NC-200 Receiver Good News/Bad News

[Jim/WA2MER]

I picked up my NC-200 a couple of weeks ago and did quite a bit of work on it in the hope of finishing it today.  I almost got there.  I replaced all of the electrolytic and paper caps.  Then I removed all of the coils in the catacomb and meticulously cleaned all of the pins and chassis contacts according to Carl's instructions.  I fired it up on the BC band and was treated to some of the nicest sounding audio from any receiver I've owned so far.  Finally I did a by-the-book IF alignment, which went just fine.  OK, so that's the end of the good news.

The bad news is that one band (Band C) is completely dead.  I couldn't believe that I somehow failed to clean the pins properly, so I pulled the coils for the band and discovered that a winding on one of the coils (C1) was fried!  I can't imagine what may have taken it out, but the wires from pins to the coil form are gone, melted away.  The coil wire in the form is intact, but all of the insulation is gone and the coil is shorted, so there's no use in trying to put pigtails on the remainder of the coil to connect it up with the pins.  The winding is toast (it's the winding on the lower part of the coil form, the fine wire that was cloth-insulated).

I have two choices, either find a replacement coil from a parts radio or rewind the existing coil.  Any chance  I'll find a replacement, or should I just go ahead and rewind the existing one?

73,
Jim

[k4kyv]

I'd try to re-wind the existing one.  Very unlikely you will ever find a replacement coil anywhere close to identical to the old one, unless from another National receiver with catacomb assembly.  The form is most likely bakelite.  Sounds like the antenna or plate winding, not the grid winding, the one that resonates with the tuning capacitor. Maybe lightning, or if not the first coil, maybe a tube or something shorted and the full +HV was bridged across the coil and burnt it out.

Carefully remove the coil and count the exact number of turns on the burnt winding.  Try to find some replacement wire the same size. You could measure the diameter with a micrometer, and use wire tables to find the gauge. Probably 20-something. The coil is probably wound in the same kind of pattern as that used on rf chokes.  It would be very difficult to duplicate that pattern without special winding tools, but it should be ok to scramble-wind a replacement  coil using the same  number of turns.  I have done that with AM broadcast sets and the new coil worked just fine.  If the main coil (the one that resonates with the variable capacitor) is intact, the repaired coil should be good as new.

If you have some small rf chokes in the junk box, you might even find one wound with the same size wire, or something very close to it,  that could be salvaged as a source of wire.

What frequency range is Band C, and was it the rf, mixer or oscillator coil?  I might even have a replacement here.  I have the remains of a coil catacomb assembly round here somewhere that I picked up years ago at a hamfest, and there might still be an identical coil on it.  I think they used the same coils on several, if not all, of those receivers.  I pulled one coil off to replace a damaged one in the NC-81X I was working on, and it turned out to be identical.

[KM1H]

The coil forms are not bakelite but rather a high Q (mica filled I believe) material that is not hydroscopic.

Glad to here the rest of it is coming along. I dont see too many of those sets showing up as parts and I havent had to rewind a coil yet but have had to in several HRO models.

Carl

[Jim/WA2MER]

Don and Carl, thanx for the replies.  As soon as I found the damaged coil I closed up shop and went upstairs without looking further into the matter.  Just before work this morning I looked at the print and determined that the coil is in the RF amp circuit, specifically the winding that's across the antenna terminals.  You seem to have nailed the cause Don, something came into the antenna input that the radio didn't like.  Judging the nature of the damage I don't think it was lightning.  There's no indication of an instantaneous catastrophic event like I've seen associated with lightning strikes in the commercial world (two-way radio). Instead it looks more like a much slower event, like a heating up followed by melting wire.  My guess is that someone transmitted into the antenna terminals.  Band C covers the 40 Meter band.

Fortunately the winding is a single layer, about 10-12 close wound turns of what appears to be No. 28 or 30 wire. Considering its function and where it is in the circuit it's not a terribly critical winding.  I should have no trouble rewinding it.  Just a minor PITA.

Many thanx for your input, guys.  Whatever it takes, this radio is definitely worth the trouble.

73,
Jim

[k4kyv]

Sounds like someone zapped the input coil with RF.  They might have simply had a receiving antenna too close to the transmitting antenna and there was enough  RF pick-up to toast the antenna coil.

The coils are made of some kind of phenolic composition material, usually yellow and quite possibly filled with mica. It needs to be non-hydroscopic, rf-transparent and temperature stable.  Something National made in-house.  I believe even the HRO coils are identical to the ones in the catacomb assemblies.  Later models of the HRO use some kind of clear plastic for coil forms, polystyrene maybe, and the plug-in coil contacts are embedded in ceramic material rather than the bakelite-looking stuff.

[K7MCG]

Jim-
I note that the main tuning capacitor capacitance is the same as for the HRO.  And the bands tuned are the same.  Could it follow that the coils in the NC-200 labyrinth are the same as the coils in the HRO plug-in unit ?  Ifso, you may be in luck, as the winding details for the HRO coils are available in the UK Royal Army HRO manual !  Or you could consider sacrificing a HRO JC coil set to get a coil to try.

