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RF Amplifiers Have Lethal HIGH VOLTAGE Inside

I acquired a Henry 2K in pristine condition, however when first powered up I could not get an Ip (Plate Current) dip. Everything appeared to be working, HV was good, blower fan was strong, both 3-400Z were lit and appeared healthy. However when keying the amplifier with 30 watts drive, Ip was about 300ma and there was only about 200 watts RF output, no Ip dip.

Troubleshooting:

 

Checking the following components: HV RFC, TUNE air-variable, door knob capacitors, tank coil and 3-400Z tubes, everything checked good. The only thing left was the band selector switches. Visually looking over the band selector switches, they all looked good, no burn spot or bent contacts.

 

Jim N6MJD suggested that I disconnect the AC power from the amplifier, then connect my MFJ-259B Antenna Analyzer to the amplifier's RF output SO-239 connector. I can then either leave the 3-400Z tubes installed, or remove the tubes and use a 3.3Kohm resistor clip-lead from Plate connections to chassis ground (see picture below). This is required to provide enough capacitive resistance to the circuit.

Mechanically place T/R RY3 relay in transmit position and place the band selector to desired band. Tune the MJF-259B to the Mid Band Freq output of the desired band and begin to try and tune the output tank Circuits (Plate Tune and Load on the AMP) for proper SWR 50 Ohm SWR 1.1 (resonance). You should have the same results like when trying to acquire Ip dip when the amplifier was turned on with drive. The MFJ-259B is a much safer way to troubleshoot the issue. 

 

Issue Identified:

 

What I found, the 5 wiper contacts all needed to make good connection all the time. If pin 1 is selecting the desired Coil Tap, then Contacts 2 thru 4 need to short the remaining coils as it will affect tuning if they do not make a good connection.

 

All four wafer switches MUST track together, so when 20 meters is selected, all four wafer switches are in the 20 meter position. After further investigation of the band wafer switches, the band selector switch S3/S4 was not registered correctly, it was off by one contact point. When 10 meters was selected for example, the lower S4 wiper was correct and the upper S3 wiper was sitting on 15 Meters. Also, some of the wafer wiper contacts were not making contact on the unused coils causing tuning issues as well. 

 

Once the switches were aligned, using the MFJ-259B was a really nice way to test Band Switch s3/s4 and it was great to see the Bands Tune. This again is testing the output tuning area s3/s4. See the below picture of the big coils and the bottom side is s4.

 

 

The band selector is composed of four separate wafer switches, two for RF input and two for the RF output. The two wafer switches for the RF input select both sides (in/out) of the tuned input coils. The two wafer switches for the RF output select the two different high and low band RF tank coils. All four of the wafer switches are ganged/connect-together using pulleys, cables, drive shafts and a 90 deg gear assembly. See the following picture.

 

 

Band Switch Alignment Procedure:

 

Place the band switch into the 10 meter position. Using a Bristol wrench, loosen the set screws for the wafer switch shaft. Rotate and align the wafer contacts by hand so all four wafers are selecting the correct 10 meter circuit, so when the 10 meter input coil is selected, the 10 meter RF output tank coil is selected. Tighten the Bristol set screws, then rotate the band switch and check each band for alignment. Repeat the same alignment procedure for other bands if necessary until all bands track and are aligned correctly. Make sure there are no wafer contacts in mid-position between contacts when rotating the band selector switch. Be sure to also cleaned and re-tensioned all the wafer contacts if required. Once the wafer contacts are all aligned, use the MFJ-259 to test and validate the circuit before applying HV and RF power.

 

Conclusion:

After the band switches are correctly aligned together, the amplifier will now being working great with full 1000+ watts RF output on all bands. There will be a very smooth and pronounced Ip dip, where before there was no Ip dip.

 

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Henry 2DK-5 and 2K-Classis

 

Henry used this type of pulleys, gear chains and gears assemblies for band switching on many of their amplifiers, not just the 2K. If you have a Henry amplifier where you are experiencing similar issues, where Ip does not dip or there is little to no RF output, check all the band wafer switches and make sure they are all aligned together for the band you are operating. There is a high probability one or more of the Bristol set screws have come loose the the drive train is no longer true.

 

2DK-5 and 2K-Classic The band selector switch is also connected to a camshaft which adds/removes door-knob capacitors in/out of the RF tank output circuit using three insulated cams on a metal shaft. The Bristol set screw for the camshaft has been known to come loose, resulting in the capacitors not being properly sequenced in the circuit. If the amplifier is not tuned correctly, or the camshaft has slipped alignment, there will be high RF current and heat generated across the switch contacts. This high heat will cause the insulated material for the cams touching the switch contacts to soften and melt. Over time the cam will shorten in length until the cam no longer pushes the contact(s) fully closed.

2DK-5 and 2K-Classic

 

Life is too short for QRP!!

Happy DXing

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