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I have a grip about an item I just sold via eBay. I sold my near mint NC-183D receiver just recently for $949 on eBay. It might seem like a very high price, but that is was it was worth to me and I wanted it to go to a good home. I did not want to see it get shipped overseas like much of the American made equipment has ended up. I knew that if someone was willing to pay that high price it would be someone who really wanted it and they would take care of it because they would not be able to make any profit.

Here is my grip; the A-hole in New York who bought it just sent me an email today complaining that it didn't work very well. He said it had low sensitivity, the resistor tolerances were way out and the audio sounded distorted, etc. Well, I used the receiver all last winter for SW listening because of the great high powered audio quality via the two 6V6GT’s in push-pull and it never sounded distorted. Anyway, the NC-183D’s are known for having very low sensitivity on the higher bands and the receiver was all-original. It had all of the original tubes; resistors and capacitors except for maybe the electrolytics in the AF final section. I think I did replaced those years ago, but I can’t remember for certain.

Its common sense that when you have a receiver made back in the 1950’s such as the 183D the components are going to have drifted. Nevertheless, none of the caps in the receiver were shorted and everything was working and nothing was missing. The receiver was in excellent cosmetic condition with only a few (3 or 4) very light scratches on the sides.

Its obvious this idiot must be a newbie and is probably using the receiver with the AVC turned off which is why he is getting the audio distortion. He probably has no experience with the old antique gear and just assumed it should work like a modern ricebox. I already received the money via PayPal minus the eBay and PayPal fees, but if anyone knows who this A-hole is please purchase the receiver from him so it will have a good home.

90% of the carbon composition resistors of that age will have drifted off the original values, even new old stock that have never had a soldering iron touch their leads.

But over 90% of the ones in a circuit that have drifted in value will have no discernible effect on operation. I change them out only when they actually cause degradation in performance, which is very seldom. I suppose the drop off in performance could be cumulative, but I have never seen any improvement in anything that I have "repaired" by replacing drifted resistors. Mine usually increase in value, sometimes by as much as 50%.

If it's a drastic change, which happens sometimes, that is a different story, and usually fixes the problem.

Also, I have found the drifted values of old resistors to be just about as stable as they ever were. The change seems to take place over an extended period of time. I have several boxes full of composition resistors. For handling any significant amount of power, I prefer them over film resistors - films seem to be about as delicate as solid state devices, and are not always guaranteed to be non-inductive. If I am building something and can't find one that is still in tolerance, but I run across a different value one that has drifted within range, I use it, and this has never given me any problem.

In restoring antique equipment, I have been known to re-paint old body-tip-dot resistors to represent the measured value of replacements.

My dad's NC-183D developed weak reception a few years ago. The main culprit was the rf and if amplifier screen grid dropping resistors going up in value. Every resistor of that value and appearance in the set was bad. (There appeared to be 2 distinct vendors of that value.) There was one weak tube replaced but not a problem. The radio was never worked on since new.

I found ALL of the audio section capacitors and resistors bad - first audio, phase inverter, audio output stage and even the output feedback components.

Also the audio output transformer looks like it has a design problem. I could only get 3 1/2 watts out. Many many troubleshooting test were done. The transformer is not shorted or something like this. I came to the conclusion that there was another transformer vendor (or lot) and that the turns ratio is not correct. Some units do put out 10 watts of audio I believe. The one other ham that did check his NC-183D audio output power for me also came up with 3 1/2 watts out.

I have a 1947 NC-183 (not D), that I have had since 1958. When I was a young ham I thought it had great sensitivity, that it seemed to have lost over the years. As Don has noted, most of the resistors had changed in value. I replaced mainly the screen dropping resistors and all of the ones in the audio stages. Also had some leaky paper capacitors, which were all replaced along with a power transformer that burned up because of some leaky caps. The receiver then worked as I had remembered it. The NC-183D does not have low sensitivity on the higher bands if it is aligned correctly and working correctly. I believe it has two tuned RF stages like the NC-183 has. I don't believe there is any obligation for sellers on ebay to replace resistors on old equipment unless it is claimed that it has been done recently.

As an ebay buyer, all I really want is an honest description and it sounds like you certainly provided that to him. With this older gear you can have all original with generally less than perfect performance, restored properly with good performance, or (unfortunately too common) a hack job restore and poor performance. For his $900 it sounds like he got a very nice looking NC-183D that could be restored, if he wishes, to pristine electrical performance. Probably a bit of buyer's remorse or as commonly referred to in marketing, cognitive dissonance which is very common after buying something "pricey".

