NC-183D engineering error? HRO-60?

[KM1H]

The pictures you posted in this thread are the ones that take up excessive bandwidth, they take forever to download and then really show nothing that a few words wont accomplish. Others post the small jpg's as attachments that can be clicked on to expand. Those download fast.  Learn how to resize your photos for attachments.

Im stuck on dialup until my T-1 microwave radio comes back from repair. DSL isnt even an option in this area due to Verizon putting the fiber upgrade on hold and I dont have cable. That limits me to what a US Robotics 56K external modem can eke out of a poor phone line that runs around 33K tops and sometimes folds back to 19.2K.

Its part of the benefits of rural living on top of a hill and at a dead end road.

I'll appreciate your cooperation.

Carl
KM1H

[KD6VXI]

The pictures you posted in this thread are the ones that take up excessive bandwidth, they take forever to download and then really show nothing that a few words wont accomplish. Others post the small jpg's as attachments that can be clicked on to expand. Those download fast.  Learn how to resize your photos for attachments.

Im stuck on dialup until my T-1 microwave radio comes back from repair. DSL isnt even an option in this area due to Verizon putting the fiber upgrade on hold and I dont have cable. That limits me to what a US Robotics 56K external modem can eke out of a poor phone line that runs around 33K tops and sometimes folds back to 19.2K.

Its part of the benefits of rural living on top of a hill and at a dead end road.

I'll appreciate your cooperation.

Carl
KM1H

Having lived on top of a mountain, in the middle of nowhere, using nothing more than wifi with a yagi and parabolic reflector, I can sympathize with Carl.

But, I find it funny that people who are stuck in the backwardness of non-high speed internet always want everyone else to post low bandwith.

Carl, we'd appreciate it if you'd move.  Somewhere with high speed internet.

Thanks for your cooperation

(tongue firmly in cheek).

--Shane

[KM1H]

What, move and give up a kick butt location Shane? Its worth 10-20 dB over the bottom dwellers.

The only real bad thing is that no 3 phase service is available to build amps with tubes containing five zeroes as you do

It will probably be several weeks before Im back on microwave so bear with me.

Carl
KM1H

[Tim WA1HnyLR]

I have issues with the use of 47K dropping resistors for the screens. I had the use of a NC 183-D as in douche! when I was out in Sorryexcuse NY. I found that the receiver overloaded on very strong signals. When backing down on the RF gain pot the signal levels would drop off then drop off very rapidly like falling over a cliff.The S meter did not take much signal to get a reading but would pin easily.Take a meter and measure the screen voltage with no signal. Then measure the screen voltage with a strapping signal. The screen voltage was much higher . What is happening is the"Shoveling 10-1000 Against the Tide syndrome. The AVC voltage comes screaming down the line telling all of the gain controlled stages to come down in the gain department. The screen current falls off causing the voltage drop across the 47K resistors to decrease there fore causing the screen voltage to increase and therefore turning up the gain. The fix was simple : I borrowed the 105 volt  source from the 0B2 regulator. I fed that voltage through 4.7K resistors (value not critical) to the two RF stages and the IF stages. The 183-D behaved like it should after that. I could monitor my own signal without overload. The RF gain control worked smoothly. My own signal would pin the S meter. Other signals indicated were more believable . The meter behave more linearly.I have done these screen voltage mods to my Hallicrafters SX-62 and the Collins 75A4. It has resulted in better performance in dealing with a wide range of signal strengths. This technology is applicable to any high B+ receiver with improvement. Try it, you'll like it
Tim WA1HnyLR

[N2DTS]

In my homebrew receivers, I run the B+ at 150 volts, but have the screen dropping resistor at the mixer at 470k, the 1st IF at 47k, and the 2nd IF at  100K.

Maybe I should try the screens at the 105 volts (regulated) that the LO runs at, and smaller values of screen dropping resistance.

The unamplified agc seems to do fine on everything other than when I transmit, but I am always open to improvements...

Brett

[WA1GFZ]

Years ago I worked on the TPX47 IFF RADAR. It used a high value screen dropping resistor with a cap to ground at the screens. When a radar pulse came through the screen current would discharge the cap to a low value. This killed the IF gain until the cap charged up again just in time for the next closest possible pulse.
This trick was used in the six IF stages and was called antijam.
I wonder if communication receiver designers were that smart dealing with static.

[W1VD]

Since the receiver is still on the bench it was easy to have a look...

On the 1st and 2nd RF, 1st, 2nd and 3rd IF, with no signal into the receiver, the screen voltages are close to the book values in the 105 - 115 VDC range. With an input of -50 dBm (1 mV) the screen voltages rise to the range of 135 - 158 VDC. (Note there is less power dissipated in the screen resistors under this condition).

I think Tim is referring to on frequency BIG signal overload of the i-f and audio stages because of the rising screen voltage vs. increased negative grid bias. This may well be a valid concern. However, this is a different situation than being tuned to a weak signal up or down the band with little or no AVC action. If the AVC isn't activated the tubes are operating, presumably, in their most linear region and one has to accept the associated level of IMD, cross modulation, image rejection etc.

