National NC-183
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.
Repairs
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.
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.
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.
Alignment
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.
Performance
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".