NC-200 Is Jumping Frequency

[Jim/WA2MER]

My NC-200 has started abruptly jumping frequency, where it will suddenly jump up a kc or two, then it may pop right back to where it was.  it happens only on the 80 Meter band spread position; the radio is rock solid on the 80 Meter general coverage position and on all other bands.  I've not taken it out to the bench yet, but my guess is that the ceramic/mica trimmer that calibrates the dial for the 80 Meter band spread has gone squirrelly.  Can anyone tell me if the trimmer can safely be disassembled for inspection and cleaning, and if so what's a safe cleaner that I could use?

Thanx & 73,
Jim
W2BVM

[W3RSW]

I've had similar and usually happens on the most used band, say 80. Most always is the band switch wafer contact for a specific band.  Both an HQ 110 and a Drake SP R4 have shown this here. Not sure how your band spread is switched in on a NC 200. Similar?

[The Slab Bacon]

IIRC, the NC-200 is one of those sliding coil catacomb rigs. If it is, you probably have a crappy contact in one of the contacts, or one of the fingers is not rubbing the moving contact hard enough.

[Jim/WA2MER]

Slab, you are correct that the NC-200 uses a sliding carriage arrangement for band switching.  When I was in there some months ago doing the re-capping and cleaning I found the finger contacts to be in excellent condition except for dirt, which I thoroughly cleaned off. I really don't think the problem lies with the contacts.  thanx to you and RSW for your suggestions though.  I'm really looking for tips on disassembling the trimmer, assuming that it can be safely disassembled, and how best to clean it.

[KM1H]

Unless you pulled every coil block out (all 18), cleaned each pin, and then cleaned the fixed fingers on the chassis you didnt finish the job. Be VERY careful of those fixed fingers as they are easily broken, near unobtaium and a royal PITA to replace. I use Tarn-X on a Q-Tip, insert thru the widest part and wipe several times. Then use a clean Q-Tip to remove the residue. A typical radio takes me about 40 Q-Tips.

I would also put a few drops of De-Oxit on each of the variable caps wiper to frame contact points and run the dial around several times to break any oxidation.

I doubt the 6J5 is at fault but try cleaning the tube and socket pins including the 6K8.

Carl

[Jim/WA2MER]

Unless you pulled every coil block out (all 18), cleaned each pin, and then cleaned the fixed fingers on the chassis you didnt finish the job. Be VERY careful of those fixed fingers as they are easily broken, near unobtaium and a royal PITA to replace. I use Tarn-X on a Q-Tip, insert thru the widest part and wipe several times. Then use a clean Q-Tip to remove the residue. A typical radio takes me about 40 Q-Tips.

Hi Carl.  I meticulously did all of the above some months ago when I first got the radio and you kindly explained the process to me then.  I recently cleaned the tube pins and replaced the 6J5 and 6K8, although I didn't think they were going to be at fault because the problem is limited to the 80 Meter band, and only in the band spread position.  I haven't yet put De-Oxit on the tuning cap wiper contacts, but I will do that over the weekend when I get the radio on the bench.  Again, because the problem lies only with one band I doubt that would be the problem, but I won't rule out anything.  In any event, tuning is smooth and without noise when moving the dial.

I appreciate yours and the other suggestions, but so far no one has answered my question as to if and how the 3-30 pF mica trimmer on the band spread coil can be cleaned in the event that I determine that the problem lies there.  I'm trying to get a heads-up before I have the radio in front of me over the weekend.

Thanx & 73,
Jim

[W3RSW]

Just read my reply and sounded stuffy.  Just cut one side of the cap loose, bridge it with an ordinary compression trimmer, see if that's the cure and go from there.


Jim, your original post mentioned nothing about your previously cleaning band contacts.
We try to be helpful.
Concerning the trimmer, those things are very brittle. Very locally, smearing nothing else, touch it up with a compound that will simultaneously loosen it, add a bit of lubrication, be non conductive, never dry out and have a very high dielectric constant, i.e., good luck.

In the real world, it's probably better to simply replace it.  If the freq. shift is still there, then what?  I'm assuming that that specific trimmer is in circuit for 80 band spread only and not  in use on any other band.