Frank...interesting tidbit. There's quite a difference on how well these various receivers handle static. Could be an interesting study unto itself...             

[The Slab Bacon]

...interesting tidbit. There's quite a difference on how well these various receivers handle static. Could be an interesting study unto itself...             

Jay,
      You said a mouthful there! ! Now that would make one hell of a study / topic of discussion!   

[N2DTS]

As an experiment, I tried various things with the screen supply to the mixer and IF amp tubes on the homebrew last nite.
The results were nil, I tried 4.7k ohms off the regulated 105 volts, and since the voltage was higher on the screens, the gain went up into the saturation point where strong broadcast signals on 40 meters maxed out the S meter and the additional signal strength agc voltage did not show on the S meter.
(signals went up to 50 over and stayed there).
Turning the IF gain down (cathode pot in the 1st IF) would lower the S meter and signal strength variations were back.....maybe I have to increase the cathode resistors...

Normal screen voltage at the tubes was around 50 volts, with 150 on the plates.
With the 4.7k resistors off the 105 volts, it was around 100 volts.

I even ran the mixer screen off the TR relay 15 volts and the gain was down but the thing worked fine!.

I then got the adj power supply out and put 40 to 50 volts on the screens, with the 4.7k resistors, but did not notice any real difference between that and stock settings.

I need to do more work, hook the scope up to various IF points and see what stage is saturating with real strong signals, if one is, I dont hear any distortion, but the S meter stops moving up past some point in signal strength.

I also put the BFO on, set it to hetrodyne at about 30 Hz, and tried adding a .47uf cap to the agc line to slow it down, I heard no difference in the audio. I then added another .47 and heard no change, the S meter was very sluggish, but no audible change in the 30 Hz or broadcast audio quality.

Not sure what is up there....the normal AGC time constant seems about like medium on the R390a, looking at the meter.

Some day I will pick up a distortion analyzer and run amuck with it...

Brett

 

[Jim, W5JO]

The RF stages and the second and third IF stages of this receiver are grounded cathode.  This makes for a lot of gain.  Where I have seen most of the saturation has been in the third IF.  A strong signal is hot where it gets there and the additional gain really pumps it up.  This is also where the AGC is sampled so it is seeing a very hot signal

On the early model radios the S meter was driven off the AGC line and in the later models they modified the AGC a bit and used 6SN7 as phase inverter and as a meter driver.  In the first model it was very easy to pin the meter as has been stated, but in the later model it did not pin as easily.

One could insert cathode resistors in these stages to help reduce the gain throughout the receiver as in the HRO 60 then apply other fixes as desired.  There was a 2 or 3 part article about this topic in ER some years back.

[KM1H]

Since the 183D is designed to work up thru 6M that explains the gain availability.

I guess a few here arent aware of that or the fact that the radio has a RF gain control that works rather well.

An external step attenuator is another option.

I can also see where slightly leaky tubes in the RF and IF stages could cause an overload condition. Nothing like little or no grid bias to make things hot

Carl
KM1H

[W1VD]

Are you saying this because the screen current decreases under that condition even though the voltage increases?

All I'm saying is lower voltage drop across the resistor means less power dissipated by the 47k 'suspect' resistors.

Many of the old tube receivers suffer from overload especially in the 2nd RF stage and not just in the IF stages. When the cathodes are tied directly to ground it is always a problem.

You're going to have to help me with this one. In the HRO-60, for example, there are cathode resistors on each of the RF and IF stages. However, they are bypassed. Stage gain should remain the same in that case.

Carl...I'm with you on the proper use of the rf/af gain controls on the old receivers.

[WQ9E]

Since the 183D is designed to work up thru 6M that explains the gain availability.

I guess a few here arent aware of that or the fact that the radio has a RF gain control that works rather well.

And the NC-183D also has a deck of the band switch that has resistors to control the gain band by band.  When I got mine, a previous owner had rewired that section so that it ran at full gain on all bands-once I corrected the out-of-spec screen resistors I also put it back as National intended.

Rodger WQ9E

[KM1H]

There is nothing wrong with adding cathode resistors to the 183D as in the 11/04 ER article. The author is quoted as saying it is an alternative to using the RF gain control which he didnt care to use. However that has a long term effect as tubes age. The HRO-60 uses a selected resistor in the 3rd IF; I used a trimpot in mine and set the full IF string gain as per the manual.

The 6BA6 is a pretty poor tube above around 20M and a 1st RF stage change can make a big difference. Going to a 6GM6 with suitable component changes and reducing the 2nd stage gain to compensate will really lower the noise and wake up the higher frequencies. Making that change in a HRO-60 brought the 10M AM sensitivity to under 0.5uv for 10 db S+N/N and that is mighty good.

Most hams in the 50's had pretty simple antennas and overload wasnt so much an issue as today. I use Beverages on 160-40 and overload is not an issue. I also use Beverages on the modern gear for 160-80 and even 40 at times.

Carl
KM1H