National NC-100F? HF Receiver

[Tom WA3KLR]

Just for information, here is a rare HF receiver, the National NC-100F, rack mount, black crinkle-finish front panel.  Self-contained coil tray, 5 bands covering 0.54 to 30 MHz., self-contained a.c. supply.  Classic National PW dial, but with gray-blue coloring.

It’s not one of the prized high performance boat anchor receivers, but an interesting radio.  The NC-100 family has many variants.  This unit was designed for the CAA (now the FAA).  It is capable of crystal control and has an avc-operated squelch.  The crystal box extends from the upper right part of the front panel.  The box can contain 2 crystals.  There is a switch on the box to turn on crystal control.  I presume that the receiver would first be tuned to the approximate receive frequency, then would lock on to that close crystal when the switch is thrown to ON.

In the early days of aviation radio, the two standard voice frequencies were 3105 and 6210 kHz.  This would require crystal frequencies of 3561 and 6666 kHz.  Both of these receive frequencies are on the third band of the radio.

The basic NC-100X has 12 tubes and crystal filter.  Single-conversion to 456 kHz. i.f.  Features push-pull 6F6’s in audio output, 6E5 magic eye tuning, 6K7’s in rf and two i.f. amplifier stages, 6J7’s in BFO, local oscillator, mixer (1st detector) and avc detector-amplifier, 6C5 audio detector (2nd detector), and 80 rectifier.  No VR tube, no audio noise limiter.  Introduction in 1936.

The NC-100 came without the crystal filter.  Later NC-100’s had an S-meter in place of the magic eye.

The NC-100F has a single 6V6 audio output, 11 tubes and draws 61 watts.  This receiver has no crystal filter, no magic eye tube, but does have the amplified avc like all -100’s and had an additional “INS” squelch circuit like the RCF.  (INS means Inter-station Noise Suppression.) On my receiver, the audio level holds +/- 3 dB over a 100 dB r.f. range.  Accessed at the back of the radio, there are 2 pots for the squelch circuit adjustments.

There are 2 B+ filter chokes instead of one, unlike the other -100’s which use a speaker field coil/filter choke on the speaker and no internal audio output transformer.  The -100F has an audio output transformer inside where the second 6F6 would be.  The output impedance is 20,000 and 600 Ohms.  There is a relay for audio transformer output muting.

I have a Radio Shack 10 Watt 70.7 Volt speaker transformer (32-1031B) connected to the receiver’s 600 Ohm output to match to my 4 Ohm speaker.  It is a good matching transformer for the purpose.

The RCF is basically identical to the -100F except for not having the crystal-control capability of the -100F.  It does use an additional tube in the audio chain for the squelch function, for a total of 12 tubes.  National apparently found a way to get rid of that audio tube in the -100F.  They are probably controlling the 6V6 for the squelch.  I never had an actual NC-100F schematic and I haven’t found one on the web.

The power consumption of only 61 Watts is somewhat remarkable.  My HQ-120X draws 102 Watts.  I don’t know if the relatively low power consumption was a contract requirement or a by-product of a cost goal.  The schematic is noticeably devoid of the liberal use of by-pass condensers.

[W3SLK]

Tom, what you have is a RCP. These were like the 'band-switching HRO' but they are a little broad. Originally they were used for what was called the A&N system at airports. Early runway alignment was dit-dah or A to the left of the runway, dah-dit or N to the right and a carrier when you were down the center. The RCP was used to detect if the carrier was still on the air. It has a squelch circuit built in. When the carrier dropped, the squelch would close the receiver as well as activate a set of contacts on the back. This would light a red lite on top of the tower to notify pilots that the runway alignment system was down. Duane KK4AM has one that works, and he uses it regularly on 160 when the band is in decent shape. I have one that needs a going over. I think there are several variants to this. Duane has the manuals and he might be able to secure you a copy, (I'm going to ask him because I need one too).

[Tom WA3KLR]

Hi Mike,

There is no serial number or nameplates on the chassis or cabinet.

Well I don't think I have a RCP.  I did notice that 4 of the 5 cans have "RCF" stamped on them near their bottom edges.  The relay does not operate as you say, but appears to be wired like on a RCF schematic - the relay coil comes out to a set of terminals on the back labeled "6VDC".  When the relay pulls in, the audio output is muted by shorting the output of the audio transformer (believe it or not).  I operated the relay armature manually.  6 Volts d.c. does not operate the 700 Ohm coil though.  12 Volts makes it move about halfway, so I think that 24 Volts to the coil terminals probably would do the job. 

It would be good if Du-ane can compare my top view photo to his manual though.

So what I have may be a later model of RCF with the crystal control added and the one less audio tube simplification.  Perhaps some other variant RXX designation?  The squelch in my radio may actually mute the audio detector tube.   Since the avc detector is separate this can be done.  The a.c. input power does not drop at all when squelched, so it looks like the 6V6 does not get cut off.

* Added later by edit - The receiver i.f. bandpass is not broad.  Moving 5 kc. off channel does give a reasonable amount of rejection.  Moving 10 kc off channel gives quite a bit of rejection, fair AM signals will completely disappear.

