Hallicrafters HT-20

[kc2dhu]

Greetings all. New to amfone here.  I've recently acquired a Hallicrafters H-20, I'm told it has been dormant for over 20 years. Replaced all electrolytics. Fixed some funky looking soldering and repaired the milliamp meter among the usual looking over and cleaning.

I am not an electrical RF wizard by any means, just a mechanic that loves boat anchor transmitters.
The problem I am having at this stage is the rig loads up 100W+ however I am not seeing that pronounce dip that I usually see in the plate current. There is a dip but not a sharp one, maybe this is normal for this type of transmitter.  However I am getting arcing at the Antenna Loading Coarse switch. Looks to be arcing to the center shaft. I have the front panel off the unit at this point. The switch has been cleaned but maybe it is faulty..  Before I go ahead and pull the switch I am looking for any suggestions from anyone with experience with these transmitters. Maybe something is loading the switch down?  or it is just bad.

If the switch is bad any idea where to find a replacement?

Thanks for listening.  Jim KC2DHU

[Opcom]

A very stupid question - If the unit has a separate driver bandswitch and final bandswitch, are they properly on the same band? I can only say that the same thing I am implying happened to me on a 4-1000 rig and the arcing was something to behold.

[kc2dhu]

A very stupid question - If the unit has a separate driver bandswitch and final bandswitch, are they properly on the same band? I can only say that the same thing I am implying happened to me on a 4-1000 rig and the arcing was something to behold.

Yes double checked that. Oscillator and Buffer in the correct range.  The switch is arcing in position 1  .  Basically it is off not adding any capacitance to the tank circuit. I'm puzzled. But i'm a bit green at this as well.

Thanks Jim.

[wa2pjp]

Hi Jim,
Congrads on getting the HT-20, they are a rare rig and only produced for a little over a year. Their big claim to fame was in the Clipperton Island DXpedition where Hallicrafters sponsored the equipment which was the SX-88, HT-20 and HT-18.

If you are getting arcing at the loading switch, did you check to see that you are using the correct antenna output? There are two, one for the TVI filter and good for only about 50 ohms and the other is bypassing  the filter for 50 to 600 ohms. It is selectable by a jumper inside the front of the chassis. Also, I assume that you are using the tuning chart to at least get the rig in the ballpark for tuning and setting the output control to low or off until close tuning is done. You should see a good dip on the final if the antenna is properly connected. If that jumper under the chassis is lose, it would cause arcing at that switch as well. Also the course loading scheme is the same as some of the Johnson and Collins rigs, check capacitors c-125 through C-130 and I have seen these go bad. 

I was lucky enough to also come by one just before Deerfield and there bis a picture of me with the HT-20 in the back of my truck. I am in the middle of restoring the SX-88 and once I have that completed, I'm going to blow apart the HT-20 for restoration.

[wa2pjp]

Jim,

Something I forgot to mention, check the mechanical drive on the PA tuning control and that C-122 and L-109 are tracking properly. This would not really cause an arc at the loading control, but if the cap and inductor are not tracking, you may not be able to get a dip in the final. In my HT-20, the linkage from the gear drive to the cap was lose and slipping.

Good Luck,

Joe  WA2PJP

[w3jn]

The plate current dip really isn't that sharp, but it is pronounced.  Be advised, NEVER change the loading switch with the rig keyed (true of any xmitter).  That'a s good way to ruin loading switch contacts.  Check the output filter to see if it's set properly for 50-75 ohm output, rather than high impedance.

Mine is virtually like new and possibly wasn't used at all.  I keyed it up on 10 meters and clouds of smoke started pouring out the top.  Both of the parasitic suppressor resistors had cooked; replaced them and the same thing happened again.  I finally copied the Collins 32V parasitic design and had no further issues.

Another gotchya is the fiber plate tuning shaft.  Busted mine using the crank a bit enthusiastically.  I think it's fiber so it doesn't pass RF thru the compartment, but TVI is no longer a concern so it probably should be replaced by a metal shaft (ensuring that it doesn't touch its mate thru the insulated coupler).

Beyond that, it's as bulletproof as any transmitter I can think of, and it looks great as well!

[kc2dhu]

Thanks for the replies.  I believe someone was hot switching the coarse loading control years ago when this rig went dark. I have checked all mentioned above from replies.

I have removed the coarse loading switch and it has one contact burned off the arcing I was seeing was when loaded and switched to high. It arced to the tuning shaft. looked like the phenolic material probably was damaged years ago and also the two nuts on the screws that hold the entire switch together wer just about falling off.

I scoured the inter-web for hours for a replacement and found one from a Johnson viking that looks like I may be able to substitute. Anyhow waiting for it to arrive to inspect its condition and see if I can make it work.

Thanks again for the replies and I will keep you posted.
Jim KC2DHU

[w3jn]

Post a pic of the damaged wafer, chances are someone here has one in a junkbox that they'll donate to the cause.

[kc2dhu]

Update.  Received the Johnson Viking wafer switch today. Cleaned it all up with De Oxit, took it apart and turned the wafer 180 degrees and it made a perfect fit with a little doing. It even indexes properly.
Now the old gal loads up 98W. I am happy at this point.

Now on to modulation, I have none. Swapped the 807's with spares I had here, still nada.Modulation Cathode is reading 105ma. 
I will start checking caps, resistors etc.  If anybody has some hints or a little pearls of wisdom they would like to share, I sure will listen.

Thanks Jim KC2DHU

[Steve - K4HX]

Start at the mic end and trace for audio.

The resting current for the 807s should be 60 to 80 mA. So you need to adjust tap on the HV bleeder resistor.

[kc2dhu]

Steve, thanks for the reply.  That is exactly what I did and found C145 shorted to ground.  Replaced that and had audio.  I am using a D104 so I swapped out R1 4 4.7M to a 10M as for the 2 capacitors I did not have on hand 0.47UF I did have 1.0UF 630V and changed them out as well. So far it is sounding good.

Thanks Jim KC2DHU

[KB2WIG]

It sounded ohtay this afternoon. I didn't have the HQ145 on, and so your audio , was a  bit restricted on the ricebox, as I received it.


KLC

[kc2dhu]

It sounded ohtay this afternoon. I didn't have the HQ145 on, and so your audio , was a  bit restricted on the ricebox, as I received it.


KLC

Thanks Kevin. I talked to Bob KBW he had a good copy on me and said the same about the audio. I ordered the correct caps for the pre amp stage and go from there.

Thanks for the report.  Jim KC2DHU

[w3jn]

Outstanding, congrats on joining the much-envied and exclusive HT-20 club!