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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sun 15, 2020 10:11 am 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 345
Location: San Jose, Ca.
Scott, I agree with you about the 45v on the cathodes in the Sams schematic. That is not going to happen unless the Sensitivity is turned way CCW or the cathode resistors have gone way high in value. Otherwise, those resistors and tubes are going to be very hot. Your reading and what's in the manual is what I would expect to see.

Jim is right that you need to focus on the caps on the avc buss.

Having the avc buss biased a little positive is a good way to delay the effectiveness of the avc action until the received signal is strong enough to overcome internal and antenna noise.

Regards, Larry


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sun 15, 2020 3:07 pm 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1727
Location: Rochester, NY.
Read the instructional text about control positions on the Sams schematic page. The sensitivity control IS turned fully CCW (minimum) for the 45 v (to ground) reading.

Also:
Set to: Band A.
ANL: on.
Selectivity: off.


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sun 15, 2020 4:54 pm 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Thank you Larry and Wally,
The more explanations I get the better. I have trouble reading all parts of the Sam's PhotoFact on my computer as I have to blow up the text and then read sections at a time. However I like the clarity in the Sam's diagram. I plan on concentrating on the resistors and caps in the AVC buss.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Fri 20, 2020 8:34 am 
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Joined: Mar Tue 10, 2020 5:11 am
Posts: 91
Location: Mission Viejo, CA
I finally got around to working on my own NC-98 this afternoon and tonight. I decided not to replace every single resistor that measured out of tolerance, but instead to just replace those that were needed in order for the radio to play. As I examined my chassis, I slowly came to realize that someone had previously ALREADY replaced some of the resistors; they were bright and shiny, whereas the obviously-originals are dull-appearing and usually slightly-odd physical sizes... one watters might be somewhat longer than the 1-watters most of us are used to, some of the 1/2-watters are similarly slightly-longer than a more-modern counterpart, etc. I did measure all the old resistors and wrote the measurements down for reference. As I've mentioned before, almost all of mine were somewhat higher than they should be, some grossly-so (a 150K measured 470K, etc.).

Anyway, after finally running her up on the variac, I started measuring voltages. All the B+ voltages are high, and that'll require further investigation but I suspect right now that that's the case because the vast majority of the various dropping resistors are too high in value, and so the various tubes aren't drawing the designed current; thus, the B+ supplies have less load than normal and so are higher than normal.

Then I got to the filament voltage... nice, it showed 6.32 VAC with 117VAC into the receiver. At this point, I began wondering why I didn't hear anything out of the headphones. Nothing, not even scratching noises when I touched various points. So I dragged out the oscilloscope and set it up; it showed virtually no audio noise at all on the output. I just picked up an old, venerable HP608F signal generator the other day, but since I don't have a low power RF wattmeter, I didn't know whether there was any RF output... nor whether the thing was anywhere near the indicated frequency... at least, not until I got out the o'scope and connected it to the generator. The scope showed the generator was working fine, so I connected the generator to the receiver, set the generator to 400 Hz internal modulation, and began probing the RF, mixer and IF amplifiers for signals. They showed some amplification although as to be expected, all the controls are scratchy or intermittent and need cleaning. There was even a signal showing up at the output of the 2nd IF amplifier... but nothing showing up on the volume control. Hmmm...

Examining the schematic again, I realized the output of the 2nd IF transformer drives the 6AL5 detector/ANL. And the filament voltage on MINE was only 1.8VAC... the two paralleled 10-ohm resistors were obviously way outta whack. Cutting them loose, I found one measured around 90 ohms, while the other is around 206 ohms... HOW the heck these things managed to drift so high is beyond me :roll:

Anyway, I installed a pair of new 1/2-watt 10 ohm carbon comp resistors (I used carbon comp only because I had some brand new ones, and they measured around 9.95 and 10.1 ohm). Measuring the AC filament voltage across the 6AL5 again, it came up to the calculated 4.8 volts... and there was noise from the headphones!! I could now tune in the signal generator on the receiver, and there was plenty of audio volume. Incidentally... those two 10-ohm, 1/2-watt resistors just barely get warm to the touch, so clearly it wasn't the power dissipation that killed them. All I can figger is National must have installed some really-crummy, cheap resistors right at the factory.

Moral: that 6AL5 isn't just the ANL circuit; it's ALSO the detector! So replace those two 10-ohm resistors, and get the filament voltage back up to 4.8VAC. (I've no idea WHY National set that voltage low like that... something to investigate further, perhaps??). Perhaps that's also the root cause of your AVC problems?

