National NC-81X Receiver Tuning Reset Problem

I can finally call the troubles in this radio Solved!
With the fall season arriving here in this part of California I fired up the NC-81X once again only to find loud crashes and pops evident spoiling the reception. As before, the problems appeared to be localized to the 6J7 Mixer stage and the 1st IF can. I got out the spare IF can once again after concluding that the 1st IF can was bad after all (Primary winding). The primary winding never did open up but it certainly is bad internally somehow-probably from corrosion. Rather than install a grid lead with grid cap to the spare IF can I decided to rewire the 1st IF tube socket for a single ended 6SK7 tube. This wasn't too difficult to do although the pin outs are quite different. The installation went well. I ended up putting a pair of new B+ and Plate leads (for the 6SK7 tube) on the xtal filter can as well as one new longer lead on the terminal board of the replacement 1st IF can for the control grid of the 6SK7. Then I got the set all back together for a try out. It went well. The crashes and pops were greatly reduced and the new 1st IF stage with the 6SK7 tube was stable and performing well. There were still some residual popping and crashing noises and this appeared centered around the tube socket for the 6J7 Mixer tube. I considered replacing the socket but I didn't have any new ones (National) and they were riveted in place in this model. I got out my Nu-Trol cleaner/lubricant spray and decided to try cleaning the original socket contacts. I applied some of the Nu-Trol to the socket contacts and also to the pins of the 6J7 tube. I worked the tube up and down in the socket as usual to clean them. Well, it worked! Previously, I had done the same treatment to the grid-gripper for the 6J7 Mixer tube and there had been some improvement also. Now the set was performing well and quietly! No more crashes to date.

It is interesting to note that the cathode of the 6J7 Mixer tube is connected in series with a tap on the Local Oscillator Grid coil. Before cleaning the Mixer tube socket contacts I had noticed some change in the local oscillator frequency as I shifted the Mixer tube around in its socket. It appears that a poor connection at the socket was causing the the shift in frequency that I had noticed some time ago and was the original complaint about the set. I am not seeing a change in the tuning frequency now. When I come back to it and turn it on it is on the same frequency (3870 kHz) as before. So I think it is safe to call it "Solved!" now.

73, Rod  WB6FBF

I made another attempt at checking on the NC-81X intermittent that I've been describing including taking another look into the 1st IF can. Something about the 6J7 Mixer stage and the 1st IF can seemed to be at fault. Looking it over carefully, I couldn't find any fault with the solder joints under the chassis or in the IF can. Likewise with the IF trimmers. They resemble Hammarlund APC style variable capacitors. I didn't see any trash in between the plates that could be causing an intermittent short. I did clean the rotor contacts on the trimmers as in the past I have found that APC style caps sometimes develop poor contact there and can benefit from a cleaning. 

Well, all this investigation didn't really help much in regards to improving the set operation. The intermittent crashing and drop off in sensitivity continued. Finally, I tried a different 6J7 tube in the mixer stage. This simple change did help amazingly. The crashes stopped-at least for the time being. The new tube was a later production RCA 6J7. The old tube was a very early production Arcturus 6J7. I wondered if it really was at fault. I have to admit that a while back I had changed around several of the many 6J7 and 6K7 tubes in the set trying to get stronger tubes in place of the older ones that had been in there for a long time. Maybe this was a bad idea and caused problems. Really, only the 6J7 tube in the BFO stage had gotten very weak over time and was causing problems with BFO operation in the set. A replacement there did help. The others probably did not.

I setup my Hickok 540 tube tester to evaluate the Arcturus 6J7 tube. I couldn't find any intermittent shorts in the tube even with vigorous thumping during testing. Likewise, I couldn't find any intermittent problems during the quality (Gm) test. Then I looked at the pins of the tube for corrosion and found that they were shiny and clean. Next, I looked at the grid cap and it did appear to have a layer of crud on it as I have found sometimes deposited on the metal surfaces in an old radio. This stuff can be a very good insulator and hard to break through. Contact Cleaners won't remove it. I started scarping it off and then finished by polishing the grid cap with one of the eraser pencils I bought a while back. It is nice and shiny now. I replaced the Arcturus tube in the set and gave it another try. It appears to be working well now.