The first real piece of vintage gear I bought was an NC-183D back in 1983. It looked great but performance was pretty weak and at that time vintage gear wasn't collectible so it cost me all of $10. That receiver had more WAY out of spec resistors than anything I have seen since. I remember in particular the 47K screen dropping resistors, of which there were many, were all consistently within 10K of 270K and I wondered at the time if a receiver could have been built and gotten through National quality control with mis-marked resistors. There is a note in the owner's manual from the original purchaser noting poor gain and he changed all of the band switched gain adjust resistors so that it ran full gain on all ranges. Once I went through and changed all of the out-of-spec resistors it really came to life except for the top frequency range. I finally traced that to inadequate oscillator injection on that range due to a damaged coil.

I don't recall ever measuring the sensitivity on my 183D but I do know that peaking the trimmer causes a definite increase in background noise on all ranges so in my fairly quiet rural location it has all of the sensitivity and gain it needs. I am still amazed how well the calibration holds across the bands on my NC-173, 183, and 183D receivers.

Rodger WQ9E

My first real "communications" receiver was an NC-173. I used it to replace the converted AM broadcast receiver with SW and LW bands with homebrew add-on BFO.

I used the 173 for several years until I acquired a 1935 vintage HRO in the late 60's. The HRO was so much better that I ended up giving away the 173 to a friend, just to get it out of my way. But the HRO didn't perform well on 10m, and was made before the advent of 15m, so I had no bandspread coil for that band, and besides, it used the same coil for 15 as it did for 10. The HRO was superb for 160-20, but it was built before the technical design for 10m reception had been perfected.

The 173 was sensitive enough on 15 and 10, fairly selective, but its frequency stability and image rejection SUCKED.

Quote from: K3ZS on June 12, 2009, 04:27:45 PM

They added the extra RF stage in the NC-183 to improve image rejection. The 183 image rejection still sucked. Didn't they use the NC-173 on the Kon-Tiki expedition. Maybe that's why they had trouble with communications.

There was a nice article in an early ER about the NC-173 used on the Kon Tiki and it got a salt water bath but did come back to life; not too shabby.

The image rejection on the NC-183D is pretty good since it operates with dual conversion (1720 Kc first IF) on the higher bands. One of my NC-183 receivers would make an awful crackling sound after a few minutes of warmup; it turned out to be an air dielectric temperature compensating cap across the oscillator section of main tuning cap was shorting. I guess this cap was an attempt to address the drift issue but it sure had me searching for awhile to find the problem. It appeared to be factory given the mount for it was integrated with the main variable cap frame. It looked a lot like an old style neutralizing cap.

Rodger WQ9E

"a-hole from NY".. there are a few there. I guess that was one of them. Maybe it's a perception due to the different culture.

If someone wants it or thinks it's the best, the willing buyer is the key and they have a right to complain -only- if they did their research and were mislead. Not radio related, but I'm about to lay out the bucks for a pair of Wesco "boss" boots made to measure. Sure there are way cheaper engineers' boots out there, but there's no guarantee they'll fit well or be tolerable to wear all day. (I'm an engineer, I should have the boots right?)

High-dollar stuff is a long term item, like that radio will be for that New York gentleman. Same as the person who lays out $20K for a restored car, or $3000 for a sweet-looking Zenith shutter-dial console that "plays well". It's not necessary, but it's what they want. People have spent more on stupider stuff, like $5K worth of 24" rims for the proverbial $300 chevy caprice. His money..

The guy probably felt the same about the particular radio, was in love with it or really needed it somehow, but he didn't take the time to ask about it and discovered its design imperfections after receiving it due to his own ignorance of the model itself and is trying to blame the seller.

I never try to be Mr. Style. Huge useless drain on the funds. Hate the pointy and the silly/weird too. It's like the designers of so-called fashion are trying to see who can con the sheeple into buying the most outlandish stuff.

Not sure about being in it? deep, I just want to try the best for once and the choice is conservative. I don't like cupcakes either. Leave them for the ladies. I like pi.

I had a 183D for a while so I can opine that whatever version, it was a great SWL radio for fun back in the day, but it is not the best model if performance expectations are high. The HRO Sixty is a much better radio and the only reason I do not have it any more is it was traded for an R390A. I eventually traded the 183 for an SX-28 and I am more pleased with that. Maybe it's subjective. Either one can make good use of a 10dB preamp for the high end if a bandpass/BC filter is placed before it.

I'm surprised of the comments about this double-conversion receiver with 2 r.f. stages being insensitive and having poor image rejection.

When I overhauled my dad's NC-183D I did a full re-alignment including the optional plug-in FM adapter. I don't recall a sensitivity problem. I did use my r.f. swizzle stick (brass tip one end and ferrite tip on the other end) to prove that the coils were perfectly aligned and tracking perfectly.

In a couple days I will be visiting my dad’s QTH and will dig into that receiver's file folder to see if I recorded any sensitivity and image rejection numbers. He has a HP-606A signal generator so maybe we will make some new measurements. I will report back.