Hope to work you on 160 today, tonight or tomorrow.  The last 2 evenings have had no static crashes!

[W3SLK]

Tom,
It is my intention to get on 160 tonight. With Ernesto clearing the atmosphere of crap we should have good conditions for a while. We will discuss this at length then.

[KA8WTK]

Hi Tom,
  I have the CAA version of that receiver designated RCQ.
  The crystal switch work just like you describe. I have a tuning chart for this one, which may be of use with your receiver as well. Let me know if you want a copy of it. (It really isn't bowed, the picture just came out that way.)

[w3jn]

There were MANY variants of this receiver.... I had an RCH that was pretty much identical, had the "C.O.N.S" tube (and a relay IIRC) as well.

Mine was built for American Airlines.

[k4kyv]

They are not exactly "bandswitching HRO's" because the HRO has two rf stages, with enough preselection to virtually eliminate images on 20m, while the NC-100 series has a pesky image problem on that band.  OTH, the dynamic range is better with only one rf stage ahead of the mixer.

Actually, I think they would be superior to the HRO if a good antenna tuner were used with the receiver.

I have an NC-100 XA and a NC-101XA.  The NC-101 series is very similar, except it is ham band only with plenty of bandspread.  The XA  series uses an analogue direct-reading frequency dial, rather than the HRO type 0-500 arbitrary scale with calibration charts.

Getting those receivers running again is one of my "round to it" projects.

[Tom WA3KLR]

Bill and John,

Thanks for the responses.

Bill,

Your RCQ photo is a little different than my receiver.  Where you have a notepad area in the upper right corner of the front panel, that is where the crystal holder box protrudes.  My green pilot lens is where the middle of your nameplate is.  This receiver originally had a nameplate in the very upper left corner of the front panel.  My Dad said the receiver had no nameplate when he acquired it.  He filled in the 4 holes and touched up the panel with black paint.  The few of these receivers that he has seen, none ever had the nameplates on them.

Thanks for the offer on the tuning chart, but I modified one years ago after re-aligning the radio.

John,

I looked up a description of the RCH and came up with 80 - 560 kc. and 1.9 - 24 Mc., so that is not it.

This radio may not necessarily be a CAA receiver.  It is general coverage, so any HF communications system could have ordered it.

[Tom WA3KLR]

Mike, Bill, and all,

I see in the 3rd Edition of Raymond Moore's Receiver book that the photo of the RCE looks physically identical to my receiver.

So it would be interesting for me to have just the schematics for the RCE, RCP and RCQ receivers.

[W3SLK]

I thought that looked like it Tom. Sorry for dropping out last night but the breaker on the modulation deck kept tripping out as soon as I would key down.

[Tom WA3KLR]

Update on trying to identify my NC-100 variant:

Although the power transformer is marked NC-100F, I doubt that the receiver is actually called a NC-100F.

I have received a RCE schematic from Bill W2DGB and a RCQ schematic from Jim KA8WTK; thanks guys.

The RCE schematic is very similar to the RCF schematic I already had.  The RCE has a parallel-resonant trap on the antenna input tuned to the 456 kHz. i.f.  The RCE and many other variants feature 200 - 400 kHz. coverage at the first band position which is 0.54 - 1.3 MHz. in my receiver and the NC-100.

The tone control circuit of the RCF has 2 chokes in it, yielding an audio band-pass filter, whereas the RCE has a basic R-C tone control.

The RCE audio output transformer has just 20,000 Ohm output impedance, whereas the RCF has a tap on the output transformer for 600 Ohms and related additional complexity in the phone jack and audio shorting relay circuits. 

The RCE fuses one side of the primary inside the radio, the RCF has 2 fuses in the line cord plug.  Getting pretty picky here.  This is the differences between the RCE and RCF.

The RCE and RCF have (2) 6C5’s for audio detector and 1st audio/squelch functions.  Neither shows crystal control of the local oscillator.  Again these are 12 tube radios.

As I said in an previous posting, my radio has just one 6C5 for detector and squelch function; a good simplification.  This change must have come later.  It seems to me that you would stick with this reduction in circuitry as it works well.

The RCQ 12 tube schematic evolves from the RCP schematic.  The RCQ modifications are for the 2nd detector, noise limiter, cons. relay and 1st audio amplifier area.  The modifications are approved by the CAA on 3-30-1948.  This radio does have: single 6V6 output, carrier-operated relay (energizes on loss of signal), contacts of the cons. relay are used to mute the audio to the 6V6 grid, a 6J5 in the local oscillator instead of the 6J7 used in earlier NC-100’s and HRO’s, crystal control option for the l.o., and the avc detector/amplifier circuit had been increased in complexity to (2) 6J7’s over the earlier NC-100 variants using one 6J7. 

The first 6J7 is in a detector/cathode follower configuration with the second 6J7 providing the dc gain only.  Why the increase in complexity for the same basic avc functionality?

My guess is that there may be increased loop gain (which yields more avc control range to even stronger signals), more filtering of the i.f. energy from the avc bus, and tighter control of the basic avc time constant.

Are any of these my radio? No!  So the quest goes on.  It looks like my radio is later than the RCE and RCF, but prior to the RCP and RCQ.