All I can say at this point is that perhaps the Mixer tube grid-gripper (National item) could not make good contact with the 6J7 Arcturus  tube due to the corrosion that was present on its grid cap and thus was intermittent in operation. The Hickok 540 uses a Mueller type of grid-plate clip with a biting contact that probably was better able to cut through the insulating crud on the grid cap while in the testing phase.

Rod,
it has been known for a wire hair or metallic flake to get across an IFT trimmer too.
 I concur with re-soldering all connections in that circuit. I have found apparently good looking conn. to have a rosin pocket inside that deteriorates and the lead moves. Talk about wasted hours in finding those faults!

I want to amend my earlier statement about B+ on the IF transformers trimmers. I rechecked and found that just the trimmer on the 1st IF transformer (the 1560 kHz IF plate trimmer for the mixer stage) appears to be electrically sensitive. So I am thinking I need to recheck starting at the IF can B+ connection all the way back to the mixer plate for a poor connection. I am including another inspection of the IF can in this. Resoldering all the connections inside the if can as well as a look at the connections and contacts of the (2) air variable trimmer capacitors.

I did some more checking on the NC-81X. I improved (2) important chassis ground points that were missing a star lock washer by installing one between their respective lugs and the painted chassis. Just for good measure, I also installed a star washer on a terminal strip that had been formerly used as a ground for one of the old tubular bypass capacitors. I did this last one just in case I ever needed to use that ground again.

Still, the receiver continues to crash and cutout from time to time. I am thinking that it may be a very slight poor connection somewhere. When the output drops out it only takes a very minor electrical disturbance to restore it again. For example: as I have described before, one of the exposed trimmers on top of each IF can has B+ on it. After a crash episode, the set can temporarily be restored just by touching one of these IF trimmers with the metal blade of a screwdriver. I have switched the band change control knob back and forth several times to try to test and/or clean the connections of the band switch but that hasn't been a help.  

I also checked the various resistors under the chassis. These all appeared to be in good shape. Remarkable for 80 year old resistors.

I wanted to install a new pointer I bought recently for the slide rule dial but have decided to put that off while the set continues to act up.
I could use some new dial cord also before I attempt the job.

73, Rod  WB6FBF

You might want to check resistors in cold & hot conditions. Heat gun & freeze. They can go int. too.

The NC-81X is continuing to act up at times making loud popping, crashing and zapping noises from the speaker. The output volume also tends to drop after one of these episodes. I am not sure if the sensitivity is dropping off or what. It does come back later with the next pop. So I can't say that the trouble has been found in it. I have done some slight inspection when this happens and for one the tube filaments stay bright (series string set). Also, the B+ voltage (125VDC) stays steady. I am guessing for now that an RF or IF coil is either shorting out or opening up. I wish it would just quit so I could find it and replace the bad part. 

In the mean time, I have made some improvements to the Hallicrafters SX-99 and have added a new Hallicrafters SX-130 plus the matching Hallicrafters R-50 speaker. The SX-130 has a damaged front panel (extruded aluminum) so I am not sure what to do about that. I removed it initially from the chassis to replace the broken glass dial it came with. Then I found out the upper half of the panel was badly out of shape in the large dial glass area. The radio probably got slammed into a concrete floor at some point in its life. Otherwise it is very clean and doesn't appear that anyone else has been in it before. I do have a search on for a replacement front panel. So currently I am trying it out without one (after putting all the knobs back on). The front panel I can do without for a while but it is hard to tell where you are without a pointer for the main tuning and lack of a bandspread dial for the ham bands.

