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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sat 02, 2017 11:02 pm 
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Location: Portland, TN, USA
Thanks Rodger.

Before moving on to the RF alignment, permit me to drop back and add some documentation of the crystal filter cleaning adventure for the possible edification and interest of those who haven’t been there yet.

For those unfamiliar with the SX-42, here’s what the 455khz crystal (xtal) filter looks like nestled onto the Selectivity control switch SW-2AA a couple inches behind the front panel:

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Here it is after removal for cleaning: (as always, take pictures or good notes as to orientation and connections to save later headaches)

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As you can see, it’s held together with tiny screws and nuts.
Opening mine up, here’s the sad picture I saw – the ceramic gasket was broken into 5 pieces:

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This one shows the crystal itself alongside the gasket pieces:

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I next spent waaaay too much time attempting to fabricate a new gasket out of some hard, but still slightly flexible, plastic from a chewing gum case. I’ll show the result here to demonstrate why I next elected to attempt super-glue repairs of the broken gasket:

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Unfortunately, I failed to take a picture of the repaired gasket. It wasn’t a pretty sight, but I thought it had a better chance of working than my creation.

After a final careful cleaning with 91 proof ISO and keeping my fat fingers off of things, I very gingerly reassembled the filter using the repaired gasket.

After letting it age for several days and finding the resonant frequency stable, the filter was reinstalled and appears to have survived my ministrations.

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Want Hallicrafters S-76 with good cosmetics; Can fix electronics issues.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sat 02, 2017 11:29 pm 
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Chuck,

Almost every one of these I have taken apart had cracks in the gasket, they must have been designed by the father of the engineer who developed the cylinder heads for the GM 6.2/6.5 diesels :)

But your gasket was definitely cracked worse than most. I have been putting these together with super glue but since the filter seems to go for years with a gasket that could have easily have been cracked for decades I think the most important function served by it is a non-yielding spacer rather than a seal so pretty and sealing integrity probably isn't that critical. I am glad your repair worked out.

For those restoring vintage receivers with these single element filters keep an eye out for FT-241 crystals which are in the category of ignored and often thrown away items at hamfests and estates. Several of these are close enough to 455 Khz. to make a good replacement for any receiver using a single element 455 Khz. IF filter and realistically with most receivers the IF can easily be realigned to anywhere in the 450-460 Khz. range at minimum to fit a different crystal without any noticeable calibration or tracking issues. Here is a good web link about the FT-241 crystals that explains the frequency/channel markings: http://people.csail.mit.edu/sw/html/NS1W/ft-241.html

Another possible source is some of the early SSB CW transceivers used 455 Khz. range crystals for carrier generation/BFO and some of these same sets are a source of 455 khz. filters. You will also find these crystals in some communications receivers although usually one good enough to use a crystal controlled BFO isn't a receiver you want to junk unless it is down to a parts carcass already.

Although a 455 Khz. ceramic resonator can sometimes be substituted it has a much lower Q than a good crystal filter so circuit performance will drop and I would use one only as a very temporary measure.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sat 02, 2017 11:50 pm 
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Location: Costa Mesa, California
Chuck, let me know if you need a good gasket. I have one from a holder where the crystal was not working and all attempts to get it working have failed. I did eventually find a replacement holder and crystal. As Rodger stated, a 455 KHz resonator is small enough to work and I used one while waiting for a new crystal. The resonators are dirt cheap, so buy a quantity--not that you will ever use more than one or two, but because they vary widely in their Q. I went through a batch of ten and and found one with a much better Q than the others-which was the one I used.

Norm

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Mon 04, 2017 1:18 pm 
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Location: Portland, TN, USA
Thanks for the kind offer, Norm.

However, so far so good with the original, not very pretty repairs I made to this one. It settled back down to very close to its original frequency, so we'll see how it does.

Today I hope to tackle the issue of insufficient HFO trimmer range by playing with the bandspread cap baseline setting as suggested by you and Rodger. It will likely be slow going, as their bi-directional bandspread scheme is interesting to say the least.

I'm assuming that the "Set" or "Zero" point is with the bandspread cap rotor completely unmeshed. The tweaking will start from there.

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Want Hallicrafters S-76 with good cosmetics; Can fix electronics issues.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sat 16, 2017 2:11 am 
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Between chasing my tail and chasing rabbits, I've had a frustrating couple of weeks with this project.

After moving the bandspread dial about 13 log-scale points CCW, I was able to align and calibrate bands 1 & 2 without too much trouble.

The chasing my tail involved attempting to align band 3. Somehow, I set up to align the HFO for low-side injection and could not get things to adjust properly.

After several hours over several days of wrestling with it and trying several values of capacitance in parallel with C43, I was about ready to call for help again. Then one night it dawned on me that I needed to be aligning for high-side injection instead of low-side. :roll:

It still took a couple more sessions of experimentation to get the HFO to behave, because C43 still ended up needing about 3pf in parallel to bring the HFO frequency down to calibration. At the moment I'm having to use two 5pf cap's in series until I can scrounge up a single cap.

The whole process was drawn out, as usual for me, by my stopping to make up some sniffer probes for my frequency counter so I could look at the HFO frequency directly. The DX-400 digital receiver is great for monitoring the HFO when you're sitting still, but it's much handier to watch the frequency change real-time when you're having to experiment with changing values like I was.

I finally had another thought come to me, that my MFJ-259B antenna analyzer has a frequency counter built in. By the time I remembered this, I'd already built enough sniffer probes to equip both counters which really made the operation almost idiot-proof. And, if you haven't already guessed, when I'm working with me, that's needed.

Next up, band 4 RF alignment. Remember Chuck: LOW-Side injection for band 4.

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Want Hallicrafters S-76 with good cosmetics; Can fix electronics issues.


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