National NC-183 receiver

National NC-183 receiver

National's NC-183 is a 5 band single conversion communications receiver with two IF and two RF stages covering AM broadcast and shortwave from 540 KHz to 31 MHz plus the 6 meter band from 48 to 56 MHz. The IF is nominally 455 KHz with the actual IF being matched to the crystal filter.

National NC-183
National NC-183

The NC-183 was introduced in 1947. Price at introduction was $269 including the 10 inch matching speaker. Ads in later years attempted to keep close to that price but made the speaker an optional extra. Then NC-183 was replaced by the NC-183D which employs dual conversion. Both sets use push-pull 6V6GT output tubes for "Hi-Fi" sound. Both have tone controls and an RCA jack for phono input and are known for excellent sound from a matching speaker. Both use VR-150 voltage regulator tubes. The NC-183 uses all octal tubes such as four 6SG7 for the RF and IF stages while the NC-183D uses 7 pin miniature 6BA6 tubes for those functions.

December 1947 ad for National NC-183
National NC-183 ad


After unpowered safety checks and inserting a proper fuse, early tests showed that the radio was functional even with all of its wax paper capacitors. In these tests I always monitor the current draw while ramping up the voltage slowly. All three toggle switches and the RF and AF gain controls needed a deoxit treatment to solve intermittent contacts.

I replaced all of the wax capacitors including all the 0.05 caps that looked like mica caps but were in fact just molded paper caps. I also installed a new 3-wire power cord. I removed a couple of modifications that were likely intended for an unknown device which plugged into the accessory socket.

National NC-183 caps
Some of the caps removed from the NC-183

I noticed that the stationary pointer on the main tuning dial was occasionally rubbing the tuning dial rivets and bending. Also the antenna tuning control had a dead area for part of its rotation. The first thing to do then was to pull the cabinet, a difficult task because several of the knobs would not easily come off the shafts, including the bandspread knob. For the stuck knob on the phasing control, I was afraid to pull too hard for fear of breaking the phenolic shaft. After removing the knob set screw. I put a bit of deoxit in the set screw hole. That lubricated the knob and shaft friction enough to allow removal without excess force. Pulling the cabinet also allowed better access for cleaning the chassis thoroughly.

National NC-183 cabinetNational NC-183 cabinet

Replacing the antenna trimmer capacitor
The antenna tuning variable capacitor proved unrepairable. Somehow the blades on the stator had warped. No adjustments would remove the short on part of the rotation. I located a variable cap with the same mounting style in the well-stocked "box de junque". Its shaft was about a quarter inch too long and it had about three times more blades than the original. Full capacity was 150 pF, also about three times what was required. Using a rotary cut-off tool, I sliced off the excess part of the shaft and also a large part of the stator and rotor section. I then removed a few more rotor and stator blades with pliers. The replacement ended up a near-perfect match for the bad antenna tuning cap. Removing the original cap and mounting the replacement in very tight quarters took considerable patience.

Replacing the antenna trimmer cap - National NC-183 chassis
National NC-183 chassis

The tools used to remove and then replace the two mounting screws in a very tight location are shown lying on top of the power transformer. Those were a Philips-head bit and a quarter inch wrench with a magnetic pick-up tool to help hold the bit in place. I soldered longer tinned wires onto the replacment before mounting it, partly to hold it in place while mounting.

I carefully followed the alignment steps as written in the manual, making sure that the IF was aligned at the crystal frequency. Even the Six meter band aligned well. The original alignment was close except for the six meter band.

1951 ad for National NC-183
National NC-183 ad

With all the work completed, the NC-183 proved to be an excellent performer, as "hot" as advertised in the 1947 ad above. With this NC-183's longevity, National is truly "Makers of Lifetime Radio Equipment" as also touted in the ad.

Given today's higher socket voltages, I use the set with a bucking transformer to keep the power transformer relatively cool. For some details on a cheap bucking transformer, see the writeup for the Hallicrafters S-76 .

6SG7 substitutes
The set as purchased came with a couple of plug-compatible 717A tubes in place of the 6SG7 RF amplifier pair. I could not easily determine any differences in sensitivity in the upper bands. The 717A was designed for UHF service and was often used to replace a 6SK7 or 6SG7 for improved performance on the highest frequencies. I decided to leave them in place as a useful but easily reversible period modification.

Manual and schematics can be found on the BAMA site

More National sets
I have repaired and documented other National radios. A list and links to those National radios can be found here.

Date 8-21-14

A National NC-183D receiver was the previous project "on the bench".

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