I remember several items I worked on having ground issues from loose rivets. They can be a killer pita to find too. Often I would just shotgun the grounds and solder all of them, even if they looked good.
Great job solving your problem

Good  troubleshooting. Glad you got it going.

As a follow up, I got busy this morning and decided to replace the first IF can with the lone spare that I had on hand. These large National 1560 kHz IF cans are not very common at all. I haven't been able to get any other spares since I've had the set. 

I had determined last night that the first IF can was most suspect from having thumped each IF can vigorously to see if I could get the set to act up. The other IF cans were quiet during this procedure but the first IF can would make a popping sound each time I thumped it. Not the most scientific way to troubleshoot a set but from that I decided to replace the first IF can and see what happens.

I got the old IF can out and dissembled it and compared it with the spare. The spare IF xfmr was meant to work into the 6C5 detector stage and did not have a grid cap lead out the side of the can as required. I needed a new grid lead for the NC grid cap gripper. The old lead would not reach down far enough into the spare IF can to make the connection as the internals were slightly different. Also a somewhat longer lead would allow the use of a glass 6K7GT tube in that socket. I have wanted to try out a 6K7GT tube in the set but the grid leads were just too short to reach as the 6K7GT tubes are somewhat taller than metal 6K7 tubes. 6K7GTs were specified for use in the IF stage sockets of the war-surplus BC-312 and BC-342 receivers I used to own and operate. I liked the fact that the designers choose them for the set. They did use metal 6K7 tubes in the (2) RF stages. 

After the replacement IF can was prepared I reinstalled it and then got the set ready for a test.

I had to do bit of touch up on the alignment of the new first IF can. At first, I readjusted it for maximum noise and that bought up the background noise level with antenna attached to about normal. Then I got out my solid state GW INSTEK signal generator set to 1560 kHz and retouched it up. I had hooked the generator output lead directly to the grid cap of the 6J7 mixer tube. I forgot to say that after I powered up the set it was fairly quiet-no loud violent crashes evident as it was prone to do before. It has been running for a while now and is doing ok. Hope that got it. Time will tell.

I've set aside the Hallicraters SX-99 for the time being and got the NC-81X going again. To do this I had to disconnect the speaker leads for the SX-99 at the speaker and reconnect the separate speaker leads (with phone tips on the ends) for the NC-81X. The NC-81X requires an output transformer at the speaker which of course I left in there as I only had to disconnect the leads going to it when I needed the speaker for the SX-99. I have another speaker coming tomorrow (Hallicrafters R-50) which will help for trying out these receivers.

Well, on firing up the NC-81X I was met by loud crashes indicative of IF transformer failure. I had heard these before when I had the 1927 Wirt cone speaker hooked up to it for testing. At the time I didn't know if the Wirt was breaking down or the receiver was at fault. Now I know the receiver is having the trouble. I've started to isolate it but haven't found the problem yet. There are (5) IF transformers in the set to check including the Xtal Filter and AVC cans. These crashes can get very loud if the volume is turned up. So right now I am continuing to operate the set until hopefully it breaks down for good so I can begin to isolate the problem stage.

I haven't looked inside the 1560 kHz IF cans yet but I believe they are tuned by air variable caps so silver mica disease is not to blame. Instead I am probably looking at IF transformer coil breakdown from corroded coil windings either going open or shorting. The B+ supply voltage is remaining steady so far. I can check this on top of the IF cans as there is B+ on (1) of each pair of IF can trimmer adjustment caps. I don't know right now if the B+ voltage present on top of the IF cans represents the B+ side or the plate side but I am willing to guess that it is the B+ side. 

I won't be able to check plate voltages and grid returns without turning the set over and removing the bottom plate to get at the connections. I have been thinking that injecting a 1560 kHz signal for testing each stage might help find the trouble if voltage checks don't do the job. I don't have an oscilloscope here to help with isolating it.  I do have a spare IF can and a spare AVC can. I probably have enough parts to rebuild the Xtal filter can if necessary.

