  it should not degrade
    with keying, assuming you are using the relays.

    The voltage fed to the 1626 oscillator should be
    no more then 200 volts.  Yes, I know the AN/ARC-5 
    manual say the plate will test about 230 volts
    in the original setup.  Nevertheless,
    higher values can cause drift.  Lower values reduce 
    oscillator output.  Since the bias on the 1625s comes 
    from the oscillator output, it needs to be "right,"
    or at least proportional to the high B+.  
    If you run 400 volts on the final, 
    180 volts on the oscillator will probably work OK.

    The original design uses a voltage divider network 
    to derive screen and oscillator voltages from the 550 volt B+.  
    This is still the best way to do it.  
    The oscillator stage is very stable 
    if the bathtub caps are replaced and the voltage is right.
    A properly operating 1626 oscillator with 200 volts
    supplied will draw around 20 milliamps.
    An exception is the AN/ARC-5 T-18 2.1-3MC transmitter.
    Its oscillator will draw around 18 mils.

    If you're going to run AM, regulate the 1625 screens
    down to 150 volts with a VR-150 bypassed with .5 ufd.
    Connect it between the screen voltage lead and ground.
    The screen divider resistor will also current limit for it.
    If you screen modulate, you must tune the transmitter to 
    full output on CW before switching in this regulator.

I hope this is helpful.

73 DE Dave Stinson AB5S