I was at my dad’s QTH today and read through his NC-183D file folder – no notes on the sensitivity or image rejection from when I overhauled and re-aligned the receiver a few years ago. So I fired up his HP-606A rf generator and did some testing.

The receiver has plenty of overall gain and is capable of driving the speaker with noise to a loud level without an input signal on all bands. I recorded the level at which you could begin to hear (but not straining) the 1000 Hertz 30 % modulated signal AM signal.

For image rejection I used the S-meter at S-4 or S-5 for reference.

Results:

Frequency Audible signal image rejection
3850 <0.1 uV 75 dB single conversion
7200 0.2 uV 118 dB (compression?) double conversion
14200 0.35 uV 93 dB double conversion
29000 0.35 uV 58 dB double conversion
51000 0.5 uV 40 dB double conversion

The performance looks o.k. to me.

Brian,

Sorry to hear about the -606 transformer. I will look at my manual to see what would be the specs. I don't know off the top of my head of sources for exact replacement except to canabalize.

I have been looking at NC-183D ads in the old ARRL handbooks. My NC-183D manual had no specs! However it did state that the input impedance is an average of 300 Ohms. The input is a balanced link input. I have a 50-to-300 Ohms balun and may try that next time I am at my dad's QTH. That should give a voltage step-up and may improve upon the numbers. I will do the proper 6 dB S/N measurement.

The late 1950's ads quote 2 different specs for AM sensitivity:
1. "3.5 uV for 10 dB S/N"
2. "1.5 uV for 6 dB S/N, entire range".

Brian,

I have my HP606B manual out here. The power transformer has three secondaries with no center taps. Each secondary runs a bridge rectifier.

I feel that those early silicon diodes were unreliable then and especially now. Shorted bridge diodes is probably what took out the transformer; was the fuse size proper? Replace all of the diodes at any rate (if you get a transformer) by putting in (2) 1N4007's in series in each leg of the 3 bridges. No matter how high the PIV rating of replacement diodes, if you just use one per leg and they short, things will cook.

The 3 windings are 315 V, 220 V, and 31 V. Remember this is my 606B manual. There are 3 series regulators then giving +300 Vdc, -200Vdc and +25.6 Vdc. I don't know the currents, but there is a fuse in the +300 V supply bridge from the
(-) output to ground - F2 1/4 Amp.
Gives you an ideas on the 315 v winding current.

The bridge for the -200 V supply (220V winding) has a fuse from the
(-) output to ground - F3 1/8 Amp.

No fuse in the +25 V volt supply.

Unfortunately an odd combination of secondary windings. Well there should be junker 606s around. The part number for the transformer in the HP-606B is 9100-0375, manufacturer coded as HP.

Recent receiver specs.

Icom 756

Sensitivity (typical):
Frequency range (MHz)   SSB, CW, RTTY (at 2.4 kHz BW)   AM (at 6 kHz BW)   FM (at 15 kHz BW)
0.50-1.799   --   13 µV   --
1.80-27.99   0.16 µV *1   2 µV *1   --
28.0-29.99   0.16 µV *1   2 µV *1   0.5 µV *1
50.0-54.0   0.13 µV *2   1 µV   0.32 µV *2

10 db S/N for SSB, CW, RTTY and AM, 12 dB SINAD for FM
*1=pre-amp 1 is ON, *2=pre-amp 2 is ON

Yaesu FT-2000

Sensitivity:

SSB/CW (2.4 kHz, 10 dB S+N/N)
2 uV (0.2 - 1.8 MHz)
0.2 uV (1.8 - 30 MHz)
0.125 uV (50-54 MHz)

AM (6 kHz, 10 dB S+N/N, 30% modulation@400 Hz)
3.2 uV (0.2 - 1.8 MHz)
2 uV (1.8 - 30 MHz)
1uV (50-54 MHz)

FM (BW: 15 kHZ, 12 dB SINAD)
0.5 uV (28-30 MHz)
0.35 uV (50-54 MHz)
(with RF2 amp on)

Older Receivers

Collins 51J-4

Band 1 - Less than 15uV gives 10 dB s/n
Bands 2 to 30 - Less than 5 uV gives 10 dB s/n

SP-600

The m-c-w sensitivity of the receiver to any signal within its tuning range is two microvolts or better, for a signal-plus-noise to noise power ratio of 10 to one at the receiver audio output terminals.

Quote from: w5hro on June 20, 2009, 10:51:53 AM

I'm talking about injecting a signal to the antenna input and looking at the last IF stage's output before detection using a spectrum analyzer. You want 20dB above the noise floor at that point all the way down to 1uV for a tube receiver and less for SS receivers. If you are only 10dB above the noise floor it will often get covered up when a noise source is injected and that floor rises.

