A Long Overdue Fix - April 13, 2017

It's been almost 20 years since I got this AN/URM-25. It's the first thing I ever bought on eBay. The first time I turned it on there was no output. Turns out some of the precision resistors in the step attenuator were fried. The guy who sold it was very apologetic and gave me a partial refund, and I was happy to order the resistors and replace them. I used it back in the 1990's to work on my very first R-390A. It got me through a basic alignment, mostly, but I was never really sure about the quality of its output.

So this time around I can tell it needs some work. Going through the IF section on the R390 I could see once the output level dropped below 1 mV the waveform was distorting. Intermittent oscillations were happening where the frequency would just take off somewhere else. Varying the audio modulation level sometimes pulls the frequency with it.

I vaguely remember having the box opened up with the subassemblies out a long time ago. But I don't think I wound up doing anything to it other than replacing the attenuator reistors. And I've still got Dallas Lankford's original technical notes. On paper. So I think it's time to open it up and see if we can make things better.

Already Capped - April 14, 2017

Huh. Obviously this unit has already been recapped. And by me no doubt although I don't recall doing it at all. Looks like I did the Dallas Lankford orange drop replacement.

With nothing obvious to replace, I did a tube check and found V107 the 6AH6 Modulation Cathode Follower tube to be extremely weak. With no handy spares I subbed a 6AU6 in and powered the generator back up. Frequency now stayed solid while I varied the Audio Modulation pot. And on the scope the sinewave looked good on levels below 1 mV. That would be nice if all this thing needs is a single tube to get it going again.