When I first picked up this very nice spectrum analyzer a couple of years ago, I plugged it in to try it out and to my dismay, there was a problem. The high range (2-22GHz) had a couple of spurious signals and very high attenuation of the input signal. I let it sit overnight and tried it again, and happily found that it was working properly. A few weeks ago, the problem returned.
I checked the frequencies of the spurious signals and found they were the product of the 3.3GHz oscillator in the second converter and the LO mixing to give the IF of 321.4MHz. This oscillator isn't supposed to be on when the unit is switched to high range, it's just supposed to mix with the upconverted first IF at 3.621GHz when the unit is switched to low range (0-2.5GHz). The PIN diode switch in the second converter must have been switched incorrectly too, to account for the high attenuation of input signals.
Luckily Keysight (formerly Agilent, before that HP) still has service manuals on their website for some of these older instruments. Unfortunately, the scan quality isn't great on some of the schematic pages, so this is what I get for the circuit that controls the 3.3GHz oscillator and the PIN diode switch:
Some of the component values and designators are unreadable, but I can see that there a couple of test points on the board at the output of the oscillator drive circuit. After moving the spectrum analyzer to a bench where I can stand the units on their sides to access the test points and removing the bottom cover, I test the voltage at the oscillator drive: -7.1V for both high and low ranges. I test the voltage on the PIN diode switch: 19.6V in both ranges. Something is not switching the way it should.
I remove the board from the spec an and test a few things - no shorted transistors, resistors and caps read reasonable values - a puzzle. I try lifting the resistor that connects Q21 to Q4 (R8, I think?) and replace the board in the unit and power it back up. Now the PIN diode switch control is at -9ish volts and the oscillator drive is high (as this signal is emitter current for the oscillator, a low voltage turns the oscillator on). No switching between ranges, but I don't expect it to as I've cut the signal that connects the controller to the second converter.
I probe around a bit more, but don't find anything unusual, so I decide to remove Q21, a plastic-packaged (TO-92) NPN transistor. I test it with the hfe tester in my multimeter and can't get a good reading - perhaps this is the culprit? The C-E junction isn't shorted but it could be conducting enough to turn on Q4. I replace Q21 with a 2N3904 and try again - no luck though. I check the input signal to the card (A6A10, pin 34): 0.7V low, 2.5V high. The high level should be high enough, but is 0.7V too high? It could be turning on Q21 just enough to drive Q4, so I decide to add a 4.7k resistor between Q21's emitter and base - this will increase turn-on voltage at the input to about 1V. Try again: no luck.
This time, I decide I need to see what's really going on with Q21, so I remove the board and solder a couple of fly wires on the base and collector of Q21. With the board back in, I get 0.4V at the base and 10V at the collector in high range, 0.65V at the base and 0.2V at the collector in low range. The collector voltage supply is 20V, so something is going on to give only 10V when the transistor should be turned off (and 0.4V at the base should not turn on a 2N3904). I decide to pull the capacitor across Q21's C-E junction (C9, as far as I can tell from the schematic). I test it with the multimeter - open circuit, so it shouldn't cause problems.
Replacing the board and turning on the spec an, I see that the problem has gone away - high and low ranges both work! The collector voltage at Q21 is now 19.7V in high band and 0.2V in low band. C9 must have been acting as a zener - conducting when the voltage reached 10V, but not at the voltage my multimeter tests at. I replace C9 with a surface mount 1206 0.47uF 50V cap and close up the spectrum analyzer (after removing fly wires of course).
Success! I have a fully working 0-22GHz spectrum analyzer again! (Actually, in range-locked mode, it can be pushed over 25GHz...useful for those 24GHz Gunn diode units.)
