SX-100 stability problem

[W8UJX]

Just bought an old SX-100 MARK 2.  It received two BCB band stations when I got it.  RF and both IF's were totally screwed up.  Did a complete alignment and this RX is really hot. 

Problem:
Mechanical stability is horrible.  I can zero beat WWV on 20 MHz and just barely touch the back of the main tuning cap frame (not the plates) with one finger and the RX will move completely off WWV's frequency.

The slightest movement of the front panel (RX out of cabinet) will cause the same problem.

I'm thinking of removing and cleaning the main tuning and bandspread cap but I am not sure this will fix the problem.

Any ideas?
Jerry, W8UJX

[kg8lb]

 Cleaning the sliding contact and clean/lube the bearings should help a lot. Still, it sounds like there may be other "issues" to deal with. Make certain all electrical/mechanical connections are solid, especially the frame of the air variable.  Should be a reasonably stable RX once sorted out.
The SX-100 is a wonderful receiver, a real bargain compared to it's boutique counterpart, the SX-115.

[Pete, WA2CWA]

With the receiver out of the case, I would loosen all the nuts and/or screws (if there are any) holding the front panel in place. See if the mechanical instability stops. If it does, then tighten them one at a time and see if the instability reappears. This would provide a clue as to where the mechanical stress point(s) are being generated.

[w3jn]

The SX-100 was never noted for its mechanical stability.  Every one I've had that's sat for a while has had problems similar to yours and it's pretty easy to fix.  I can't remember the exact construction of these caps, I've gone thru so many different receivers, so these are general tips.

First, inspect the brass wiper on the oscillator sections of the variable capacitors.  There's the general coverage cap and the bandspread cap.  A bad contact at either or both will cause this.  Could be there's a cold solder joint to the cap, or the brass wiper could be dirty/loose.   Inspect to make sure it's soldered securely to the frame of the cap, if it's supposed to be.

Next, wet a piece of paper with some DeOxit.  Don't try and use anything else, I've never had luck with anything but DeOxit on this.  Slip the paper between the brass finger and the variable cap rotor, and crank it thru a couple times.  Change position on the paper a few times so there's a clean spot burnishing the contact.  Do this for both the gen coverage and bandspread caps.  If you can't slip a piece of paper in there, wet a Qtip with some DeOxit and hold against the rotor/finger junction as you crank the cap thru several times.

Also the end bearings could be loose/dirty.  Use some teflon grease (available at Radio Shack) to lightly lube the end bearings.  If it has an end bearing loading adjust, you can sometimes take that apart and clean the ball bearings inside.  BE CAREFUL you don't lose any!  The proper way to set the end bearing loading is so that there's equal space between the rotor vanes and the stator vanes.  I'd advise you though not to mess with the end bearing as too much can go wrong in the process.

Good luck and let us know how it turns out.

[KM1H]

While the SX-100 is quite sensitive its mechanical stability was poor right out of the box. I almost bought one but the demo at Harrison Radio in NYC put a fast end to that. The chassis actually flexes, it is that thin.
And yes, I have a SX-100 MK2A, it just doent get used much. Its another example of great Hallicrafters looks hurt by design by the accountants.

As for JN's paper and DeOxitit suggestion I use automotive feeler gauges to deposit the stuff where it can do its work after a few rotations of the rotor.

The SX-115 is one of Hallicrafters best performers; mine is used every week for vintage SSB chats. No drift, no chassis flex and looks good next to the HT-32B behemoth.

[WQ9E]

You have gotten a lot of great suggestions thus far and I will add a little bit to JN's suggestion.   Set the dials so both the main and band spread capacitors are fully meshed.  This will prevent the rotor from shifting enough for the front bearings to come loose.  Then remove, clean, and lubricate the thrust bearing in the end of each of these capacitors.  I use WD-40 and a cotton tip to clean out the old lube and synthetic bearing grease for the replacement lube.

Then tighten the thrust bearing holder until the rotor plates are exactly centered between the stator plates.  I guess you could use a feeler gauge but I do it by eye looking straight down from above to eliminate parallax error.  Once it is centered tighten the lock nut while holding the thrust bearing holder setting with a screwdriver.  This procedure cured the mechanical instability problem with my SX-101A.

