Time to Check Your Work - Radio Magazine

Time to Check Your Work

We’ve reached the last step in our transmitter rehabilitation project
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We�ve reached the last step in our transmitter rehabilitation project. In our penultimate installment (April 2016 issue), I discussed physical issues with the RF output stage. For the purposes of this article I�m assuming you�ve completed all the steps mentioned previously in the series.

As I�ve said in the previous five installments: Working with transmitters that use any kind of high voltage (whether AC or DC) is inherently dangerous. Never work on transmitters when alone or tired. Take every precaution, and then take every precaution again.

THE MOST COMMON PROBLEM WITH ANY TUBE AMPLIFIER

The half-wave cavity lacks the HV blocking capacitor wrapped around the tube anode. However, the high voltage still needs to be kept isolated. Keep all surfaces that isolate DC clean. You can waste a lot of time analyzing problems that are actually caused by a bad tube. It�s imperative before going on that you know the tube you have for the transmitter is serviceable.

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The best way to tell is to install it in another transmitter. If it works normally, it�s ready to go back to the transmitter being fixed.

If it doesn�t, stop the project until you can get another tube. (You probably know this, but for those who don�t � a good source of large rebuilt vacuum tubes is Woodland, Calif. -based Econco, and they�ll take the �dud� off of your hands as well.) The tube you use for testing can have low filament emission � that�s fine for testing.

THE SECOND MOST COMMON PROBLEM

The vacuum-tube amplifier operates at a very high voltage, and low current (the ratio of which is, of course, the plate resistance). The output network is used to match this high impedance to the low impedance of the load. By necessity, the plate is connected to the high-voltage DC, but a quarter-wave matching network presents a short for DC. Obviously a capacitor is needed, then, to block the DC and pass RF. This capacitor couples the RF by being physically wrapped around the anode of the tube, and the entire plate supply appears across it.

A common problem is a failure of this capacitor. Before you even energize the plate supply in this transmitter, inspect this capacitor for obvious problems. Actually, if a �new� capacitor is available, it wouldn�t be a waste of time to simply replace the old one.

Inspect all high-voltage standoff insulators before energizing the plate supply. One-half-wave cavities lack the blocking capacitor, but the network is still energized by the plate supply. Clean any part of the output circuit that is used to isolate DC.

LOAD AND DIRECTIONAL COUPLER NEEDED FOR TESTING

A Bird �drop� slug is an ideal way to sample RF after the low-pass filter. Use caution when connecting to your spectrum analyzer � typically 20 to 30 dB of attenuation is called for. I hope you aren�t planning on using an antenna as the load for testing purposes. You�d have to do testing in the middle of the night then, correct? That would violate the cardinal rule of transmitter work: Avoid it when you are tired.

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If you don�t have a dummy load, stop the project until you can obtain one; and when you do, make sure it�s reasonably close to 50 ohms. Sometimes the load resistors can be cracked (and open) causing the load presented to be higher than 50 � which could complicate your amplifier testing.

When making a temporary connection from the output of the transmitter to the dummy load, it�s best to insert a directional coupler in-line. It�s not absolutely necessary, especially if you have one going to your main antenna (because at some point you�ll be connecting this transmitter to it).

However, you may not know how well the meters on the transmitter are working at this point in the test. If the plate current meter, or the output meter are giving you bad information, then again, you might be wasting time analyzing a problem with the amp that doesn�t really exist.

TIME TO TEST YOUR WORK

We�ve discussed many steps in this series, and now we�re about to put everything together. The tube is in its socket and ready to be energized; the amplifier output is seeing a good load. You have an exciter that provides enough RF to drive the IPA so that it drives the grid of the final.

At this point I always have the power control turned down, but I caution you: If you turn it down too far, sometimes you�ll think that nothing is happening at all. Some power controllers virtually turn the amplification off and beyond. You can energize the plate circuit, though, just to make sure it comes up and stays on.

So here we go:

DOUBLE CHECK that the final amplifier compartment is closed, and that all interlocks are closed.

PLATE ON: The power supply should come up, and high-voltage should be indicated. At this point you may not have much plate current, or any output at all.

GRID DRIVE. Check the grid current. Are you getting the appropriate amount? Check the input tuning and increase the drive if necessary.

POWER CONTROL. Turn the power controller up some. You should see some plate current and screen current (if using a tetrode) indicated.

POWER OUT. Hopefully your directional coupler is indicating some power. Use plate tuning (dip plate current) and then the loading control to find the peak in output power. At this point, the goal is maximum smoke (maximize the power output).

RAISE POWER. As you turn up the power controller, tuning steps need to be done again. This is an iterative process. Continue until reaching your output power goal.

If you�ve reached this point, and the transmitter is humming along � well, congratulate yourself, because you�ve succeeded.

SPURIOUS AND HARMONIC
CONTENT AND NEUTRALIZATION

Ideally, for this test you�ll have access to a spectrum analyzer because this is the quickest way to look for �junk� in the output of the amplifier.

If you have inserted a directional coupler in the line, hopefully you can insert a �drop� sample slug in it to give you a direct sample of the RF after the low-pass filter. (Use caution driving the S.A. input, though; you�ll still have a substantial signal coming out of this drop sample.)

Neutralization is the process by which the amount of coupling from output to input of the amplifier is minimized. If there is too much coupling from output to input, the amplifier may become unstable. Common symptoms would be oscillation (in other words, there is still output, even without exciter drive) or perhaps the presence of spurious signals (varying in frequency and level) as seen in the output of the amplifier. Consult the transmitter manual for the specific neutralization procedure.

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