For what purpose? Comparing receiver sensitivity? Alignment? Some other reason?

Not sure what the purpose of looking at the last IF stage with a spectrum analyzer might be. If you do this for the purpose of measuring ultimate S/N, you're overlooking at least three whole stages of the receiver - the detector, first audio, and audio output - any or all of which can contribute significantly to the noise level and thus the ultimate S/N ratio. Every test method I've ever seen, or done, uses the audio output from the receiver. Anything else, in my view, is neglecting many variables and is tantamount to comparing apples to locomotives.

Moreover, it's relatively hard to measure noise power in a spectrum analyzer but relatively easy using an audio power meter. Also using a spectrum analyzer with a 50 ohm input impedance will load down the stage considerably unless you're using active high impedance probes (which still could skew results due to introducing extra capacitance in teh circuit).

A 10 dB S/N is as good as 20 dB *for the purpose of comparing receiver sensitivity*.

Quote

Everyone whom I have ever worked with has used 20dB as the standard.

I've been doing this professionally (repairing, using, and spec'ing/purchasing high performance receiving equipment for my employer) for well over 25 years and very seldom have heard of anyone using a 20 dB standard except for VHF/UHF gear. In any event, for HF receivers, NF is relatively unimportant as atmospheric noise generally overcomes any internally-generated noise (unless you have a receiver with a crappy synth).

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Forget what most of the comm service monitors and amateur gear specs tell you because you can’t see the whole picture unless you’re using one of the really high-dollar modern monitors which has a built-in spectrum analyzer, tracking gen, etc. You also need a noise source.

Pretty sure the ARRL lab and manufacturers use professional grade test equipment to run their figures. For my own purposes I use a late model Agilent 8935/E6380A which has all of that, and more (except the noise source), as well as other sundry test equipment including a HP8640B, HP 3577A network analyzer, Tek 2460 scope, and a Tek 495P spectrum analyzer.

You need a noise source to measure noise figure, but not to measure S/N, S+N/N, or SINAD.

I did a study today on my Hammarlund HQ-120X receiver tuned to 3900 kHz. It’s i.f. bandwidth is comparable to the National NC-183D. I measured sensitivities at 6 dB and 10 dB audio S+N/N plus a subjective level of “just audible but comfortable audio tone“ that I used in one of my previous postings on my dad’s NC-183D "sensitivity" and image rejection measurements. The idea here is to provide a comparison to my previous test and a feel for real world minimum readability performance compared to the 6 or 10 dB S/N sensitivity numbers.

I used a HP8640B r.f. generator and the ac voltmeter function of a HP331A distortion analyzer. The speaker was set to a comfortable listening level. The ac voltmeter was connected to the speaker terminals.

I could hear the 1000 Hertz tone as low as 0.13 uV at the just perceivable level. I found that the 0.2 to 0.3 uV level as probably like what I recorded as “just comfortable audio tone” in my sensitivity levels at my dad’s QTH.

The levels for classic AM 30 % mod. sensitivity were:
6 dB S/N = 0.69 uV
10 dB S/N = 1.1 uV.

Next I played a tape recording of a QSO into the r.f. generator; modulation level adjusted with a scope first. The QSO was of 2 stations received here with excellent signals. Both stations were just at a 5 readability with 0.4 to 0.5 uV. (a few dB less than the 6 dB S/N sensitivity level).

For a more comfortable listening level though I picked 2 uV. The S/N at 2 uV measured 17 dB.

When I test HF receivers, this is the kind of results I usually get; I can hear the 1000 Hertz tone in the noise floor around 0.14 uV. If I get that, I don’t bother actually finding the 6 dB or 10 dB S/N level. We are used to knowing that we can hear a signal less than 0.1 uV on SSB mode but the same noise figure receiver on AM mode boils down to a 1 uV 10 dB S/N sensitivity number. So this is not poor, this is a good AM sensitivity; you have to get used to the correlation.

We are also used to the sensitivity numbers being specified in a 50 Ohm system. That is what I have been quoting. For a receiver with a 300 Ohm input impedance like the NC-183D, to put the same r.f. power to the receiver input, you need to apply more voltage by the ratio of the square root of 300 Ohms/50 Ohms which is 2.45:1. So for the NC-183D, the comparable sensitivity performance would be 2.7 uV (1.1 uV x 2.45) for 10 dB S/N. Agree?

Quote from: w3jn on June 20, 2009, 01:27:30 PM

If one is interested in how ARRL conducts its testing of equipment, the measurements that are used, etc., the ARRL Test Procedures Manual is here for all membership to review:
www.arrl.org/members-only/prodrev/testproc.pdf

Brian,

I see what you are doing. Somewhat of a bizarre test, but it seems like a test to spot i.f. deficiencies, AVC noise problems.

