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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Oct Mon 21, 2013 5:01 am 
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Reviving this old thread in the hopes that someone has the schematic for this amp. I am in need of a copy.

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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Oct Tue 22, 2013 9:44 pm 
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I don't have a schematic for the STA-24 but I found some information about a National Kits model 2200, a mono version. You can download the schematic, voltage and resistance chart, and parts list here:

https://app.box.com/s/7v3ai7zoxhib4fxr7em0

Attachment:
National Kits 2200.jpg
National Kits 2200.jpg [ 31.41 KiB | Viewed 1629 times ]


I'm certain the STA-24 is the same for each channel but with a common power supply.

Dave


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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Oct Wed 23, 2013 2:42 am 
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I forgot to post here that I have the schematic, I got it from Tom Bavis. Thank you though I may have a use for this as well.

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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Oct Wed 23, 2013 6:00 pm 
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I had never seen an un-equalized preamp before except for microphones and instruments. Magnetic phono cartridges and tape heads definitely require equalization. I wonder what they had in mind.

-David


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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Oct Wed 23, 2013 6:18 pm 
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dberman51 wrote:
I had never seen an un-equalized preamp before except for microphones and instruments. Magnetic phono cartridges and tape heads definitely require equalization. I wonder what they had in mind.

-David


I can't speak for the 2100 but the STA-24 supposedly uses NARTB.

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 Post subject: Re:
PostPosted: Oct Wed 23, 2013 6:59 pm 
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lambic wrote:
Well, I replaced the caps in the multi-cap can using a 100uF + 20uF in parallel and another 15uF at 350WVDC to replace the 125uF and the 16uF. Sadly, no change to the very low signal in the B channel.

As another check, I jumpered the inputs to the balance pot to by-pass it. No improvement, so I can rule out that component. I'm wondering if I can do something similar to test the 2, stacked, volume pots...

Meanwhile, I put a meter across the new '120uF' cap and read 180V. When I measure across what I believe to be the equivalent cap on the A side, I get IIRC 320V. This seems odd, but there is visual confusion in the wiring on the A side as it's sitting near/amongst the power transformer wiring - that is, it's hard to see if I'm comparing the same sections.

I might try to check the output transformer next.


I am not sure what you mean by "A" side and "B" side, the power supply is common between the channels.

If you are tracking down channel differences, the first step (after replacing the paper caps and electrolytic caps) is to swap the tubes from side to side and see if the problem follows the tubes. Start with the small-signal tubes.

But in this case I would highly suspect the switches, particularly the source selector wafer switch. but maybe the volume control or tone control pots. They are really two separate controls on the same shaft, if any of them is dirty or defective on one side and not the other it will cause exactly this issue. And they are notoriously prone to problems, after sitting all these years, and not necessarily being all that great a quality in the first place.

Brett


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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Oct Wed 23, 2013 10:22 pm 
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In this amp A and B are the same as Right and Left. Since there is a switch to swap/reverse channels There is no "left and right".

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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Jan Tue 06, 2015 8:56 pm 
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Just came across this thread. My first amplifier back in the early 1960's was a National model 2200 which was a kit. I remember it cost $20 and used a 6X5 rectifier and smooth white knobs. I also wondered why they provided an unequalized phono input and later had to buy a tube preamp from Lafayette radio when I got my first magnetic cartridge.

A year later I had saved up enough money (I was in HS at the time) to buy a second 2200 for stereo operation. This version used a solid state diode rectifier and maroon knobs but was otherwise identical. I guess the kit was really put together with surplus parts, but at $20 it was still a good value.

I used these for almost 10 years until I bought a PAS/3X and ST70. I sold them to a friend who used them for another 10 years or so, so they were reliable.

