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 Post subject: National NC-98 Distorted Audio
PostPosted: Feb Mon 24, 2020 3:10 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
I am refurbishing an NC-98 that I bought on Ebay. On the broadcast band the audio is distorted-kind of a mushy sound. I checked all the voltages at the tube sockets, and aside for the plate voltages being on the high side, nothing was out of the ordinary. I have ordered but not yet received a 6AQ5, V-8-the audio amp, in case that is a problem. Any thoughts on what I should be focusing on?

Thanks and 73s,

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Feb Mon 24, 2020 3:18 am 
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Scott,

The NC-98 uses a "couplate" to couple the audio driver to the audio output stage, these couplates are sort of a forerunner of the IC and combine a number of resistors and capacitors onto a single substrate. A fairly common failure is for the DC blocking capacitor to short so carefully check for the presence of any DC voltage on the control grid (pins 1, 7) of the output tube which indicates leakage from the AF driver DC plate to the output stage control grid via the couplate. This may not be the issue but it can cause the problem and it is something that must be addressed immediately because the positive bias on the output tube control grid will cause it to draw excessive current potentially damaging the output and power transformer. If this is the problem, you can replace the couplate with discrete components and the values are shown on the NC-98 schematic.

It could well just be a gassy tube or another issue but check this first because it is one of those types of failures that can quickly turn into a very expensive repair.

Rodger WQ9E


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Feb Mon 24, 2020 5:39 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Roger,

Thanks for this tip. I checked pins 1 and 7 on the output tube and got just 11 mv which I assume to be essentially 0 volts. So I guess something else is causing the distortion. I notice that reducing the sensitivity improves the audio somewhat but it is still not good. I have a replacement audio tube (6AQ5) on order if that might be the problem.

Any other ideas are welcome.

73s, Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Feb Mon 24, 2020 9:36 am 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1727
Location: Rochester, NY.
The NC-98 should be capable of good fidelity and sensitivity. I replaced the couplate in mine with discrete resistors and caps. Measuring the old couplate, you will likely find drifted-high resistors and leaky caps.
You may find many drifted-high resistors elsewhere, especially the ones with a rough/dull finish. Replace the electrolytics and paper caps while you are in there.


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Feb Tue 25, 2020 2:59 am 
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Joined: Feb Sun 17, 2008 11:36 pm
Posts: 2061
Location: Dayton, Ohio
It may have a leaky capacitor in the AVC buss, allowing the receiver to be input overloaded.

Charlie


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Feb Tue 25, 2020 7:40 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Thanks for the suggestions. I will check the caps in the AVC buss. To date I have only replaced two paper caps (C-45 in the transformer primary and C-52 from the audio amp to the volume control). I will be replacing the remaining paper caps and the electrolytic caps, and I will be checking the couplate. I received a replacement audio output tube (6AQ5) but substituting the new tube for the existing tube did clear up the distorted audio.

This radio takes a very long time to warm up in comparison to my SX-99. Also after it has been on for say 5 minutes, the audio suddenly gets very loud.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Feb Wed 26, 2020 4:27 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
Scott10k wrote:

This radio takes a very long time to warm up in comparison to my SX-99. Also after it has been on for say 5 minutes, the audio suddenly gets very loud.

Scott, N6CIC


That is really strange and I bet a big clue to your distorted audio. Not a clue to ME, but a clue to someone who knows about these things.

p


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Feb Wed 26, 2020 5:00 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 7214
Does the S meter change during this increase in volume or is it just a change in audio? This will help you narrow down where the problem is occurring before you attack it with test equipment.

Capacitors that increase leakage with temperature and tubes with either gas or secondary grid emission can create this along with the distortion. A resistor changing value with heat is another common cause. Based upon the S meter reading, make tube voltage checks in the likely stages before and after the change occurs.

NOTE: a tube checker will often not catch these kind of tube faults which is one of the reasons my tube checker is under the category of least used equipment. Many tube faults don't occur until they are at normal operating temperature while operating in the frequency range that occurs in the equipment. Neither of these conditions are met in the tube checker which checks tubes at DC or AC line frequency and will never get them to normal operating temperature.