My NC-98 is very microphonic... but then again, none of the controls have yet been cleaned or lubricated. I've got some D5 coming tomorrow or the next day so I'll get to that Real Soon Now.

SteveH


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Fri 20, 2020 8:48 am 
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Joined: Mar Tue 10, 2020 5:11 am
Posts: 91
Location: Mission Viejo, CA
I don't recall whether this has been mentioned before in this topic, but here goes...

BTW, I forgot to mention... I happen to have an original NC-98 manual, dated 1955 IIRC, as well as printed copies of the Sam's Photofacts (dated 1954, IIRC). While comparing them, SURPRISE, SURPRISE.... the Sam's differs from the National manual!! The tube numbering is different, for one thing, and some of the resistors, at least, have different numbers on the Sam's.

I don't recall which R-numbers they are now, I'll look them up tomorrow when I get back out into the garage... But I also found TWO resistors in my own NC-98 that are different from the manual. One is obviously a replacement, where I, at first, thought that someone had replaced a 27K, 1-watt resistor with a 33K 1-watter. But after finding another resistor in the receiver that's different from the manual (I think mine is a 680-ohm, whereas the manual says it should be 1K), AND is buried so deeply inside the wiring that I think it must be original, I'm thinking that my receiver might be a slightly-different "vintage" from the original manual that I happen to have.

Thus, there may well be somewhat-different-value components inside your own receiver compared to what either Sam's or the original National manual say.

SteveH


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Fri 20, 2020 10:28 am 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 345
Location: San Jose, Ca.
Hi Steve, Thanks for the nice write ups on your NC-98 repairs.

The 4 or 5 ohm resistor in the filament line is used to reduce hum in the audio. There are numerous manufacturers that do that trick, National, Collins, and Hallicrafters.

Regards, Larry


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Fri 20, 2020 6:07 pm 
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Joined: Aug Sun 28, 2016 2:53 pm
Posts: 140
Location: Falls Church VA 22180
Asked and correctly answered on here about 7 years ago.

https://www.antiqueradios.com/forums/vi ... 5&t=213140


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Fri 20, 2020 6:19 pm 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Steve,

Thank you for your detailed analysis-very helpful as I am still working on solving my AVC problem. Also I had the same issue with the 6AL5 parallel 10 ohm resistors. Here is my previous post:


"As I stated before, the filament voltage to the 6AL5 detector/noise limiter was supposed to be 4.8 volts AC and I measured 2.4 VAC. I clipped out the two 10 ohm resistors which are installed in parallel to drop the voltage from 6.8 VAC to 4.8 VAC. One measured 34 ohms and the second over 900 ohms. On replacement with two new 10 ohm resistors the radio now warms up in less than 30 seconds whereas before it took well over 2 minutes before I got any audio."

I have noticed the resistors that look rough with the colored bands barely readable are very often way out of spec (over 20%) and need to be replaced. Yes-Steve-I agree that the cap and resistor numbering on the National and Sam's Photofact schematics differ. Therefore when I report on a specific component, I will mention which schematic it refers to.

I will soon be checking and replacing components in the AVC circuit as I was waiting for a shipment from Antique Electronic Supply-notably the 2.2 meg resistor (R41 in the National schematic) leading to Z3 the audio detector transformer. My AVC resistance to ground is still about 360k, whereas it should be much higher (about 1.2meg).

Steve I used Deoxit to clean all the switch contacts, and I had to replace the audio volume control as it had gone partially defective.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sat 21, 2020 4:51 am 
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Joined: Mar Tue 10, 2020 5:11 am
Posts: 91
Location: Mission Viejo, CA
I bet those guys at AES are wondering why they suddenly have a run on their 2.2M and 270K resistors :lol:

As I stared at my chassis this morning, all those old, out-of-tolerance resistors seemed to taunt me mercilessly; they sat there and each screamed in its turn "GO ON, TAKE A CHANCE AND REPLACE ME!! I DARE YOU!!" :twisted: Finally, after losing the battle of "Oh no, you don't need to replace that!!" with myself, I settled in and began replacing the ones that seemed to be crying (or yelling) the loudest. One or three others I didn't have a close-value replacement for... so what I did instead, since they all measured high, was to PARALLEL them with higher values to get the total resistance back down within the spec on the parts list and schematic. When I got done, I realized that just about all of those I'd just replaced had ALREADY been replaced, most a very long time ago when those older resistor body styles were still in vogue. They almost all LOOKED stock... but how the leads had been bent and crimped onto tube socket terminals and terminal strips was just a little different from how the factory wires had been attached to the same terminals. Except for two or three very-badly out-of-tolerance resistors, I was actually able to save most of the old ones; and perhaps at some time in the future, I'll find a use for them.

