Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jul Sat 20, 2019 4:03 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 19 posts ] 
Author Message
 Post subject: NC-100A questions
PostPosted: May Tue 28, 2019 1:55 am 
Member
User avatar

Joined: Oct Wed 01, 2014 1:20 pm
Posts: 191
Location: Wood River, Ill.
Attachment:
2DBC810A-5545-45AF-99D2-1644744BB1F7.jpeg
2DBC810A-5545-45AF-99D2-1644744BB1F7.jpeg [ 349.63 KiB | Viewed 764 times ]


Hello all,

I picked up this National NC-100A at a hamfest, a few years ago. It was pretty rough, so I got it at a pretty good price. I have not looked at it since the day I brought it home, until recently. Its turn on the bench came up about a month ago.

While it looked mostly complete, it is obvious that it has moved through a few hands in its life. The power transformer has been replaced, and there have been lots of mods done under the chassis. The quality of work done were mostly very poor, (and that’s being kind). I think I have removed at least 1/8 lb of excess solder, mostly from cold joints. It was dead from the beginning, so I pretty much had to replace almost all the paper and electrolytic caps and most of the resistors before ever trying it out for the first time. (I usually replace one or two at a time, then test.)

One of the mods was replacement of the 6K7 with a 6SK7 in the RF stage. The socket had been rewired for the new tube, even rerouting the grid lead under the chassis for the single ended tube. I decided to leave the mod in place and see how it did. Also thinking this may have been a popular mod in the day. (I might add that the work here looked very good and professionally done.)

The problem is that the receiver severely over loads, even with the AVC turned on, requiring a reduction in RF gain, with relatively weak stations. I am debating returning it back to the original 6K7, thinking the 6SK7 might have too much gain for this receiver. I believe that I can return it back to its original condition fairly easily.

Was this change a common mod in the day? If so, what was its purpose? Increase in receiver gain? Sensitivity?

Thanks for looking

_________________
73
Chris
N9WHH


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: May Tue 28, 2019 2:39 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 220
As I recall, that wasn't a common modification. I don't think your problem lies with the gain of the tube, however.

Looking at the front panel picture, I am impressed. That is a beautiful receiver.

Back to the AVC problem. Measure the AVC voltage and make sure it reaches the grid return of the 6SK7. I don't know if the AVC rectifier is the same as the audio detector, so you need to investigate that. AVC should reach the rf amplifier stage as well as the i.f. amplifier stages. It should be a negative voltage that gets greater as signal strength increases.


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: May Tue 28, 2019 3:07 am 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 6526
Chris,

The 6SK7 shouldn't be the cause unless the modifier also changed resistors in that circuit to increase the gain. Most of the " added S" (meaning single ended) tubes were pretty close in characteristics to their double ended ancestors.

As Bob noted, check the AVC circuit. You may have missed a leaky capacitor which would load down the bus or you could have a tube that is gassy or has secondary emission which is trying to positively bias the AVC bus. A quick check that will often turn up a problem tube is to measure AVC voltage on each side of the isolating resistor feeding each stage and if it is less negative on the tube side of a stage then you probably have a problem tube.

The NC-100 receiver family started the National moving coil catacomb family. I have an original NC-100 (has the HRO style dial without a calibrated dial scale) along with a NC-100A. The NC-100A was the basic starting point for the RAO series of receivers that appended an additional RF stage to the receiver.

In good working condition the NC-100 family of receivers is still very usable.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: May Tue 28, 2019 4:35 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3265
Location: Seattle WA US
Edit: <there were 4 electrolytic caps in the radio>.

Like many other national receivers, Bminus is NOT ground. Bminus, the center tap of the transformer hv winding (and the most negative point in the radio), is around 50 volts below ground. The resistor network that provides this magic is in the area of the schematic just below the AGC amplifier tube. If this network has been trashed, or the power supply filter capacitor negative ends returned to the wrong points, the amplified AGC won't work!!