I used to work on an ADF receiver that had an extreme amount of reserve i.f. gain and AGC action. When there was a bad crystal in the i.f. filter, the i.f. amps were able to completely counteract this loss of gain. Old radio designs would have had low output pointing to the i.f., but this receiver had normal output but it appeared to have low sensitivity – poor S/N for the test signal in, pointing to the front end. The problem was that the signal was greatly reduced in the i.f. due to a fault.

I stand that my dad’s NC-183D has good sensitivity and good image rejection. It is still unclear to me if the one you sold had an undiagnosed problem in it. The NC-183D also has the capability of high overall gain and has an AVC amplifier in it and the AVC bus is applied to 5 remote cut-off tubes, all 6BA6s, in the 2 r.f. stages and 3 i.f. stages, an unusual amount of AVC control range capability. This scheme could do the same thing as the ADF receiver I used to work on; a fault of increased loss in the i.f. somewhere gets hidden. Again I am unclear if what you are seeing is a component fault or a design problem. Your test seems to be for determining if a stubbornly “poor sensitivity” receiver‘s problem is due to poor front end noise figure or is due to loss in the i.f. which degrades a good S/N after the front end.

Do you still have another NC-183D or are you talking about the one you sold?

What is the level of the noise source you inject (microVolts for what BW.)?

What is the tube type you propose to use in the i.f. to fix the problem? My opinion is from what I have to go on, that instead of modifying the whole receiver, find the hidden fault.

You have quoted a 20 dB S/N ratio for a 0.1 uV signal (-127 dBm) in your work application. This implies a floor of -147 dBm. This implies a BW of 500 Hz with a 0 dB NF or a BW of 316 Hz with a 2 dB NF. Is this the work application bandwidth and NF?

I guess I still don't understand the purpose of this test, and what it tells you.

Quote

It’s not really a bizarre test because you are looking right at what the AM detector or product detector is going to be seeing minus the audio. It’s the way I was trained years ago because that level is very important. I worked in R&D helping to design receivers, etc.

Why is adding noise important? Adding noise really tells you nothing about the S/N performance of a receiver. The wideband noise will be detected by the AVC detector, and reduce the gain over and above what an unmodulated carrier would produce.

Quote

I don’t have a noise source at the moment, but what I always used was just a simple random noise generator. You basically just increase the input level until the noise floor peaks. There is a point where it shouldn’t go any higher.

So you're measuring AVC action?

Quote

As you can see when the noise floor is increased (Pic B) via a noise generator the signal is almost eliminated and in some cases the floor will be higher than the actual signal and it will eliminate and override the signal completely. This is why the IF output signal should always be 20dB above the noise floor before the noise is applied. As long as the signal is a least 10dB above the noise floor after the noise is applied everything should be ok. That is what everyone shoots for.

Quote

Right now on my NC303 I'm just using an inexpensive spectrum analyzer to see the IF output level. I'm using a Heathkit IG-42 lab generator for the signal. As long as the IF output is around 20dB higher than the noise floor on the bench without a noise source it should be ok.

OK for what? What is this telling you? How is it that you're not loading down the last IF stage with the SA?

Quote

Also, when you hook up an antenna the noise floor does in fact rise.

Which is why S/N measurements aren't done with an antenna (nor noise source) connected. All of that is external to the receiver and unless I'm missing something here, is immaterial to a S/N measurement.

Quote from: Steve - WB3HUZ on June 20, 2009, 07:32:46 PM

Yea, really. I put a signal into the receiver and get a certain SNR. I add a noise source and the SNR decreases. No kidding. The sun will come up tomorrow too. :P

BTW, I've measured receiver sensitivities using 3 dB as the SNR on professional grade/military receivers that cost $250k. And yes, signals could be seen, heard and easily detected at such an SNR. The SNR at the last IF is no different than the SNR after the detector. Only in the case of a square law AM detector will the input and output SNRs differ.

Quote from: w5hro on June 20, 2009, 07:38:38 PM

Quote from: w3jn on June 20, 2009, 07:22:54 PM

I guess I still don't understand the purpose of this test, and what it tells you.

Your right, you don't understand and its a no brainer :P :P :P

Then it ought to be easy to explain...?

Quote

No, what I'm saying is everything from the very first RF amp stage to the very last IF amp stage is part of the receivers overall sensitivity. If the IF output signal is nonexistent when a 1uV signal is applied to the first RF amp stage then the sensitivity is very low. Forget the AVC, bypass it during the test.

All true, but why all the machinations with the noise generator, and using a SA at the last IF? The detector in the radio and measuring thru to the last audio will do a better job of giving S/N, as reading just a level on a SA doesn't tell the whole story, particularly if there's LO phase noise riding on the signal.

It's a super secret measurement technique unknown even to premiere receiver and spectrum analyzer manufacturers like Agilent, DRS, Watkins-Johnson, Racal, Rhode Schwartz, etc.

