| Note: This site and the data it contains have been reposted from an archive of Nolan Lee's original web site by Steve Johnson. |
R-1051B Dial Lamp Life
The dial lamps in the R-1051B receivers are tough to find and a pain in the ass to change, especially the one under the MHz encoder. The bulbs are rated at 28 volts. Naturally, running them at a reduced voltage will greatly improve their life . Here are a few steps that I took to improve the life of mine:
The first step was to compare the position of the taps of the primary side of the R-1051B power transformer to my actual line voltage. It seems that most of the R-1051B's are set for 115 volts. My line voltage is ~125 volts or so. As a result, I changed the transformer tap to the 126.5 volt position. I'd guess that this step will prolong the life of various components in the voltage regulation circuitry of the power supply too. :-).
The voltage for the two dial lamps is fed thru two dropping resistors (A2A8R1 and A2A8R2) wired in series, located on the power supply printed circuit board (A2A8) under the main chassis near the power transformer. The original resistance value of these two 1 watt carbon composition resistors was 47 ohms at 5% tolerance on the R-1051 receivers. Dial lamp life sucked so bad that a Modification Work Order (MWO 3 for the R-1051) was issued to reduce lamp failure. It consisted of replacing one of the 47 ohm, resistors with a 120 ohm resistor. I have carried this a little further. One of my R-1051B model receivers, even after going thru a Navy refurb, had two 47 ohm resistors in stalled in this location. I replaced one of the 47 ohm resistors in that radio with a 1 watt 133 ohm wire wound did the same for the 120 ohm resistor in the other radio. When I mounted these resistors to the printed circuit board, I spaced each of them up about the thickness of a business card to cut down on the amount of heat transferred to the PC board. I've seen several boards that were badly burned by the resistors.
The voltage for the dial lamp circuit was measured at terminal E4 of the power supply printed circuit board. My lamps are running on about 16.9 to 17 volts with the receiver powered up in any mode except Standby. In the standby mode, the lamp voltage is a about a volt and a half higher since the 28 volt source powering the lamp circuit isn't regulated.
Another important thing about dial lamp life. If one of the two lamps either burns out or isn't installed, the current flow thru the two dropping resistors is cut by half. As a result, the voltage to the good lamp that is still installed increases.We all know what that means.....poof! I would NOT use one of these receivers with only one good dial lamp in it. If you do, you'll shortly need TWO replacements.
Your milage may vary. :-)