-Chuck


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: May Thu 30, 2019 1:15 pm 
Member
User avatar

Joined: Oct Wed 01, 2014 1:20 pm
Posts: 191
Location: Wood River, Ill.
Thank you Bob, Rodger, and Chuck for the quick replies.

The AGC network was trashed, as you suggested, Chuck. It was one area that required a complete rebuild. I am getting a -50 to -60 volts DC to the AGC tube, depending on if the AGC is turned on or off. I am seeing a negative grid bias on the RF, 6SK7, of -8 VDC when receiving a strong signal with the AGC turned on. It is somewhat less than that on the grids of the two IF stages. I am not sure what voltage I should expect to see.

On the 1st IF, there is a resistor in the grid lead that the schematic calls out as 500-2500 ohms. I initially installed a 1500 ohm resistor in this location. Thinking this may be the root cause of my problem, I pulled it out and connected my resistance decade box in its place, and did a Little experimenting. I got all the way up to 68k ohms before the overloading settled down. I am thinking there must be an issue in the AGC buss that I am missing, and this is just masking the issue.

In the meantime, I am going through my wiring and part changes, looking to see where I screwed up. I did a lot of component replacement while the coil catacomb was removed, and was not able to spot check my work by testing as I made the changes. I still need to take a closer look at the grid isolation resistors and caps, as Rodger suggested. I could have installed the wrong value resistor, or have a leaking cap in this area.

So I am still looking for ideas.

Thanks for looking.

_________________
73
Chris
N9WHH


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jun Fri 07, 2019 4:16 am 
Member
User avatar

Joined: Oct Wed 01, 2014 1:20 pm
Posts: 191
Location: Wood River, Ill.
Well I found the problem. As much as I hate to admit it, it was a dumb mistake on my part. Some how I installed a 6J7 in the 2nd IF socket instead of a 6K7! :oops: They both have the same pin outs, so no harm done, just too much gain. Once the proper 6K7 was installed everything settled in just like it is supposed to.

The 6SK7 in the RF stage seems to be working as it should.

Live and learn! :roll:

_________________
73
Chris
N9WHH


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Sun 07, 2019 8:24 pm 
Member
User avatar

Joined: Oct Wed 01, 2014 1:20 pm
Posts: 191
Location: Wood River, Ill.
Hello All,

When I started on this project I had no idea it was going to turn into a summer long project. Considering the condition it was in when I got it, I am mostly pleased with the results, BUT... I have a nagging issue, still with the RF gain control and the AVC action. Something just does not seem right. When the AVC is turned on the RF gain is quite low, much lower than it would be when in the MVC position with the RF gain adjusted for normal listening without overloading the receiver.

When I am listening to a strong signal, with AVC turned on, there is a point on the RF gain control near the maximum position (about 5 to 10 degrees off maximum) where the signal seems to "pop" and it plays well, without over loading. Advancing any further towards maximum RF gain, and the signal starts to over load the receiver. If I turn the AVC off to the MVC position the signal strength increases considerably, requiring a further reduction in the RF gain. In this position, the receiver is quite listenable.

I can use the receiver, and have been doing so for some weeks now, but I am wondering if this is to be considered normal. When the AVC is turned on, advancing to the "pop" position, then reducing the RF gain is working. When tuning across the band, this adjustment needs to be made again. I do expect some loss in RF gain when the AVC is used, but was not expecting it to be so pronounced. It will settle down a little when advanced to the maximum gain position, with occasional over load.

I hope this makes sense to some of you out there, as I have been trying to describe the situation as precisely as I can. If this is perfectly normal for a receiver of this vintage, then pardon my ignorance, and I will know better next time. When I went thru my NC-100X I do not recall having these issues, but it was several years ago, and could have been erased from my memory.

Thanks for looking.

_________________
73
Chris
N9WHH


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Sun 07, 2019 9:22 pm 
Member

Joined: May Sat 12, 2012 1:33 pm
Posts: 1687
Location: Rochester, NY.
Remember that the B- filter/bypass capacitor (C30) has the (+) side tied to common. The (-) side goes to the B- bus because it is more negative than the chassis. I had my first one in backwards. :oops:
Does the AVC voltage jump when you get to the point of the 'pop' with the RF gain?
With the MVC on, you hear no 'pop' in the RF gain?