P.S. I have now confirmed that the trimmers in the large 1560 kHz IF cans are in fact air variables and the B+ connection does go to the rotor making it possible to measure the B+ voltage at the top of the can. The coil windings are rather small and thin as compared with 455 kHz IF coils. The form is a wooden dowel as I have observed in NC-100 455 kHz IF cans. Not good as wood will absorb and hold moisture over the years. Also the coils have been wax dipped treated-not good either for long term life.

Glad you got it going.
A quick thermal change can break glass especially if it had a nick, crack or scratch. (Think glass cutter).
 They make a special foam tube puller to remove hot tubes with. They work even on the hottest ones I ever pulled. 
You are not a real radio tech until you have left your fingerprints and layer of skin on a hot output or rectifier tube!.

I got back to the National NC-81X receiver project Friday evening. The plan was to try to get it back to normal operation by swapping the oscillator coils out again and a checkup of the existing mixer coil. The mixer trimmer under previous adjustment showed that the plates were touching (shorting) during part of its rotation. I removed the 80 meter (spare) oscillator coil that I had put in last time and reinstalled the original coil assembly. I also removed the mixer coil assembly and inspected it. Some of the plates were a bit out of shape and needed straightening and respacing. I also looked at the pins and they needed some cleaning so I scraped them a little and used the EF eraser pencil to polish them up. I didn't make note of the 80 meter mixer trimmer's position before adjusting it previously so I used the one in the spare tuning unit as a guide to set it up after reinstalling it in the tuning unit. This worked out pretty well in fact. I screwed the bottom plate back on the tuning unit and then the bottom cover of the receiver back on the set and readied it for testing. 

I also wanted to try out a 1927 model Wirt Cone Speaker that I had bought a while back mainly for use with mid to late 1920s battery broadcast receivers. It had been sitting on a shelf for a while here. It is quite a bit larger than the Jensen speaker I have been using with the NC-81X and currently using with the Hallicrafters SX-99. Not wanting to put the full plate current of the 25L6G output tube thru the Wirt speaker I got out an old output transformer and used the primary as a plate choke to the 25L6G and coupled the AC output to the speaker with a 12uF 250V coupling/blocking capacitor I had on hand. I would have used a 2uF cap but didn't have any left. I had got in some 2uF 250VAC tubular caps recently that turned out to be defective and had put all of them in the trash. The seller sent me 12uF 250VDC tubular caps by mistake as a replacement (from China). They are good.

I fired up the set and it started working. The Wirt speaker was working and sounded ok on static. The volume was normal. Previously, I had done the usual click test on the Wirt with a battery and then an Ohm meter test to measure its resistance. The sensitivity of the receiver appeared much better and I was glad for that. I checked the tuning for 4.000 MHz and it came in at about the right spot on the tuning capacitor which by the way, has only (6) widely spaced plates in each section. Next, I set the GW INSTEK signal generator to 3870 kHz (monitored on a GW INSTEK frequency counter) and tuned the receiver for that frequency. So far so good. Next, I spotted 3870 kHz from the Johnson Viking II (my usual procedure) and found the sensitivity (from the RF gain control setting) to be back to normal. I also noticed a bit of intermittent operation and traced that down to the first IF tube. Rocking the tube a bit affected operation including its grid cap. I had replaced some tubes last time hoping to improve operation and of that particular replacement I apparently didn't checked the pins or grid cap. I put in another known good 6K7 tube. Then I scrapped the pins on the pulled tube and the grid cap a bit and put it in my NC-81X tube box. I also checked the grid cap connections on the other tubes. 

While I was doing this I decided to replace the RCA 25L6G tube with a Raytheon 25L6GT tube I had been using prior to the changes. The set hadn't been in operation long but still the 25L6G tube had gotten hot (and the 25Z5 rectifier tube next to it) so I used one of my kitchen towels to help remove it and while doing so the glass tube envelope broke. A 25L6G is a large tube. Like an 80 or 45. This has never happened to me before far as I can remember. The tube did have a crack in the glass above the base from shipping it here. I wondered how long it would last like that. The vacuum was holding up so I wanted to give it a try just for fun. It had had a lot of hours on it though so it was not a big loss.

