A small follow up to this repair project.
So I made extension cables to be able to probe the faulty board on top of the VNA. Found out that it has system power supply of +15V, -15V, +5V, +28V.
Then I took several photos with thermal imaging camera. It helped with finding suspicious regions within VCO AUX can.
Now I'll describe what I found in each can:
1. VCO AUX
RF output of this can has a clean and stable peak of 1010MHz and 10dbm.
There are 2 NE5534 opamps. I think they are dead. The negative supply rail is -14.9V, but the positive one is +25V. And I couldn't find where these 25 volts come from. There are also hot transistors and resistors.
2. VCO LOCAL
It's very similar to VCO SOURCE can. They both have MGA1 module, similar schematic but different topologies.
By default, this VCO generates nothing at RF port. There is a NPN amplifier BCP68 as power supply for MGA1 module. It outputs ~1.6V. I measured the same part in VCO SOURCE can and with the same base (13.5V), collector (14.9V), emitter voltage is 12.7V. So when I applied 12.7V from external power source, I could see a peak of 1668MHz, unstable by frequency and amplitude.
This can has a pair of NE5534 op amps on the back side. The negative supply rail is -2.5V, the positive one is 0V (short with ground). Doesn't sound right.
3. VCO SOURCE.
It has 2 RF ports. The one on the right is 10MHz reference input and used to lock at least VCO SOURCE and VCO AUX output.
The other RF port is used as stimulus generator of VNA (at least at VNA's Port 1). Within sweep region of 1-2GHz it is exactly the same, in other regions (0.3-1GHz and 2-4GHz) signal is mixed (or divided or multiplied). This can also has a pair of NE5534 op amps on the back side. The negative supply rail is -2V, the positive one is 24.6V. Doesn't sound right either, probably are both dead.
4. CANS WITH DIGITAL STUFF
There are many of them. Each can has it's power supply regulator based on BCP68 and I couldn't find any failure.
That's all for now. Still looking for service manual. There are several test points on this board and I wish I could verify them.