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Mon 08, 2019 1:51 am 
Member
User avatar

Joined: Oct Wed 01, 2014 1:20 pm
Posts: 191
Location: Wood River, Ill.
Hi Wally,
Thanks for the reply. I thought you might be on to something, regarding C-30. Wiring it in backwards sounds like a mistake I could easily make, but alas, no. I installed the new cap correctly, with the (-) side tied to the B- and the (+) side tied to the chassis.

You are right about the AVC voltage jumping at the “pop”. The voltage changes about -2 to -3 volts when it pops on strong as well as weak signals. On very strong signals the “pop” is almost indistinguishable. The AVC is running around -10 VDC on relaitively strong signals. On our 50 kw blowtorch it is running -16 to -17 VDC.

In the MVC mode (AVC off) there is no popping. The AVC voltage is 0 VDC at this point, as the switch grounds the AVC line.

Still scratching my head.

_________________
73
Chris
N9WHH


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Fri 12, 2019 1:10 pm 
Member
User avatar

Joined: Oct Wed 01, 2014 1:20 pm
Posts: 191
Location: Wood River, Ill.
After thinking this one over, for the last week or so, I think I will just have to live with it and move on. (It’s not like I can’t dig into it later.) I read all of the articles on Radio Boulevard and it sounds like the RF gain may have to be reduced a little with the AVC turned on. So, at least mine isn’t the only one.

I still have to start the refinishing of the case. I would like to get the cleaning and painting done during the warmer months. It has several coats of poorly applied paints in several places. I am planning on doing a chemical strip and repainting. It came without a bottom plate, so will be on the lookout for one, or fabricating one. Painting is not something that I have done much of. It will be a learning experience.

_________________
73
Chris
N9WHH


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Sun 14, 2019 7:12 pm 
Member
User avatar

Joined: Oct Wed 01, 2014 1:20 pm
Posts: 191
Location: Wood River, Ill.
I went to my local auto paint store and picked up some high temp. black crinkle paint, primer, and real paint remover (with methylene chloride). I have drilled the rivets and removed the etched nomenclature plates in preparation for a chemical stripping of the case. I plan to repaint, as outlined on Radio Boulevard. We will be having some hot weather here for the next few weeks, so will take advantage and leave it out in the hot sun for a good bake.

I chickened out on the idea of pressure washing the chassis. I have too much work into it now to risk damage to the rest of the chassis. Maybe the next chassis.

I still have to rewire the dial light assembly. It is all original and hope to be able to salvage much of it. The sockets don’t look too good, but have not seen any suitable replacements yet from my usual sources. I got the “S” meter working reliably with minimal coaxing. The meter adjustment pot was completely burned out, so that needed replacement.

I have replaced all the pots on the front panel. I tried cleaning and lubricating, but they were still scratchy and some were just open.
:roll:

_________________
73
Chris
N9WHH


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Wed 17, 2019 11:19 pm 
New Member

Joined: Jul Wed 17, 2019 11:01 pm
Posts: 3
The picture looks like it is missing the RF gain control, (hole on lower right part of chassis.)


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Wed 17, 2019 11:24 pm 
New Member

Joined: Jul Wed 17, 2019 11:01 pm
Posts: 3
Be very careful around the pot that sets the meter sensistivity on the chassis inside.
Usually the insulation is just cardboard around it and it has B+ in it. If it shorts to ground you will be looking for an "unobtainium" meter.
The cardboard insulator sleeve needs to be replaced before you blow the meter movement. :-)

~larry ware


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Thu 18, 2019 4:20 am 
Member
User avatar

Joined: Oct Wed 01, 2014 1:20 pm
Posts: 191
Location: Wood River, Ill.
Thanks Larry, for the heads up. :D

The meter pot was completely shot, completely burned up, on this receiver when I got it. The pot was replaced early in my restoration project. The meter does work, though. Perhaps the sluggish operation of the original meter switch saved it.