I had to wait until 10:30 PM Pacific time for a group of 3870 kHz regulars to come on for a real audio test. Conditions were only fair but of the (2) out of (3) stations that I could hear the results were good. The Wirt speaker was doing very well on shortwave. Another of these Wirt speakers that I have seen came with an adjustment control that projected out of the back of the speaker but mine does not have that feature.

Another update on the NC-81X: I got an 80 meter oscillator coil out of the spare tuning unit and successfully installed it in the NC-81X receiver. It was a fairly easy transfer. The radio sprang to life with the new 80 meter coil in place. Then the troubles started before I had any chance to assess its performance. 

I decided to align the front end which should have been fairly easy since it is a mixer-oscillator design no RF stage. The high IF of 1560 kHz was considered enough to keep images down. Well, I thought to myself: how do I do an alignment when there is no working dial indicator? I decided to set the upper end of the tuning range at what the slide rule dial showed to be the upper end for the 80 meter band. This would be approximately 4.060 MHz. I found that frequency to be coming in at somewhat less than the upper tuning range of the receiver. I adjusted the oscillator trimmer capacitor to hear such a signal at the actual upper limit of the tuning capacitor's range. Then I proceeded to adjust the trimmer cap on the mixer for maximum signal. The tuning was rather broad until I backed down the RF gain. Then I tried to assess the performance but it soon became obvious that something went wrong. For one thing, I wasn't hearing typical 75 meter signals anymore. I thought at the time that maybe there might have been a solar event while I was working, I even checked with QRZ and the ARRL to see if there was any news about one. Well, there wasn't. I concluded the sensitivity was way down-reason unknown. I could tell this by the setting of the RF gain control when I tried to use the Johnson Viking II as a signal source for locating 3870 kHz. The local oscillator still seemed to be functioning. One note: I didn't try to adjust either the oscillator coil or mixer coil at the low end of the tuning range. That requires manipulating a half turn of wire in the coil provided for that purpose. There are no tuning slugs in the National coils. 

Then I tried adjusting the mixer stage for maximum noise at around 4 MHz. That didn't help. Next, I readjusted it for maximum gain at 3870 kHz since that is where I routinely tune in. Didn't help. So that is how I left things with the NC-81X. I decided to set it aside for now while I am working on the SX-99. I also wanted to use the speaker from the NC-81X on the SX-99. It has a good sound for shortwave and high output being a vintage Jensen PM-6. It also has the required 3.2 Ohm impedance to match the SX-99.

Thanks to the service information from Carl (Burntfingers) regarding checking and cleaning individual coils in National NC-200 tuning units, I took a hint from that to try just replacing the 80 Meter Oscillator coil rather than going to all the trouble of replacing the whole tuning unit. So I am amending my list of things to try in my NC-81X. By the way I have a Hallicrafters SX-99 receiver coming here tomorrow from Washington State so I am looking forward to that. Plus a pair of C. Brandes phones and also a pair of N. Baldwin phones are coming tomorrow also.

73, Rod  WB6FBF

I took a look at those links. Thank you. One of them was to W1UJR's page on replacing NC-100 and NC-101X tuning unit rack bushings. Most of the NC-100 sets I have had came with good bushings and did not need replacing. The NC-81X was an exception and as I said I got a friend to make a new pair out of white, nylon rod stock. They fit well from the start and have not come out.