Rodger WQ9E


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Feb Wed 26, 2020 6:44 pm 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Rodger,

Yes-it takes 10 to 15 minutes, but when the audio increases in volume the S-meter goes up. At that point if I back off on the sensitivity control, the audio clears up and sounds very good.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Feb Wed 26, 2020 8:49 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
The S meter going up is a reflection of increase in RF gain, not audio (but of course it sounds louder too). Sounds like your AVC is not working and turning RF gain down (limiting RF) makes everything sound good.

p


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Feb Fri 28, 2020 11:38 pm 
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Joined: Feb Wed 26, 2020 9:27 pm
Posts: 96
Location: EU
I had similar problem with my NCX-3. It sounds OK via the headphone socket, but via the Power Supply speaker, it sounds raspy and not nice.
I was suspecting the audio transformer gone bad, and was planning to either check it or just replace it with another one if I find in similar spec from my junk box.


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sun 08, 2020 12:48 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
By now I have replaced all of the paper caps and the filter caps and C54-25 mfd electrolytic. The audio still sounds distorted until I back off the AVC and the radio takes about 5 minutes to get up to volume, compared to 30 seconds with my SX-99. At this point I am checking out the AVC circuit and also looking for out-of-spec resistors. I am in the process of replacing the 500 Kohms volume control which has developed a condition where it is still high resistance until about 3/4 clockwise. Next on my list will be the couplate.

In checking the AVC circuit, I noticed that R1, the resistor from V-1 (the Rf amp) signal grid to Z2 (2nd IF transformer) and elsewhere, is 47 Kohms, whereas in the circuit diagram and in the list of resistors, R1 is listed at 470 Kohms. Has there been a modification that National put in later, or is the circuit diagram and resistor list in error? I measured R1 at about 50 Kohms, and I am not sure if I should change it to 470 Kohms without more information.

If any reader with a NC-98 could take a look under the chassis at R1, it would be very helpful.

Thanks and 73s,

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sun 08, 2020 7:54 pm 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1727
Location: Rochester, NY.
R1 should be 470K (yl-vt-yl).
Speaking of carbon resistors, my R12 and R14 IF amp screen (100K) resistors had drifted to virtually open. I found that most of the rough or dull finish resistors had drifted way high, while the glossy finish resistors were OK. The wirewound resistors also held their values well.
Something is amiss with the long warm-up time. Mine is playing within 30 seconds and stops drifting (minor) after about an hour.
Tube socket voltage checks between cold and warm may help locate the problem if something isn't conducting well when cold.


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sun 08, 2020 10:10 pm 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Thank you Wally-I will replace R1 with the correct resistor. I appreciate you checking the correct value. Also I will check the IF screen grid resistors and any others that look very rough in their finish. I just finished replacing the volume control. That is a bit of a job because I could not locate one on eBay that fit exactly, but I got one in and it works better than the old one.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Mon 09, 2020 5:40 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
I replaced R1 with the correct 470K value resistor thanks to the info from Wally, and I now have an improvement in audio clarity with AVC on (sensitivity control fully clockwise). The radio still takes over 2 minutes to get to full volume. I checked the IF screen grid resistors (R-12 and R-14) and both have drifted high out-of-spec. These will be replaced and I will continue checking components.

73s, Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Mon 09, 2020 1:38 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
Scott10k wrote:
The radio still takes over 2 minutes to get to full volume.


Just curious, does it take over 2 minutes to get full B+? Is the same thing true with headphones? Is this also true with AVC of?

paul


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Mon 09, 2020 4:48 pm 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Paul,

Good questions. I will have to do some measurements of the time to get to full B+. Those could lead to some useful clues. Yes-with AVC off it is still slow to get to full volume. I haven't checked with headphones, just with a speaker.

73s, Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Mon 09, 2020 6:11 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
Scott,, also check to see whether it takes "2 minutes" if the radio is FULLY warmed up and then you turn it off for 15 seconds. That would be interesting and will say something about actual "warmth".

p


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Mon 09, 2020 6:39 pm 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Paul,

Thanks good idea. I just tested the time to full B+ and that is not an issue. I get full B+ right after turning on and also the voltage dropping resistors are doing their job spot on. In the meantime the radio is slowly warming up.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 10, 2020 3:59 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 4320
Location: Lincoln City, OR 97367
Greetings to Scott and the Forum:

I have a question for you. Have you ever measured the filament voltage actually on one of the tube sockets? Might be a good idea to measure them all... a high resistance connection in the filament wiring somewhere could account for the long delayed warm-up.

Regards,

_________________
Jim T.
KB6GM


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