_________________
73
Chris
N9WHH


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Thu 18, 2019 11:21 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20373
Location: Utica, NY 13502 (USA)
waywardhome wrote:
The picture looks like it is missing the RF gain control, (hole on lower right part of chassis.)

RF Gain control is 2nd knob under the S-meter on this version. I'm not sure what that hole was for. Toggle switch on left side appears to have been added. Some pictures show filler plugs in both locations.

Dave


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Thu 18, 2019 1:16 pm 
Member
User avatar

Joined: Oct Wed 01, 2014 1:20 pm
Posts: 191
Location: Wood River, Ill.
Larry and Dave,

I bought this receiver at a hamfest a few years ago from the estate of a silent key. The seller was not able to provide much information about it. It looked to me like there were some missing controls and some modifications were done to it. There were a few pots lying loose inside the cabinet.

It has been sitting around waiting for its’ time on the bench, while I gathered what information that I could find about it. After getting it on the bench, I did find that it really wasn’t missing much. The toggle switch, you mentioned, Larry, had been added. Its purpose is unknown, as the wires were cut. There was an open hole on the right side, which apparently, should be plugged. This receiver has been heavily modified with no documentation provided. Since my goal was to restore back to stock condition, I removed most of the mods. The exception being the replacement of the 1st RF, 6K7 with a 6SK7, which I left in place. The filter caps had been replaced with plug in cans with leads soldered onto the pins. I chose to leave them in place (disconnected, of course) and put new filter caps under the chassis, at least for now.

It has been an interesting project, so far. Requiring a lot of research on my part. According to what I have been able to find, this version did not have the optional crystal filter. National must have used this chassis and cabinet for several models, thus the plugged holes in the face plate. The only added holes,from the mods, are on the rear apron. There isn’t much I can do about that other than installing plugs. The mods were not well done and the quality of work was very poor, so I made no further effort in trying to determine what they were trying to do.

_________________
73
Chris
N9WHH


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Thu 18, 2019 3:02 pm 
Member
User avatar

Joined: Oct Sun 15, 2006 12:57 pm
Posts: 6412
Location: Liberty, Missouri
When I do Wrinkle finishes I set the cabinet out on an over turned plastic trash can for half an hour or more to get it plenty "warm" then I apply one coat, and let set for 3 - 5 minutes then spray a second coat. This gives time for the volatiles to evaporate and makes for a somewhat thicker coat.

I paint one flat (horizontal) surface at a time. The paint comes out very nicely be pre-heating in the sun.

_________________
Pics, Bold & underlined text, are usually links.

https://mikeharrison.smugmug.com/

Mike


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Thu 18, 2019 6:04 pm 
Member
User avatar

Joined: Oct Wed 01, 2014 1:20 pm
Posts: 191
Location: Wood River, Ill.
Hi Mike,

Thanks for the tips. I will take your approach to painting this. The weather in the Midwest is almost just right for this, with the exception of the humidity.

I am using the VHT high temperature engine paint, as I believe, you and others have recommended. Do you recommend using a primer? The guy at the auto paint store wasn’t sure, and the VHT web site was somewhat ambivalent regarding primer.

_________________
73
Chris
N9WHH


Top
 Profile  
 
 Post subject: Re: NC-100A questions
PostPosted: Jul Thu 18, 2019 9:18 pm 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 6526
Chris,

I have used VHT without a primer and it worked well for me. The VHT paint has a really nasty smell so plan on curing a couple of weeks minimum before bringing it inside. Operating the gear will help to heat up the case and speed the curing after the initial cure but don't rush it. Other than the smell, the VHT paint works very well.

Rodger WQ9E


Top
 Profile  
 
Post New Topic Post Reply  [ 19 posts ]  Moderator: Sandy

All times are UTC [ DST ]


Who is online

Users browsing this forum: Skip-wave and 4 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB