AN/URM-25F Signal Generator

I bought my URM-25F signal generator from George Rancourt at Deerchester many years ago. 

IMG 5806.jpg

I hadn’t used it much until the band-switch mechanism on my HP-606A froze up. The RF portion of the URM-25F worked fine, but the audio oscillator would cut out after a half-hour or more.

IMG 3551.jpg

This is the classic symptom of leaky capacitors in the AF oscillator. I think the capacitors are hermetically-sealed paper-in-oil, and they look to be the original ones. I tested a few for leakage, and they all were leaky. Fortunately the oil-filled power supply filter capacitor (the large silver component on the left of the photo) was still good. Perhaps it actually has a plastic film dielectric rather than paper.

IMG 3557.jpg

Replacing all the paper capacitors involves pulling the oscillator turret and bridge assembly from the front panel. This involves disconnecting a number of wires and shaft couplers. 

I followed the procedure in Section 7 Paragraph 17 of a copy of the Navy manual (Navships 92495) that I bought from W7FG manuals (now Vintage Manuals). Vintage Manuals no longer seems to offer a paper copy, but Navships 92495 is available from BAMA. BAMA also has a number of manuals for other variants of the URM-25

I  took lots of photos along the way to help reconnected everything. The procedure starts with disconnecting the three leads from the terminal board in the photo above.

IMG 3558.jpg

Unsolder the ground lead from the lug adjacent to the attenuator. 

IMG 3561.jpg

Unsolder the step attenuator lead from terminal board TB101.

IMG 3559.jpg

Unsolder the resistor from the HIGH RF output connector.

IMG 3562.jpg

Unsolder the three tube heater leads from the feedthrough capacitor.

IMG 3563.jpg

Unsolder the heater ground leads from the ground lug near the feedthrough capacitor.

IMG 3565.jpg

Unsolder the red B+ lead from the feedthrough capacitor.

IMG 3566.jpg

Unsolder the red and white B+ lead from the RF choke.

IMG 3567.jpg

Loosen the couplings on the function switch,

IMG 3569.jpg

the modulation level control,

IMG 3570.jpg

the microvolts control,

IMG 3572.jpg

and the tuning shaft. I had to pull V103 to get to everything.

IMG 3574.jpg

Loosen the clamp screw on the turret. Note the position of the turret on the shaft; replace it at the same depth during reassembly.

IMG 3573.jpg

Loosen the set-screw on the turret shaft cam.

IMG 3575.jpg

Remove the four 10-32 screws at the base of the turret assembly. I couldn’t do it without a right-angle screwdriver, and there wasn’t room for a ratcheting one. So this was a tedious task.

IMG 3576.jpg

Another painful one.

IMG 3577.jpg

I was able to remove this one with an ordinary straight-bladed screwdriver.

IMG 3578.jpg

IMG 3579.jpg

The final screw.

IMG 3580.jpg

This one was already missing.

IMG 3581.jpg

Lift the bridge assembly away from the panel. Be careful to lift the cam along with the bridge assembly to avoid damage to switch S101, which is visible just below the bearing at the top of the bridge assembly.

IMG 3582.jpg

I put a short across the power supply filter capacitor. Oil-filled capacitors are notorious for dielectric absorption, so even after being discharged they might regain enough charge to bite you.

IMG 3583.jpg

A view of the RF deck.

IMG 3585.jpg

The underside of the RF deck.

IMG 3586.jpg

Here’s a view of the tuning capacitor.

IMG 3587.jpg

The audio oscillator bridge assembly. I’ve already removed one of the paper-in-oil capacitors.

IMG 3592.jpg

IMG 3593.jpg

Here I’ve started replacing a few of them.

IMG 3596.jpg

It was a bit of a challenge getting in beneath the tangle of wires from the harness, at the lower right of this photo.

IMG 3597.jpg

There are only two capacitors that need to be replaced on the RF deck - a Black Beauty and a paper-in-oil.

IMG 3598.jpg

I replaced them with new yellow polyester film caps.

IMG 3600.jpg

There is an electrolytic bypass cap underneath the RF deck that needs to be replaced. 

IMG 3695.jpg

I restuffed it. Keeping the original mounting seemed like the easiest course of action.

I followed the reassembly instructions and miraculously it all worked when I was done.

© Steve Byan 2011-2019