Steve D


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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Apr Fri 13, 2018 1:41 pm 
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Dave Doughty wrote:
I don't have a schematic for the STA-24 but I found some information about a National Kits model 2200, a mono version. You can download the schematic, voltage and resistance chart, and parts list here:

https://app.box.com/s/7v3ai7zoxhib4fxr7em0

Attachment:
National Kits 2200.jpg


I'm certain the STA-24 is the same for each channel but with a common power supply.

Dave

Dave,

I realise that this is an old thread. But I wanted to say that I built one of these amplifiers as a teenager and I had purchased it from Burstein Applebee. That would be in the mid 60s, a long, long time ago. My reason for the reply is that I have two of them now and got one going yesterday, just as a pet project. They were supplied with various manufacturer parts and were quite crude as you know. I could not find my original documentation and an Internet search turned up this thread. Thanks so very much for still having the schematic available. My particular amplifier was branded "Merrell" as is the one that I am working on now.

I plan to take some pictures and append this thread for others to see.

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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Apr Fri 13, 2018 3:36 pm 
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Hi Don,

I look forward to your photos. Aside from the 'missing' mag phono eq, the amp design seems adequate. I guess one potential shortcoming could be the OP transformer quality? This might be limiting frequency response.

They did go to the trouble of the ultralinear output arrangement. One could argue that this saves 2 resistors and might actually be cheaper to manufacture :lol:

Keep us posted.

Tim


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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Apr Mon 16, 2018 6:03 pm 
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Just an update, I have not forgotten the pictures. I have been busy with outside work.

I let the amplifier play whilst I was working outside. I have a second one in not so great condition and I am thinking that I may strip it and build it from scratch. The second one uses a 6AX5 rectifier and "seems" to not have a 5-Volt winding on the power transformer, which is obviously not original.

Stay tuned...

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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Apr Sat 21, 2018 1:32 pm 
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Some pictures of the "Finished Project". I did not construct this amplifier but did build one years ago. I found the electrolytic capacitors to all be defective (no surprise). The can caps were left attached for visual effect and I installed one terminal strip and used another existing one to mount the replacement capacitors. Note that the schematic did not show values for the filters. I suspect surplus cans were supplied since about half of terminals were not used.

I did find that the three Sprague .047 Чfd capacitors tested excellent with no leakage. So, they stayed. Other "Good-All" paper capacitors were very leaky. They went away.

The power transformer gets up to about 132°F (55.6°C) after a few hours so that is good. It does sound better than I remembered as a teenager. Maybe my ears are not as good :roll: .

Many thanks to Dave Doughty for scanning and posting the schematic and voltage/resistance chart. That helped very much.


As they say, "Stay Tuned" because I have another one of these that I just finished stripping. That on has had the power transformer replaced and I am undecided how to proceed at this time. The construction was atrocious and I found four places where the wires were not soldered and... the electrolytic capacitors were not even soldered to the chassis!!! That is a future project...


Attachments:
Merrell1.jpg
Merrell1.jpg [ 128.45 KiB | Viewed 43 times ]
Merrell2.jpg
Merrell2.jpg [ 154.43 KiB | Viewed 43 times ]
Merrell3.jpg
Merrell3.jpg [ 158.87 KiB | Viewed 43 times ]

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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Apr Sun 22, 2018 12:34 am 
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Don, that thing is a little piece of eye candy!

As far as it not sounding as good in your teen years, remember we needed about 15 db of bass boost in those days to be satisfied.

If your second amp looks anything like its mate here, that will be a remarkable pair.

Thanks for posting.

Tim


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 Post subject: Re: National Kits Stereo STA-24 amp
PostPosted: Apr Sun 22, 2018 1:49 pm 
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I guess I am now spoiled. In my adult life, I have been able to get HH Scott LK-72 amplifiers, Sansui 1000A etc. (not to forget Sansui 990DB/9090DB, 125 Watts RMS per channel) and they have gobs more power. So, yes I am spoiled. I also don't have any really efficient speakers. Most of what I have is vintage Bang & Olufsen stuff, S75, S45-2 etc. They are rather inefficient.

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