The second link was to a page on repairing RCA's Superheterodyne catacomb sets. I did a repair on one of these years ago back when I still lived in San Jose. I heated it on a burner on my Mother's kitchen stove to melt the rosin potting compound. The problems consisted with broken wires where they came thru the potting compound going to the tube sockets. The coils in there are fairly standard looking oscillator and IF coils. Some of them do resemble the early honeycomb coils except the RCA coils are much narrower. The only other problem was an open audio transformer winding. It was the grid winding which tends to be on the outside of the coil. I peeled back the outer insulation and found the break and was able to repair it. On finishing the repair I elected to put the Rosin back in the tin can.

Yesterday, I was checking the NC-81X drum drive set screws for tightness. They were slightly loose but probably not enough to cause a problem. I did tighten them securely. Next I looked at and decided to move the mixer grid lead away from the oscillator grid lead-the spaghetti they used on the leads was touching so I moved them apart. I also moved the hfo grid lead away from the chassis as the spaghetti on it was touching the chassis. I have wondered in the past if the woven spaghetti they used had perished and was now a conductor. I don't know if that is the case but I managed to get these leads separated and in the clear. Moving these leads did affect the frequency of the hfo and I had to retune it a half turn in a CW direction to get it back on 3870kHz. 

Hammarlund may not have been the first to use the term "tuning unit" for their own unique band switched coil assembly but they did promote it heavily in their Super-Pro series of receivers from 1935 on.

Yes I remember the word catacomb, During the early 60s, It was actually a coil of wire that was wrapped in a back and forth in a pattern and on a form

And had many coils for band switching.

I used to buy both electronics magazines like Radio Experimenter and Popular Electronics  and found occasional article where that type of coil was used. Reading those articles was my first knowledge of that style of coil(s).

Another style was called honey comb coils. But they were usually only one or two.


Heres a link to coils click Here
This has pictures of a Philco with catacomb coils. Click Here

Thanks. I read over those postings on the troubles a fellow was having with operating and band switching a similar radio set called the National NC-240D. It was interesting. I may try some of the suggestions Carl (Burntfingers) described there. That forum started 5 years ago this month now as many that I have seen there often go back 10-15 years. 

I did have the tuning unit out of the NC-81X that I have here at one time in order to replace what was referred to as the carriage rod bushings. I also had a friend custom make a pair of bushings for me out of white nylon rod, after taking careful measurements of the rod and tuning unit. They worked out well for me. I don't remember doing the cleaning job on the coil contact posts and switch fingers that Carl and others described. I guess they looked ok at the time. I did clean and lubricate the rest of the assembly as best I could. 

Once when I was moving an NC-100 receiver on a cart, I had the bad luck to have the receiver slide off the cart when going down a hill with it and the receiver hit the ground hard. The band that it was on at that time never did work after that. Maybe one of the posts for that particular band's coils sheared off in the fall? I never did get around to looking at it to see what happened. Carl's suggestion of removing the coils individually for checking and cleaning would have helped there. I don't think I thought of that at the time. It would have been fairly easy to do. Sometimes these coils come up for sale on eBay. The receiver continued to function on the other bands after that so I guess the switch fingers themselves were ok. 

By the way, I refer to the coil assembly as a "tuning unit", a term used from the 1930s for certain manufacturer's multi-band receivers whether moving coil or not and do not use the term "catacomb" that was in vogue in the 1920s for the line of early RCA superheterodyne battery sets that they produced at that time.   

73, Rod  WB6FBF

Heres a link that may be or may not be useful.
On your receiver:

NC-81X Click Here
it shows one of the members called burntfingers.

He may be of some help. I don't know if he is still active or not.

"Carl
National Radio 1963-69
Service Tech, Service Manager, Sr Engr Aide
Member of NCL-2000 and NCX-1000 design teams
Collector and user of many National and other brands
I also do overhauls for others"

I thought I would put out another message on the National NC-81X tuning problem. It is still moving along in terms of my having to reset the tuning knob in a CCW direction in order to find the stations I regularly want to tune in. I don't notice the change though on any evening's worth of listening in the range of 3860 to 3890 kHz. As I said before, the change is more gradual than that. Warm up drift from a cold start is not a problem with this receiver. Regarding the broken dial cord, if I were able to restring it and get the slide rule dial working again I have my doubts about being able to keep it calibrated because of the tuning problem.