73,
Chuck K7MCG

<I have just emailed the UK data to your QRZ email address>

[Jim/WA2MER]

Fortunately the coil form is undamaged, so I don't have to deal with replacing it.  I measured the wire and it's 0.25 mm, which translates to AWG 30. Now to find a source.  I don't think the insulating material is critical, so I'll take what I can get, either enamel or cloth.  First stop, the junk box.  At one time I had some old slug-tuned coils and RF chokes floating around, and I hope I still have them and can actually find them.  Some new wire and clear lacquer nail polish to hold it in place should do the trick.

Chuck, many thanx for going to the trouble of sending me the HRO coil data.  Judging from the wire sizes and the number of turns for a given band, the HRO coils are quite different animals from what I have in the catacomb.  Nonetheless I'm very grateful to have another valuable reference for my data library.

73,
Jim
W2BVM

[KM1H]

Give me an address to mail the wire to Jim, Ive miles of most sizes that I scoffed at the final National auction.

Good recovery and spin Don

[Jim/WA2MER]

Carl: PM sent.

[k4kyv]

Good recovery and spin Don

It's a matter of what you want to call the stuff. It is all commonly referred to as "bakelite", but actually "Bakelite" is but one of numerous commercial trade names.  Look through some of the 1920s and 1930s radio magazines, and you will see other trade names for the same stuff.  Bakelite originally was black, but later a lot of phenolic products produced were brown, orange and yellow. They all use some kind of filler; the black bakelite used to make knobs and dials often uses fine sawdust for the filler.  Not sure if the national coil forms used mica, but that would be a good guess, lacking any definitive documentation from National, since it would undoubtedly make a better and more stable rf insulator than would sawdust filler.

A common mica-filled phenolic insulating material familiar to hams is called Miclex. The stuff is greyish, brittle like ceramic, and quickly wears out drill bits and saws.  Cardwell variable capacitors originally used black bakelite for the insulators that support the stator plates, but later changed over to Miclex.  One problem with bakelite was that it tended to warp over time, and the plates would become misaligned. Receiving tube bases normally used a black bakelite or similar material, but tubes like the 807 use the brownish yellow stuff; it looks a lot like what National used for the coil forms. Supposedly it is a better rf insulator than plain black bakelite. Interestingly, a few transmitting tubes, like the 802 and 808, stayed with the same black stuff as used for receiving tubes.

There is another product I see used for rf insulators; it physically resembles Miclex, but is white or yellowish-white. It has a surface texture that looks like some kind of woven material.  It cuts and drills much more easily than Miclex.  After it is sawed, there is no woven texture to the interior; it comes out smooth and uniform like regular bakelite or some other plastic.  Sawing and drilling the stuff leaves a fine white powder.  Newer Cardwell variables use this material instead of the more familiar Miclex. It seems less brittle.

Something that has concerned me a little with the white stuff, since I don't have a clue what the substance is, is that the dust from drilling and sawing might be hazardous to breathe, like asbestos and the special ceramic material used in construction of external-anode tubes. I have read precautions that breathing even a microscopic particle of the tube ceramic could lead to lung cancer; the warnings almost made the stuff appear as toxic as breathing plutonium dust.  

Wonder if anyone reading this has any idea what kind of material the white Miclex-like insulation is? It could be highly toxic as well; remember, asbestos was a common household material that didn't receive a second thought, until it was found to be highly toxic and lead to lung disease.

Regarding glossy Bakelite, NEVER use a cleaner like Fantastik or 409 to clean it.  It will dissolve the microscopically thin skin of phenolic and expose the filler, leaving a dull surface.  Once that happens, there is no way to ever restore the sheen.  I ruined a couple of antique National dials that way, and someone told me they did the same thing to the main dial escutcheon on their Collins KW-1.

[The Slab Bacon]

Just my $.02 and a few things to keep in mind.

If the un-tuned side of the coil (input or primary winding) is the problem:

1. Try, if at all possible to leave the tuned side original and untouched. This  
     way you won't affect the synchronization and/or dial tracking.

2. The number of turns on the un-tuned primary (input) is not real critical,
    anything close within a few turns will work just ok-fine. (exact match is
    better, but not critical in the primary winding.)

3. Be sure that the coils are wound in the same direction as the original
    windings

[Jim/WA2MER]

Thanx for the tips, Slab.  The two windings are completely separate from another, so I don't have to touch the good winding.

[The Slab Bacon]

Jim,
     I have repaired more than my share of them over the years. Usually, if the antenna input / first preselector coil is toasted, it has either seen a lightning strike or been transmitted into 

[flintstone mop]

Ya probably might want to consider a #47 bulb in series with the antenna input of the radio to protect from RF overloads or nearby lightning.
Installed one here for my R390A that was getting fried from stupid mistakes on my part and high RF levels coming from an aux. RX antenna. Fair Radio luckily had many RF Deck coils on hand. I bought as many as I could afford.
The neon bulb in my 390A front-end is useless.

fred

[Jim/WA2MER]

Slab, I agree as to the probable cause.  I posted my opinion on that a couple of days ago where I decided from the evidence that it was probably not lightning, but instead just a very high RF level, like someone transmitting into the receiver antenna terminals.  No matter, the damage is done and it seems like a relatively easy fix.

Fred, thanx for the reminder about the bulb.

[KM1H]

Wire went today Jim, enjoy

Don Im rather familiar with the material composition and how they were made, I worked at National after all.

The 802's here have ceramic bases.

Carl

[Jim/WA2MER]

Thanx very much Carl.  I'll post an update when I'm all done and it's up and running properly on all bands.

73,
Jim
W2BVM