The AM Forum

Sometime in the past I read an article about installing a fuse to protect the power transformer on a National receiver. Well, I didn't save the article, and now I have a National NC-240-D on the bench with a bad power transformer. I have a good transformer to install. but I'd really like to install protection for the transformer. Does anyone remember this? Or am I imagining things again lol (wouldn't be the first time).

National never made a NC-240D receiver.

They did make a NC-2-40D receiver.
If you review the manual and schematic for the NC-2-40D receiver, you'll see the primary of the transformer is protected by a fuse. It's also listed in the parts list. There is one fuse if you run it on 115 volt AC and one fuse if you run it on 230 volt AC.

WHY would you want another fuse  ???

Ray,

You may be thinking about some of the 1950s era National sets that were built with a .1uf capacitor from one rectifier plate to ground.  This capacitor isn't needed and many sets lost their transformer due to it shorting; it wasn't used in later production.  At least some of the NC-183, 183D, HRO-50, and HRO-60 models had this capacitor.

Some people were putting a lower value fuse in the B+ center tap to protect it but removing the cap is best because the fairly light gauge wire in the secondary could open before the fuse.

Rodger WQ9E

Don't see any AC line capacitors on the primary side of the power transformer.

I have that same receiver on the bench now. It actually has two fuses in the primary side with a switch for 115~230VAC. In an interesting note, it is designed to work with AC voltages 110-120VAC. They even went as far as measuring the drift at the higher voltage.

The only thing that would take out the transformer, aside from the transformer itself having problems, would be the filter caps or one of the chokes.   You could put a 100mA fuse between the rectifier's output and its connections with S2B and X2 pin6.

(https://photos.smugmug.com/NATIONAL/NC240D/New-Rs-and-Cs/i-jVtTvWQ/0/ecfccacd/L/PsCrop-L.jpg) (https://mikeharrison.smugmug.com/NATIONAL/NC240D)

National wasn't consistent with their use of a .1uf cap from one rectifier plate to ground, it shows up in some schematics from the 1950s and not in others.  The earliest I found it was in a HRO-7; my NC-183D came with a note and receipt in the manual showing that the original transformer was replaced by National and the cap (not shown on the schematic) was removed at that time by National's service department.

So I check any National receiver from this general time era for the killer cap when it comes across my bench.

This is from the schematic from the HRO-50T-1 showing the troublesome cap from one rectifier plate to ground.  National mounted them right at the rectifier tube socket so it is an easy check to make.

Rodger WQ9E