Quote from: w3jn on June 20, 2009, 08:23:23 PM

Quote from: w5hro on June 20, 2009, 07:38:38 PM

Quote from: w3jn on June 20, 2009, 07:22:54 PM

I guess I still don't understand the purpose of this test, and what it tells you.

Your right, you don't understand and its a no brainer :P :P :P

Then it ought to be easy to explain...?

Quote

I asked

Quote

Are you talking during alignment, or rated sensitivity?

You replied

Quote

As this didn't answer my question, I then asked

Quote

For what purpose? Comparing receiver sensitivity? Alignment? Some other reason?

You replied

Quote

As this still didn't answer my question, I asked

Quote

I guess I still don't understand the purpose of this test, and what it tells you.

Why is adding noise important?

You replied, with an ad-hominem attack

Quote

Your right, you don't understand and its a no brainer Tongue Tongue Tongue

Making one last attempt, I asked

Quote

All true, but why all the machinations with the noise generator, and using a SA at the last IF?

And you repllied, never answering any of my questions

Quote

I did explain it in great detail and it does not take the place of normal receiver alignment tests as specified per the receiver's manual. However, there are things you cant see unless you probe around and look at various points within the receiver and the noise floor and IF gain are very important.

You explained what you did, but never why, nor what it's measuring, nor why it's important.

Tests, schmests, the real "proof of the pudding is in the eating ! ! ! !"

no arguments to the accuracy of some of these tests and the gear used to make them, but...................... the best test equipment is somethain that we all come with as standard equipment! Our ears and eyes! ::)

I have seen many times that the published specifications dont tell the whole story.
If a receiver is deaf or lame, you will notice it the first time you set in front of it and use it. If you are a knowledgable operator, you will take notice of it immediatly!!
If a receiver is "HOT" you shoud also take notice of it as soon as you set in front of it. If a receiver is good or bad you will usually take notice of it rather quickly. If you dont, I question your prowess an an operator!!

I have a 183-D and have never noticed anything out of the ordinary about it OTHER than it's rather good sensitivity on the higher bands. (noticably better than the plain 183)

however 2 receicers come to mind for this post (I have 18 of them) An NC-300, and an SX-28. The NC-300 has good overall sensitivity,(even on the higher bands) but a piss-poor noise figure. The internal noise from the 6BA7 first mixer is absolutely horrible. A 40+ incoming signal never completely quiets it out.

The SX-28, with one small modification has the absolute best signal to noise ratio of any receiver I have ever owned!! that modification is just to swap out the 6SK7 first IF amp to a 6AB7, It is a little lacking on sensitivity on the highest band (what do you want for a 1939 receiver) But the incredible signal to noise ratio makes it my favorite receiver for the lower bands!! (And receiver of choice)

The bottom line in my guts is to use all of your best test equpment to align them, but use your eyes and ears to evaluate their performance afterwards!!

The Slab Bacon

Hmmmm................ I guess I'm "the other guy" ;D

Comparing a 183-D to an HRO-60 is a pretty unfair comparison. Kinda like comapring it to a R-390. I still maintain that a 183-D is a pretty descent receiver for what it was. As far as sensitivity goes one never knows what to expect in the real world. A while ago I got the surprise of my life while repairing an HQ-140 on the bench: With nothing more than a clip lead hooked onto the antenna terminal, I was listening to the goof balls on 11m! I thought that was pretty good sensitivity for a radio of that era and quality level. (what it cost back then)

Many of the published specifications were published to keep up with the "pecker matching contest" race between equipment manufacturers. Many of them mean a lot less in the "real world". If you spent a little more time operating some properly maintaind versions of this stuff your opinions might change rather dramatically.

Quite often the ones that put up the biggest arguments here are the ones who spend the least time at the mike.

the Slab Bacon

So, how would injecting noise make it easier?

Quote from: w5hro on June 22, 2009, 10:06:06 AM

Quote from: Steve - WB3HUZ on June 22, 2009, 08:51:46 AM

So, if your test is 20 dB SNR w/o the noise source, what is the purpose of later adding the noise source?

Because its very hard to get 20dB above the noise floor without noise injected on the higher bands like 10 meters on many of the old tube receivers. The losses within the long wires, the big rotary switches and the limitations of the tubes themselves make it really difficult even with the better receivers. If you can get it close then it wont matter and you don't need any noise source.

Quote from: The Slab Bacon on June 22, 2009, 01:17:46 PM

The HQ-140 is a real sleeper. It has great AM selectivity, better than NC-183 or HQ110 (I have all of them), and good sensitivity and relatively cheap on the used market. It doesn't drift much and has no paper capacitors (as far as I could tell) to go bad. But, it is single conversion and has poor image rejection above 20M.

