A 1N4148 will never be able to work at 250MHz.

Rtaher try 1N3595 or equivalent.

Charles a écritÂ*:
Just finished working on this old beast.Â* The display was flashing,
indicating an open amplitude control loop.Â* The output vernier had no
effect.Â* There is an op-amp error amp and it is socketed (741) so that
was easy to try first.Â* No joy.Â* Looking at the schematic it seemed the
peak detector diodes would be easy to check.Â* Turns out that they are
under two covers but still not too hard.Â* One cover is glued down but
was easy to pry off.Â* One diode was open.Â* I tacked in a 1N914/1N4148
substitute and amplitude control was regained and no more flashing
display.Â* The output at 250 MHz is within 1 dB.Â* Not too bad.

To fix it right, I am looking at a MMBD452LT1G.