Then I fired up the radio again; sure 'nuff, the audio was somewhat louder (thanks to that 6AQ5 cathode resistor having drifted up to almost 750 ohms from it's specified 270 ohms! I replaced it with a 2-watt unit I happened to have). Another quick check with the DVM was more gratifying: the B+ voltages now measured pretty close to what the schematic says they should be!

But band C is just as far off frequency as it appeared to be last night (no, Virginia: I did not expect resistor-replacement would magically-restore frequency display accuracy!). My quick-and-dirty measurements of late last night (nearly 1 am, IIRC) had been not far off the mark: 12 MHz indicated on the dial was actually more like 10.8 or 10.9 MHz on the signal source; and 14 MHz indicated was actually around 12.3 MHz. I measured and confirmed the signal source's frequency on a frequency counter I'd built nearly 25 years ago. After fiddling around with confirming the poor dial frequency display accuracy, I decided to check band D. Nada; nothing. The scope shows that the 6C4 is not even oscillating on band D. There was some mention about this problem re: 6C4s and 12AX7s in that older, 7-year-old post; so apparently, it's not an uncommon malady. I'll just have to look into it further, perhaps even begin checking out some of the capacitors. I have a very nice and accurate kit-built digital display LC meter I just built about 6 months ago, and while changing out the resistors, I did check those caps that could easily be measured without further unsoldering; the few I checked measured very close to the specified values, however. I have a corner on the world's stock of silver mica capacitors; a few months ago, on another forum, someone advertised and pictured a USPS Priority-mail box chock full of new silver micas; of course, I bought the pile. So I'm pretty-well set up with a reasonable range of values that'll last far longer than I'm certain I'll live :lol: But first, I think I'll try substituting other 6C4s to see whether they will oscillate.. I think I have one or two others.

SteveH


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sat 21, 2020 9:51 am 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 345
Location: San Jose, Ca.
Thank you, Brad (wb1bgb), for pointing that thread out to me. I appreciate it. It says that reducing the filament heat and thus the cathode heat, that the voltage drop from cathode to plate is reduced. Is that your take on it?

If that's right, which it seems like it to me, then it seems that in a limiter circuit, the benefit might be less audio signal loss and less audio distortion. What do you think?

Regards, Larry


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sat 21, 2020 3:53 pm 
Member

Joined: Aug Sun 28, 2016 2:53 pm
Posts: 140
Location: Falls Church VA 22180
Quote:
There was some mention about this problem re: 6C4s and 12AX7s in that older, 7-year-old post; so apparently, it's not an uncommon malady.


Hammarlund, a few National, and other models using the 6C4 for the HFO have had this problem for decades and I have not read of any clear cut reason why.

Many have found that the Sylvania brand is the best choice for the plain tube and the 6C4WA/6100/6135 for the premium versions. I have not owned/serviced a NC-98 but several Hammarlunds have responded well to the above choices.

Some other brands may work at first but seem to return to their old tricks fairly soon.

Brad


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sat 21, 2020 5:07 pm 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Quote:
"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." Wally58

Thank you Wally for this advice. I have now ordered the caps and resistors needed to replace the couplate. Note to Steve-thank you for your last informative note. I too have had to make some adjustments to come up with the proper resistance combinations to replace some of the originals which drifted high-namely the 10 meg resistors R34 and R45 (National schematic). I have now checked and/or replaced all caps in the AVC buss and my AVC is still not functioning properly. Resistance to ground from the AVC buss (measured at C24 (National schematic numbering) is still around 300k-way too low.

Next I plan to check the tube sockets. Sam's Photofact has a table of resistance values at the tube sockets. I hope that will give some hints as to what may be the problem.

Well-since I am sheltering at home due to the Coronavirus pandemic working on my NC-98 is a good activity. Please stay safe everyone!

Scott, N6CIC
Quote:


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sat 21, 2020 5:34 pm 
Member

Joined: Aug Sun 28, 2016 2:53 pm
Posts: 140
Location: Falls Church VA 22180
Quote:
Thank you, Brad (wb1bgb), for pointing that thread out to me. I appreciate it. It says that reducing the filament heat and thus the cathode heat, that the voltage drop from cathode to plate is reduced. Is that your take on it?

If that's right, which it seems like it to me, then it seems that in a limiter circuit, the benefit might be less audio signal loss and less audio distortion. What do you think?