I have finally replaced all the original tubular paper capacitors. Just recently I cut out the last one that was a cathode bypass for the second IF amplifier. I haven't replaced any of the micas yet. There are very few of them in the whole receiver. There are (3) micas in the 1560 kHz IF crystal filter unit, however I recently replaced the whole crystal filter assembly with another one and haven't noticed any change as far as the tuning resetting problem goes. The 1560 kHz IF transformers are tuned with small air variables. I haven't looked inside the IF transformers. I know National wound their 455 kHz IFs on wooden dowels. I had to replace a bad one of those in an NC-100 receiver. 

Most of the other micas are in the 6C5 Plate Detector (Second Detector) circuit and the 25L6 Audio Output circuit. There is only (1) mica in the 6J7 HFO circuit and it is the usual grid type (.0001uF) in parallel with a 20,000 Ohm grid leak resistor. This grid capacitor/resistor combination is mounted above the chassis and is connected on one end to the tuning capacitor oscillator section and the other end goes to the grid cap of the 6J7 triode connected HFO tube. I've wondered in the past if I should replace this .0001uF (100pF) mica grid capacitor but haven't done so because of appearance reasons and possibly the need for a mixer-oscillator realignment of the receiver. I have checked the tuning unit coil box and I don't see any micas or any other fixed capacitors in there.

Not knowing how to trouble-shoot this problem I've been thinking of trying the following:

1. Replace the HFO grid capacitor/resistor and wait and see if there is any improvement. This would be the easiest change to make.

2. Replace the two section tuning capacitor with a spare and see if there is any change. This would be fairly easy to do. 

3. Replace the moving coil tuning unit with a spare and see if then there is any change. This would take a lot of work to do. I'd rather not do this step as it affects the band switching mechanism.

4. Examine the tuning drive mechanism. It is of the pinch roller type. The tuning knob acts on the dial drum rim thru a pinch drive setup. The dial drum is in turn coupled to the tuning capacitor.

Well, that's all for know. 

73, Rod  WB6FBF

Tough problem.
Things that come to mind are:

 Tuning capacitor rotor loose on shaft (Saw this one time) 

 Silver mica disease causing IF to shift a bit. Not sure what your IFT's look like but some had a padder in the base and some had it in the can.

 Dial cord slippage.

 Bearings in tuning capacitor bad or a ball bearing missing.

 Possibly a defective, new tubular capacitor in the Osc. section.

 Mica capacitor, most specifically a paper mica, or any El Menco brand one. (I disagree with Cliff on the mica cap issue). Having run into so many bad "Domino or postage stamp"mica caps, I just automatically replace them all with new stock silver micas, epoxy encapsulated. 

It might be an osc. tube issue, but not too likely.

 Just guesses at this point. HTH

I was lucky enough to get an original manual and speaker with the radio. The slide-rule tuning dial pointer does not move due to the broken dial cord. I can still tune the receiver with the large (weighted) tuning knob. The dial cord system's sole function is to indicate the frequency being received. Regarding the pointer slider, it does bind somewhat in spots. It is made of some kind of black plastic. The same plastic was used for the tuning unit bushings. In fact, I had to have a friend make a new set of nylon bushings for my receiver as the originals were shot. This set does not have the two-speed tuning drive. It may not have been used on the ham band models. 

I have also wondered if some kind of gradual change is occurring with the tuning coils located in the tuning unit. They look good. I do have a spare tuning unit I could try if I was willing to tackle the chain drive band-switching mechanism again. I had to adjust it after getting the receiver in order to be able to change bands. Well, I gotta go. Hope to get some more comments.

Rod

Do you have the Manual?
Click Here for Manual
If not down load the link and read pages 5 & 6 it talks about maybe the issue you are having? It does say that that model may have a two speed dial which should be investigated for operation.

