Sx28 help

[Detroit47]

I know someone on this site is the SX28 Guru. I have two of them with low receive. I have done just about everything to them except recapping the IF strip and the local oscitator. This seems to be a rather audacious task. I was wondering if anybody has experience with this. I have seen a few restoration articles on the web, but I am always open to suggestions.
73 N8QPC

[w3jn]

Google is your friend.  Look for Phil's Old Radios site, it has a nice step by step method of doing the RF compartment.  The IF section should be easy, the caps are all out in the open.

[Jim, W5JO]

Lightning may have opened the input link on the antenna coils.  You can check this with an ohm meter by hooking it acorss the antenna terminals.  If you have continuity the problem is electronic, if you don't have continuity then the link is open and you will have to replace that wire.  Meaning you will have to get into that box to do that. 

Brian, WA5AM just finished a restoration on a 28A and he found two of the transformers with problems like that.  I did one years back and two were open.  So when you get in that first RF box, look for transformers with burned wire.  Some of them are hard to see.

[WQ9E]

I had to replace fixed mica caps in some of the IF transformers in both units I have restored.  In my case they were easy to locate because the transformers tuned very broadly without the noticeable peak of a good transformer.

It is also common to have to clean the crystal to get proper operation from the crystal filter.  A lot of the older National manuals have instructions on how to do this but basically disassemble the holder and clean with the purest isopropyl alcohol (lowest water content) you can acquire.  Then dry and reassemble the holder.

You can use your signal generator and scope (or a sensitive RF voltmeter) to check the crystal response before and after to make sure you have it assembled and working properly.  Connect the high side generator lead to one crystal pin and the scope input to the other and connect scope ground to generator ground.  As you tune the generator through the crystal frequency you should see a very pronounced peak. 

This method can be used to check the true frequency of those crystals that are marked with a channel instead of the actual frequency.

The crystals most in need of cleaning are those where a previous owner went wild with a cleaner/lubricant spray.

[KM1H]

Dont expect any halfway decent performance without a total recap and replacing out of tolerance resistors. Then do a complete socket voltage and resistance check.

Then you are ready for the real problems!

When you do the RF section caps also do the silver micas and resistors, its not an area you want to work on twice.  Im about ready to start on another SX-28 for a customer this week, its not bad after the first half dozen or so.

Carl
KM1H

[W1GFH]

It always surprised me that for such a nice looking receiver, Hallicrafters used so many low quality parts. Check out those 60 year old resistors. Thanks to the failure of the waxy caps, many of the associated dropping and biasing resistors will change value over time. "Low receive" is a typical result.