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Author Topic: Too much RF gain  (Read 11026 times)
W4OP
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« Reply #15 on: April 26, 2012, 11:54:13 AM »

Thanks for the comments.  What you describe works fine and is what I too have found works...it just may be this model exhibits the problem worse than others I have worked with in the last few years. 73

You're right Darrell- if he is talking about AM also , then there may be an  overall AVC issue.

I had followed the thread regarding another poster who also suggested that for SSB/CW he turn down the RF gain- and assumed, the positive repsonse from the original poster confirmed this.

 Dale W4OP
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G3RZP
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« Reply #16 on: April 27, 2012, 02:41:25 AM »

If there is an AGC problem causing AM distortion, then winding back the RF gain will help. Possibly worth while checking the resistance to ground of the AGC line - there could well be a leaky capacitor.

It is ironic that designing equipment some 40 odd years ago, we would choose capacitors that we were sure wouldn't go leaky or whatever. 40 odd years later, we find we were wrong!
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KAPT4560
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« Reply #17 on: April 27, 2012, 10:11:13 AM »

 I am baffled with a National NC-98 of mine with the similar rf or if overloading problem. Bringing up the RF gain control on AM in AVC mode, the S-meter will nearly peg with or without an antenna attached and has a severe audible or inaudible oscillation. The AVC voltage goes to minus 16 volts during this oscillation.
 If I reduce the rf gain, flip to standby and back to receive, the receiver will return to a somewhat normal state of operation and the S-meter will work normally. Like a tube was blocking.
 Before I recapped the paper/foil caps and reresistored it, it behaved with much more stability. The 100K screen dropping resistors were at 24 meg before replacement.
 If I detune the if transformers slightly it will work almost normally, but with less sensitivity and poor tone.
 This National doesn't have the selectoject filter, but does have a crystal selectivity circuit before the 1st if. Even with that function off, it still does this.
 Note: the 2nd if transformer did have a frozen slug and the cardboard winding tube was damaged slightly in replacing the slug. Could this be the culprit?
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K7MDO
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« Reply #18 on: April 27, 2012, 07:22:41 PM »

My set does not oscillate but sure sounds lousy on any strong station with the RF gain up too high...  AM or CW or BFO'ed SSB.  However, I still wonder a bit about the fact that turning down the live voltage to 115 seems to aid the little set.... so there must be some voltage too high....  was just hoping someone had a common solution...

PS, I am currently working on an NC-183D restoration and am finding virtually every resistor so far out of tolerance (up to 10 times) that it is hard to believe it played at all... and oddly the thing was pretty darn good at least through 40 meter band... 

I have great hopes for the 183D to respond well to my ministrations of the components!

Take care, Tom 73
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KAPT4560
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« Reply #19 on: April 28, 2012, 01:27:58 AM »

 If it is not oscillating but has a harsh tonal quality, make sure that the power supply filter caps are 40 mF. I used what I had on hand (68 mF-more is better?) and the set did not like this. Besides the slight delay in powering down after shutoff, it brought the B+ up and the audio quality suffered. After installing the correct values, the set sounded good.
 On your 183D, C-65 off the B- line should be installed 'backwards' (plus side to chassis)as the chassis is more positive than B-.
 There are some excellent threads in the National Archives on the QTH.net website that address fixes for issues that others have found:
http://mailman.qth.net/
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K7MDO
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« Reply #20 on: April 28, 2012, 05:30:37 AM »

Thanks for the info on the cap (C-65)....  really the 183 is a marvel to work on as there is a lot of room for the work....  nice big open spaces for the solder sucker...  no end of fun.

73, Tom
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W2WDX
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« Reply #21 on: April 29, 2012, 08:03:05 PM »

I know of no new, factory built multi-sections out there. 

Actually there are several including one company using the original Mallory equipment. CE Manufacturing. Only thing is they are very expensive, at over $30 a piece. They make one that is the same values for radios like the Hammarlund HQ100.