I believe that link said it all, Im not about to try and second guess them.


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sat 21, 2020 9:30 pm 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
All of the tube socket resistances for the control grids of the RF and both IF amps are well below the resistances specified in the Sam's Photofact table:

RF amp 6BA6 Measured 83k--spec. 1.8 meg
First IF amp 6BD6 measured 162k---spec. 1.8 meg
2nd IF amp 6BD6 measure 334k---spec. 1.3 meg

I made sure I used the protocol specified by Sam's. All other socket resistances were within 20%. Now I need to track down the cause. Any ideas are welcome.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Mon 23, 2020 3:35 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Now I believe I have corrected all the problems I reported earlier with my NC-98. the radio is performing very well and I am ready to do an alignment. Here is a summary of the steps I took in case it would be helpful to others.

Initial problems were: distorted audio especially with AVC on, weak AVC, poor sensitivity all bands, bad volume control, very long warm up time-over 2 minutes, and oscillation and microphonics on Band D above 16 MHz.

Initially I checked all the tubes on my Sylvania tube checker. This did not give all the info I needed as I later discovered. I also measured the tube voltages before doing replacement of parts. The big news here was the filament voltage of 2.4 VAC on the 6AL5 detector/AVC/noise limiter vs the spec of 4.8 VAC.

Next all of the paper caps were replaced (7), the three filter caps, one electrolytic cap (C54), and four ceramic caps which initially I thought were leaking but now I am not so sure. I measured all of the resistors and replaced the 13 which were higher in value by more than 20%. The couplate (Z4) resistors were on spec. and the couplate did not seem to be leaking as indicated by no DC voltage on the control grid of the audio tube 6AQ5. I also replaced the bad volume control.

Replacement of the two 10 ohm resistors in parallel to drop the 6.8 VAC filament voltage on the 6AL5 detector to about 4.8 VAC immediately corrected the long warm up time. The two bad resistors measured 34 and 900 ohms, respectively, once they were clipped out of the circuit.

With all of the above replacements done, performance was improved, but the radio still suffered from weak AVC as measured by the AVC buss to ground resistance of about 360k ohms, some audio distortion, low sensitivity, and oscillation above 16 MHz on band D.

So I repeated all of the tube socket voltage measurements. All were reasonably close to spec except for V3 (1st IF amp 6BD6). The control grid measured 8.3 volts vs. spec. of 1.5 volts and the screen grid measured 14 volts vs. the spec. of 92 volts. So this tube must be bad even though my tube tester did not show it. I replaced it with another 6BD6 and now the voltages were close to spec., and the AVC buss resistance to ground now measured 1.2 megs.

Operationally all of the problems with the radio described above were corrected. Lesson learned-measure and then measure again! Thanks to everyone for your help as this story unfolded.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Mon 23, 2020 7:50 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!

Glad to hear you got it all worked out. Wow, a screen to control grid short not found by a tube checker. I think I'd look for another tester.

Anyway, good trouble-shooting! I wish you much enjoyment of your new NC-98.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Mon 23, 2020 4:50 pm 
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Joined: Aug Sun 28, 2016 2:53 pm
Posts: 140
Location: Falls Church VA 22180
Congratulations on a good ending Scott.

Quote:
The couplate (Z4) resistors were on spec. and the couplate did not seem to be leaking as indicated by no DC voltage on the control grid of the audio tube 6AQ5.


They will leak sooner than later and often take out the 6AQ5 and output transformer if not caught in time.

Brad


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Mon 23, 2020 5:50 pm 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Brad,

OK thanks for the advice on the couplate. I will plan on replacing it with individual components. Better safe than sorry.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 24, 2020 12:24 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Brad, Wally, Rodger, and anyone else who has used discrete components in place of the couplate: I notice a significant difference between the diagrams for the couplate connections between the NC-98 schematic and the Sam's Photofact schematic.

In the NC-98 diagram, the 250 pf bypass cap on the audio amp side goes to ground along with the bypass cap on the side to the audio output tube.

In contrast the Photofact diagram has the 250 pf cap on the audio amp side going to the 470k resistor that in turn leads to a B+ connection.

Which do you think is correct? I am inclined to believe the NC-98 schematic but would appreciate your thoughts.

Thanks, Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 24, 2020 2:07 am 
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Joined: Aug Sun 28, 2016 2:53 pm
Posts: 140
Location: Falls Church VA 22180
For starters
http://pacifictv.ca/schematics/centralabcouplates.pdf

Google is your friend for other brands.


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