[ARC5] Chirpy BC-459

[Kenneth G. Gordon]

First of all, your power supply should be able to provide 250 mA of HV (600V). If it will not 
proivde that current, the voltage drop at the transmitter under key-down conditions will 
cause chirp. Measure the voltage-drop at the HV input to the transmitter when you 
key-down. It should not be more than a few volts at most. 5 V drop is good. Anything more 
than that is not.

Secondly, the voltage to the oscillator should be as close to 200 VDC as possible, as this is 
the "sweet spot" for the 1626 triode oscillator tube. If you are not using regulated voltage for 
that load, you should be. A pair of VR-105s will give you 210 VDC which is good, or a 
VR-105 and a VR-90 will give you 195 VDC which is also good.

And oscillator voltage should not be gotten from a voltage divider on the main HV, but 
should be from a separate supply, if possible.

If you must use a voltage divider from the plate supply, this is another reason your plate 
supply must be very "stiff" and not sag under load.

Thirdly, bad chirp can be traced to a weak oscillator tube. If you have several 1626s, 
swapping them around, then trying the keying might show that one tube is far better than 
another or others.

Heating of not-quite-good connections to the loading coil can also make the load on the HV 
supply vary enough to make chirp worse.

You might also wish to use some DeOxit on the oscillator tube pins and socket. If the 
connections there are the least bit "iffy" chirp will result. In fact, DeOxit the pins on the finals 
too.

If your finals are oscillating, this can feed-back into the oscillator and cause chirp.

One of the BIGGEST causes of chirp in your setup often comes from RF from the 
transmitter getting back into the oscillator via the wiring between the power supply and the 
transmitter. All power leads should be bypassed to ground at both ends with something like 
0.01 MFD 1 KV disc ceramic caps as close to the entrance and exit of the wires as possible, 
and the power supply itself should be well-grounded. So should the transmitter for that 
matter.

Do you hear bad chirp when you kill the voltage to the finals and only key the oscillator? 

If you don't, then either the load on the final is affecting the oscillator's voltage, OR RF from 
the final amp is getting back into the oscillator, and if this latter, then that RF is coming in 
via the interconnect cable between the power supply and transmitter, since the internal 
shielding of the transmitter itself (assuming you have ALL the covers on) will prevent this.

Also, sometimes if you are listening on a nearby receiver to your own keying, overload of 
the receiver's input can make it sound like your transmitter is chirping, when your receiver is 
actually doing the chirping. You need someone at least a mile or so away to listen to your 
rig and tell you what it sounds like, or even play it to you over the telephone.

In your situation, the first thing I would do is to remove all voltages (except filament) from 
your final amp tubes, and key JUST the oscillator to see if the VFO is chirping. Then, if by 
adding the finals, your chirp gets worse, you then need to look at some of the information I 
mention above.

It is also very possible that RF is getting into your filament power supply. Make sure its input 
and output leads are bypassed to ground, then ground the case.

The signal quality output of the BC-459 transmitter should be perfectly good by modern 
standards. If it isn't, then something is wrong.

You may also wish to review what Dave Stinson told us several years ago about this issue.

Go here:

http://www.w7ekb.com/glowbugs/Military/arc5pages.htm

and read the first two PDFs shown there.

Ken W7EKB

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