I do this with almost every older receiver that I repair and I have yet to have the front bearings come loose in any of them.  Just don't move the capacitor while you have the thrust bearing loose.  This clean/lubricate operation often provides a marked improvement in tuning feel.  The thrust bearing is generally a standard size so if you find one that is worn/flat spotted from use and dried lube you can easily replace it at any bearing supply house.  With some receivers, you will find the thrust bearing screw cup difficult to access with a regular screwdriver.  In this case, use one of the tips from those multi-tip screwdrivers inside a small box wrench to gain access.

The SX-101 series have higher mechanical and thermal stability than the SX-96/100 but from your description your SX-100 has a problem.  I used an SX-100 with my Ranger/Desk KW for several months after I decided the HRO-50 selectivity wasn't sufficient for my use as a net control station for a regional AM net.  Although most operation was on 80 and 40 I did operate the station on 20 meter CW and I didn't have any problems even with the narrowest IF bandwidth setting.

[W8UJX]

Thanks for all the suggestions guys.  I removed the front panel from the RX and the problem is still there.

Since I can barely touch the back of the main tuning cap to cause the problem,
this leads me to believe the mounting lugs on the cap frame are not making good contact with the chassis.

I am going to remove the mounting screws and slide a piece of sand paper under each mounting lug on the main and bandspread cap to clean the chassis and bottom of the mounting lug.  Then reinstall and tighten all the cap mounting screws.

If this does not work I will look at cleaning the cap bearings with DeoxIT gold. 

Jerry

[kg8lb]

Thanks for all the suggestions guys.  I removed the front panel from the RX and the problem is still there.

Since I can barely touch the back of the main tuning cap to cause the problem,
this leads me to believe the mounting lugs on the cap frame are not making good contact with the chassis.

I am going to remove the mounting screws and slide a piece of sand paper under each mounting lug on the main and bandspread cap to clean the chassis and bottom of the mounting lug.  Then reinstall and tighten all the cap mounting screws.

If this does not work I will look at cleaning the cap bearings with DeoxIT gold.  

  
Jerry

  You are probably on the right track. The mechanical end of servicing some of these old boatanchors can be just as important as the electrical end.
A fine receiver for AM work . Not as "pretty" as it's overpriced "boutique" cousin (I have owned 4 SX-115s now) but a real nice radio.

BTW, Jerry. Did you pick the SX-100 up in Westlake OH last Saturday ? I saw one for sale there.

[kt4ae]

My limited experience suggests that in the day, these receivers weren't hugely mechanically unstable.  We're seeing them fifty-odd years after they were made and things wear and loosen up.  I had an SX-117 that was wore slap out and mis-aligned which wasn't much better when I got it.

I have a Mark 1A I got through the generosity of one of our number which appears to have been used very little.  It has all but one original Halli tubes and is pretty much unmarked. 

After tightening all the screws and nuts, cleaning the tube pins and making sure the tubes and shields are settled, and cleaning the controls, switches and wipers you have to hit the table pretty hard to make it jump.  Yes, slapping the receiver makes it jump, but I'm a lot more likely to bump the table.  If it hurts, don't do it.

Normal activities near the SX-100 don't bother it.  I can operate switches on the nearby Ranger, take notes, operate a key and even change the Selectivity on the receiver without any problem.   

Pay particular attention to the band switch.  It has to be clean and even then it helps to work it some to settle it down when changing bands.   Before the last time I cleaned the band switch, even moving the volume control would change a CW note.

I don't think touching the main tuning cap is a fair test.  Even waving your hand in that area will change the frequency.

A post years ago in Boatanchors@theporch mentioned a very stable SX-100 which had a #10 or #12 wire strut soldered to that shield waving in the wind between two of the main tuning cap sections.  He said that the receiver had other mods and he wasn't sure that the struts alone did the job.

Harry, KT4AE

[KM1H]

Were all 100's built the same?  With several revisions over its life I wonder if the flimsy chassis metal gauge was a constant.