Sometimes I simply leave the old can in and solder in the right values underneath inside the chassis to the appropriate points. Most modern electrolytics are small, compared to what was available in the 50's & 60's. If you use the same values they are very small.
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KD0ACY
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« Reply #22 on: April 30, 2012, 12:53:28 PM »

i Guys
I hope we can clear things up here! At this point I have checked most, if not all the cap and resistors for correct values and position and find  all to be correct. The audio out is now at, what I think to be normal. The BFO seems to work as it should, however , I have only tried it once or twice. The AVC does not work as I have experienced , in the past.   When I tune to a very strong station, either BCB or SW, in the manual mode and then turn it to AVC the audio is reduced to almost nothing and when I do the same on a weak station and turn to AVC, there is no audio. I have changed the 6SJ7 and triple checked the connections.  another item, the S meter shows about 2 to 3, even on strong BC close by and it does wiggle, when in AVC. Checked the ground on the AVC strip, under the chassis and it is solid ground. I re-soldered the connections. All caps and resistors are new and have been checked.
When I switch to BFO the voltage goes up a lot at  6SJ7 -P 6 and does not  increase in eather of the other modes. I didn't copy the voltages down and cannot remember what they were.
Hope you can think of something else for me.
Mike 
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KD0ACY
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« Reply #23 on: April 30, 2012, 12:58:43 PM »

I think My reply has gotten on the wrong post!
Mike
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K1TWH
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« Reply #24 on: May 11, 2012, 01:36:18 PM »

Tom,
     Looking at your NC-190 radio's schematic, the most likely culprit is an aging R15 (18meg Ohms marked brown-gray-blue) resistor from the +145 supply to the AGC line, its on the schematic below V4.  As these carbon resistors get old, some succumb to moisture and drop in value.   If that resistor dropped far enough it would inhibit the AGC and force all of the stages into full gain, making even the cathode RF/IF gain control difficult to use.
      Once the AGC is working, this was an advanced design that included AGC for CW/SSB and a product detector.  This radio switches from a 2215KHz 1st IF on upper bands to a 230KHz IF on lower bands, something that current radios like the IC-7000 or TS-2000 still do (over a much wider freq range).
      Any value from 15M <> 20M would work fine.  In fact, the radio might do just fine without it?
           73 & good troubleshooting.    Tom Howey  WB1FPA
                                                        =       =       =
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K7MDO
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« Reply #25 on: May 12, 2012, 04:15:12 PM »

Tom Howey  WB1FPA, I am on a trip for a couple of weeks but when I return I will check that resistor.  I did go through the power supply and audio sections completely but can't remember if I changed that resistor or not...

For whatever it is worth, the resistors I have replaced all seem to have gone way up in value, not down....  but anything could happen and I did find a few parts replaced by previous owners that were not correct at all....

Anyway, thanks for taking the time and I will let you know in 2-3 weeks if there is any improvement.

73, Tom
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G3RZP
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« Reply #26 on: May 12, 2012, 10:44:15 PM »

I generally find that carbon comps go up. But W8JI knows of cases where they go down. I suspect that there are different mechanisms involved and maybe temperature and humidity come into play.
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K7MDO
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Posts: 325




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« Reply #27 on: June 04, 2012, 09:37:51 AM »

Tom Howey  WB1FPA, back at work on the NC-190 and have some progress.  The 18 meg resistor was almost dead on 18 but another resistor that dangled from the same terminal was 3 times it's correct value and while I was desoldering to replace it I found the coupling cap that was nearby also deteriorated. 

After replacement and a trying it out I found it to be significantly improved.  In fact I am beginning to suspect that it is now OK.  As long as I turn the audio up and only use enough rf gain to get good reception it isn't too bad.  The distortions that I originally heard seem much improved if not completely gone unless I tune the strongest local AM stations on the broadcast band.  Even there it is controllable.  Interestingly it is a "hot" receiver in that 4-6 feet of wire gives good results clear up to 14 mHz. 

I think maybe the only other disappointment I have in it is that it seems to continue to drift in frequency a bit, even after 5-10 minutes.  The NC-183D I have settles in very well in frequency after only about 4-5 minutes and seems to stay there.  This is not a big deal if you are on AM but on SSB it is obvious as you continually have to touch up the dial to stay on tune on SSB.

In any event, I now have a really nice operating old tube radio and in fixing it up I have learned a lot. 

Thanks again for the help and interest, Tom, 73
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