I'd replace it temporarily with a compression trimmer and see of that is a fix.  If so, gently remove the original and work on it outside of the radio.  Is the freq. jump always the same cycle shift or all over the place?  Resolder all the connections in that bank.  Suspect fixed value caps, spray them with chiller, all the usual stuff.

And last but not least, even though you cleaned all the contacts does not mean an  connection will be fixed.  If anything, you may have widened the contacts.

[Jim/WA2MER]

Rick:

Thanx for your reply and suggestions.  FWIW, I didn't find your reply "stuffy," so there's no issue with that.  I do appreciate all of the suggestions I received, and suggestions are always welcome.  I hope that I didn't come off as being less than grateful.  However, it took three days before someone answered my question, much less acknowledged it.  Regardless, your comments and suggestions about the trimmer were what I was looking for from the outset, i.e. is it practical to mess with the trimmer or not?  Apparently not, unlike the trimmers in Collins gear that can be disassembled and cleaned if one is very careful.

Yes, you are correct that the problem is confined to the 80 Meter band spread position, and the trimmer in question is in use only for that band.  The NC-200 has four band spread positions, and each uses a different set of coils and trimmers, so that's how I decided that the problem probably lies with a specific coil/trimmer assembly.  Anyway, when I get in there I'll recheck everything right down to the contact tension, but I suspect the trimmer.  We'll see.

73,
Jim

[KM1H]

What is the value of that cap? I might have some. Ive also had to replace a fixed mica at times.

Here are the fixed fingers and they should all look like this. Its easy to bust one if the carriage is over them when replacing the coil packs.

[Jim/WA2MER]

Thanx for the photos Carl.  I have a closer look at the fingers once I get back into the radio over the weekend.  I did make sure that when I removed the coil assemblies that they weren't engaged with the fingers; I recall your original warning about that and I took your advice.

The manual indicates that the trimmer (C60) is 3-30 mmf.  OK on having to replace micas, but there are none in the BS coil assemblies, only fixed ceramics, the ceramic/mica variable and an air variable.  At this point I'm not in a position to rule anything out; ceramics go bad, too.  I've targeted C60 only because it's it's a likely suspect because of its construction, but any of the caps in there could be the problem, as well as a mechanical issue with the air variable.  Hopefully it won't take very much poking around to find the culprit.

[KM1H]

I dont think it would hurt by applying a little fast evaporating brake cleaner so that it gets between that caps sections and giving it a few turns if it proves to be the culprit. It might take a bit of time to frequency stabilize so dont rush it. When I do things like that I go to bed

[Jim/WA2MER]

I found and fixed the problem.  It was the trimmer that I had suspected from the start.  BTW, the manual refers to the trimer as mica, and I had been referring to it as ceramic/mica because I had never done a close examination of it.  It's construction and shape are pretty much the same as commonly seen ceramic trimmers, but on closer examination it appears to be made of a plastic composite which I assume is some sort of molded mica compound.  Anyway, back on topic.

Very slight pressure on one side of the trimmer caused the frequency jump.  The trimmer had a hairline crack that I very carefully spread apart just enough to get a bit of Krazy Glue into it.  It's all good now, rock solid.  Whether or not the repair will stand the test of time remains to be seen.  I'm optimistic.

Many thanx to all who responded with suggestions.

73,
Jim
W2BVM

[W3RSW]

Great! Hopefully it's still variable after all that crazy glue 
Don't you just get that "did it myself" glow after fixing stuff?
Almost no greater feeling...

[KM1H]

Sometimes a little creative license is needed to keep this old junk running. Super Glue, JB Weld, and even a little bit of Henrob 2000 torch work has come thru for me at times.

[ke7trp]

Glad you found the problem. A trick I use to find such gremlins is to set the BFO just off center so you can get a tone on the calibrator or sig gen signal.  Then poke around with a plastic pen.  When you hit that bad component or loose connection, you will hear the BFO tone zing off beat.

I found bad/loose bandswitch terminals this way on my SX42.  Repairing them was another story.

C