Hi, Fortunately I have not had this problem but I did have to deal with a destroyed SRD in a HP 5110A synthesizer driver a few years ago. The SRD in that device is maintained at an elevated temperature in a little oven and as a result of a thermostat failure it was reduced to a charred blob. With no possibility of finding an original replacement I set the harmonic generator module up on the bench ( no easy feat!) and tried a range of different diodes to see if any of them would work. After a lot of experimentation I finally hit on a power varactor diode that was the final frequency multiplier in a junked cellphone transmitter from the original analog FM system that was eventually replaced with the current digital one. That diode was made by National Panasonic and is a gold plated stud mounted device. I had to fiddle around to fit it in but the 5110A has not missed a beat since. Of course, in the 5110A it does operate at a much lower frequency than the 5061A, but it's original use was in the upper VHF range I think. If you can't find a replacement you might have to resort to similar experimentation . Morris > May I know if anyone knows the part number or the specification of the > step recovery diode in the harmonic generator for the old HP 5601A > cesium frequency standard? > --- This email has been checked for viruses by AVG. http://www.avg.com