Quote from: w5hro on June 22, 2009, 03:06:19 PM

That is exactly my point, there has always been the impression that the NC183D is a great receiver and compares to other rcvrs like the HRO60, etc. The problem is when you finally get one you quickly find out that it does not, it’s not even close.

Strange, I have an early model 183D and 2 later model HRO 60s and all compare in the sensitivity and selectivity departments. The 60 are better at rejection but not by a great amount. There isn't enough sensitivity difference to notice for me.

I align/test by using an isoTee and hooking the antenna to the direct connection in the Tee, then the receiver to the isolated connection. In this fashion you read the sensitivity as a result of local conditions at the time.

My 183D will hear a 1 Kc signal down to ~ .5 uv on 75 and 160 to ~.7 on 10 meters. I do have a quite location here and can discern signals much better than my brethren in a city/suburban environment.

This is the second 183D for me, the first was a much later model and a bit more sensitive. Adjacent frequency rejection was/is near the same as the 60 and aligned corectly with the AGC adjusted right overload is not a problem for my ears.

This doesn't square with your previous statement. If you don't need it when the level is less than 20 dB, then why you now say you do need it. Which is it?

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Because its very hard to get 20dB above the noise floor (without noise injected) on the higher bands like 10 meters on many of the old tube receivers. The losses within the long wires, the big rotary switches and the limitations of the tubes themselves make it really difficult even with the better receivers. If you can get it close then it wont matter and you don't need any noise source.

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There is in fact a noise floor which increases when you connect an antenna to the receiver. If the IF signal is too low the noise floor will cover the signal up..

This had nothing to do with the sensitivity of the receiver. Noise external to the receiver will always be present. But it will not be the same when considering location and the antenna in use. The reality is that if you hear an increase in the noise on your receiver when you connect the antenna, then your receiver is sesnsitive enough. So, you are correct in that a receiver may not hear signals as small as the stated sensitivity when connected to an antenna. So what? That's a given. But that's because over most of HF, the SNR of a received signal is defined by noise external to the receiver. It matters little what the level is at the output of the final IF. That level could be huge, but if the SNR is negative, guess what, the SNR out the output of the RX is negative. Injecting noise into a receiver front-end when making sensitvity measurement tells you nothing about the receiver's performance. You can arbitrarily change the so called sensitivity be changing the level of noise.

Quote from: w5hro on June 22, 2009, 04:43:23 PM

Quote from: Steve - WB3HUZ on June 22, 2009, 03:48:33 PM

So, how would injecting noise make it easier?

It would not make things any easier, but if the output level is too low it will help you see if it’s still at least higher than the noise floor when noise is injected. If it’s at least a few dB higher with a certain uV level injected into the antenna input then the detector should be able to detect it at that uV level when the antenna is connected, etc. I like to visually see what’s happing on the analyzer. You could probably just listen to the tone test with noise injected, but to me having a visual reference is better.

One of the receivers brands I have never gotten into are the HQ’s. I always wanted to try a couple, but I’ve been so happy with my HRO60 and now my NC303 that I just don’t care anymore. Those two are good enough for me and I doubt if I will ever buy another receiver again unless it’s for parts.

The cathode resistors in the HRO-60 aren't there for bias (since bias is applied from the AVC circuit) so much as they are for degeneration purposes. This will necessarily reduce gain (not a big deal, since there's a ton of gain available to begin with). The real culprit in both receivers are the 6BE6 mixers. Noisy, and prone to IMD. Nevertheless, either receiver in my experience works satsifactorily.

The reason I "picked apart" your posts, as you put it, is that I repeatedly asked you questions which you were unwilling or unable to address. I still don't know why one would *inject* noise when checking S/N. Calculating noise figure, yes. Figuring S/N - can't think of a reason.

I visited my dad’s QTH again today and did some more measurements on his NC-183D. I did some proper AM sensitivity measurements with an ac voltmeter connected to the speaker for AM 30 % 1000 Hertz modulation 10 dB S+N/N measurements on 160, 80, 10 and 6 meters. An image rejection measurement is added for 160 meters. Any image rejection measurement over 80 dB I now put as >80 dB since the generator level is probably exceeding the r.f. dynamic range of the receiver for those ratios.

Not stated in my previous posts, the r.f. generator 50 Ohm output was run via an RG-8 cable directly to the receiver input terminals, one antenna balanced input terminal was tied to chassis ground, the receiver selectivity was set to maximum bandwidth and the tone control was set to highest frequency response.

