RCA K80 CHECK OF ANTENNA AND OSCILLATOR COILS USING THE URM-25F U.S. NAVY SIGNAL GENERATOR
How to Restore an URM 25D Signal Generator
These units are historically significant due to the fact that they are still in use today by boat-anchor, antique-radio and ham-radio enthusiasts. Those wishing to restore these units often face difficulty finding information. These are some tips on restoring the URM-25x series of military signal generators build during the cold-war era.
Check the fuses are all okay.Replace any blown ones.
Check all switches are set correctly.
- The "X-MULT" switch should be set to the band being used.
- The "SET CARRIER" or "RF OUTPUT" control should be set to indicate full scale on the meter with the "Microvolts" control set to max (fully clockwise). Then slowly adjust the "Microvolts" control down to the optimal setting.
Check if the meter works in the 400/1000 Hz range.This will help you determine if the the audio oscillator and modulation circuits work properly.
Check if the meter reads RF in the CW position.This will help you determine if the RF section works properly.
Check if the meter reads RF output on all bands.If not, the intermittent contact on the wheel may require cleaning or lubrication, or the spring fingers may require adjustment to ensure they are making good contact.
Test if all switches move smoothly, or if they require cleaning.
Check that the power plugs inside the front panel are connected.
Test the tubes on a Dynamic Mutual Trans-conductance tester.There are 9 tubes in the 25D model.
Inspect the wiring connections.Some spots may require re-soldering.
Check for broken or failed line feed-throughs.These units are located on the underside of the unit.
Check that none of the moulded paper capacitors are leaking.They may look like Mica. Replace any that are.
How do I replace a dial lamp LED?
Is a schematic for the URM 25D Signal Generator available?
- Dial Bulbs: in some models a #323 incandescent 3v 190 ma bulb T1 1/4 clear w/screw base is used. These are hard to come by and may be replaced with an LED and series resistor.
- Attenuator contacts require no lubrication.
- Shaft end-play on the frequency setting knob can affect frequency setting. A setscrew on the shaft sets ball-bearing clearance, held by a lock nut.
- Modulation can drop off after sufficient warm up period. R169 may need to be backed off a touch from maximum under cold measurement.
- Backlash in tuning can make interpolation difficult in setting carrier RF, but a BNC output jack provides constant level signal feed to an external frequency counter (see picture).
- Dial illumination in some models is provided by two #323 screw base bulbs. These are difficult to obtain, and modifications are usually necessary to repair, such as with a clear LED and series resistor.
Video: AN/URM-127 Signal Generator
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