Since I have never seen one on the inside, its kind of hard to help. But I would clean the dial pointer track and pointer unit where it contacts the top edge of the dial plate. It may be gummed up from harden grease. Also clean any pulleys for the dial string. Either use a drop of high quality oil such as gun or clock oil including sewing machine oil or use lithium grease (used for chains and gears). If it has a spring tension on the dial pulley that may also have an effect. Some dial pulleys have two geared plates with a spring to keep the plates from binding up and to reduce backlash. If that is the case then if any crud is in between the plates they also need to be cleaned and possibly lubricated. If the dial string is old it might have some elasticity that is now effecting the tuning.

Of course temperature could effect tuning as it warms up. But if it is a continuous issue then moisture itself could effect the coils.
Then there is the issue of tubes, some change internally because of expansion and contraction of tube element spacing. It may be time to change tubes especially in the RF-IF areas.
Also consider Solder connections on the tube bases. It may be a rerouted wire that effects tuning or a capacitor that is getting leaky. (probably a paper capacitor, unlikely to be a mica Domino style)



I'm just guessing at some of these as possible issues, but leave no stone unturned.

I have been having a mysterious tuning problem with my main receiver to which I am now turning to the forum for help to get it resolved. The set is one of National's late 1938 models that I have admired and enjoyed using for at least the last 10 years ever since I located one up in Ontario, Canada. It arrived with some shipping damage to the slide rule tuning dial. I guess the box must have gotten smacked on that side. The packing the seller used appeared to have pressed on the dial area and that contributed to the damage. I mention this in case it has some bearing on the problem. 

I'll state here that I have owned a number of National models from the 1930s and 40s with the moving coil tuning unit design. The receiver in question here is a ham  band model NC-81X. It is a very nice single dial control receiver somewhat like other National sets of the period.

Well, after looking the radio over I noted the changes and modifications. They mainly consisted of someone adding a homebrew pre-selector to the set. I proceeded to remove the modifications and get it back together like it was when it left the factory. It mostly works very well ever since then. By the way, this set has the optional power transformer although it still has the tube filaments wired in series like an AC/DC set which most of the other NC-80X and NC-81X model were.

One of the continuing problems I haven't fixed is the very unusual dial cord setup National used. The dial cord was broken and thus the dial inoperative. For the slide rule dial, they used a two piece cord design. The ends of these (2) cords are riveted to the pointer that rides on the slide rule dial. The fact that the pointer cannot be removed from the dial for servicing is a problem. The other ends of these dial cords are attached to the dial drum with a very small sheet metal screw and a spring (far as I can tell). Back when parts and service were available I can only conclude that an owner with a broken dial cord would have to return it to the factory for repairs.

Not having wanted to tackle this I have used one of my transmitters to spot check the frequencies (3870 kHz) I listen to and tune the receiver in with the BFO on. 

Now for the problem: The nets and stations I regularly listen to slowly and steadily move away from the expected point on the tuning knob over a period of days and weeks. I've been keeping track of the tuning knob's position by observing the relative angle of the set screw on the knob. This change has been a continuous process and I find myself retuning in a CCW direction from time to time to find the stations. I'll state here that it is not normal drift encountered in operating older receivers with a HFO. I have wondered what could be causing this and think it might be some kind of mechanical problem that leads to a shift in the tuning capacitor setting. Turning the tuning knob in a CCW direction engages the tuning capacitor to a greater degree thus tuning the set to a lower frequency. 

I have only encountered one other receiver that had a somewhat similar problem. That was a Hammarlund BC-779 (Super Pro) of WWII vintage. That set exhibited a rapid change however. It would just run away from you. After much head scratching I eventually found that the thrust bearing adjustment on the back of the bandspread had gotten loose. After a careful re-tightening of the bearing that problem was solved.

So I ask now any thoughts or experience with this kind of tuning resetting problem?

73, Rod WB6FBF