NC-183D Updated Sensitivity and Image Rejection Table 6/24/09

Frequency Audible AM sensitivity image rejection
Signal 10 dB S+N/N
1900 1.5 uV >80 dB, single conversion
3850 <0.1 uV 1.0 uV 75 dB, single conversion
7200 0.2 uV >80 dB, double conversion
14200 0.35 uV >80 dB, double conversion
29000 0.35 uV 0.95 uV 58 dB, double conversion
51000 0.5 uV 1.8 uV 40 dB, double conversion

Image Rejection Specifications from the 1958 ARRL Handbook catalog page 35 ad:
“Image Rejection (At high end of band)”
“Band Image ratio”
“A 40 dB” (This would be 55 MHz.)
“B 65 dB” (This would be 31 MHz.)
“C 80 dB” (This would be 12 MHz.)
“D 80 dB” (This would be 4.4 MHz.)
“E 80 dB”. (This would be 1.55 MHz.)

Some of my measurement equal or exceed these specs and some don’t quite meet them.

The 1955 ARRL Handbook catalog page 33 ad merely states:
“Image rejection
Signal/image ratio better than 55 db at 30 Mhz.”

This is met.

Brian,

The sensitivity and image rejection numbers I have measured are reasonable for the design. You stated particularly bad image rejection on the lower bands which is odd since the lower frequencies should fundamentally have the best image rejection performance. Your particular NC-183D must have not been properly aligned or working properly.

Did you truly do a full alignment on that NC-183D or not?

Brian,

The starting level for the image rejection was probably the point at which I found the 10 dB S/N sensitivity or close to it. I use the s-meter reading for reference and I remember the S-meter read S-7 in one case. On the NC-183D S-7 is low level, not the 13 uV S-7 is supposed to be today.

Well it seems you did not realign the one you just sold and now it sounds like you are actually describing IMD products rather than poor image rejection. That would explain why we are going around in circles over the so-called poor image rejection rap.

The image for the 40 meter broadcast band is in the 25 meter broadcast band. I suppose there can be a cumulative problem of both IMD and signals at the image frequency but I believe now that the proper labeling of your performance complaint of the NC-183D would be due to the rf dynamic range being exceeded by the strong 40 meter broadcast signals.

I have never noticed this with what listening I have done on our NC-183D. But this is one of the possible down-sides of a 2 r.f. stage receiver. The way to overcome this IMD problem is to simply put a 20 dB pad at the antenna terminals or switch to a more minimal antenna under those conditions, rather than modify the whole receiver.

Please read what I wrote, especially the part where I wrote "for any given bandwidth." Of course, if you reduce the bandwidth, you can improve the SNR. But there is no amplifier used in amateur grade HF receivers that improve the SNR. If the signal is above the noise floor of the receiver (really just the RF stage), then the SNR is defined solely by noise external to the receiver. There is no way around this. This has been understood in the HF world for many decades.

From "HF Communications, A Systems Approach" by Nicholas Maslin:

5.1.7 Condition for External Noise Limitation Consider an incoming signal of mean power S at the antenna and an incident noise power N_o in a 1 Hz bandwidth. Let the (omnidirectional) antenna have efficiency n at the frequency considered. After reception by the antenna the signal power is nS, the noise power is nN_o and the signal-to-noise ratio remains S/N_o. At the receiver input other losses such as those caused by imperfect matching reduce the signal strength to n'S and the noise power to n'N_o. As the signal passes through the receiver, the receiver noise power N_r, per Hz contributes to the total noise power and the final signal-to-noise ratio becomes n'S/(n'N_o + N_r) (5.4) If conditions are such that N_r << n'N_o (5.5) then the signal-to-noise ratio at the receiver output is effectively the same as at the antenna. The inequality (5.5) can be expressed in logarithmic units as n' + F_a >> F_r (5.6) where n' dB is the loss in the receiving system, F_a is the effective antenna noise power factor (in dB) and F_r is the receiver noise figure (in dB). Inequality (5.6) is the condition that the receiving system performance is limited by external noise. Provided that (5.6) is valid the receiving antenna efficiency is unimportant. The condition in (5.6) is almost always true, except when using a poorly matched receiver at the lowest frequencies in the HF band under conditions of low external noise.

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Quote from: Steve - WB3HUZ on Today at 10:35:36
Over most of HF, the SNR of the signal (for any given bandwidth) is determined external to the receiver. A receiver cannot improve this SNR, whether it has a tuned front-end or not.

Unfortunately that is incorrect. Both the RF stages and the IF stages have their own separate signal-to-noise ratios. Good filtering that produces more narrow bandpasses increases the RF energy level being produced by the stages and better attenuates adjacent frequencies and noise outside of the designed bandpasses. One of the results is a better signal-to-noise ratio. That is one of the reasons for using analyzers such as the HP noise figure meter when designing RF and IF amplifiers. You do it both without and then with the noise source applied.

The AM Forum

THE AM BULLETIN BOARD => QSO => Topic started by: w5hro on June 12, 2009, 01:55:31 AM