I got the email but figured I would reply here so all can see; from the email, and it reminded me I haven't looked in on the EEVblog for some time:
another member here suggested you may have just a "bit" of insight on the 8753C. I recently picked up a unit and unfortunately the A5 "Sampler/Mixer" card on Port A is dead. I confirmed this by swapping it with Port B, and the problem followed the card. My question is, if I replace the card (mine are all labeled 5086-7413 on the sampler) with a random unit, what kind of trouble can I expect if I don't do have it lab calibrated? This is just for home-lab use, nothing critical and nothing that requires traceability.
Will the standard user Open/Short/Load/Thru CAL's get the unit close enough for most use? Or is there enough variance from part to part that it can cause issues?
Short answer: It should be fine.
Long Answer (digging back to 1983 when I designed that sampler...) The diode bias doesn't change much but does effect the absolute conversion gain. The different samplers will have slightly different frequency responses so that they might not pass the absolute conversion loss test or the frequency response test. But these limits were set based on the HP8754 which did not have a computer or user error correction and at the time the thought was "some people will want to use the raw performance just like the 8754 so we need to make it just as good". So this was the first time anyone had put in a "factory calibration" with numeric correction in an instrument (well VNA anyhow).
However, since you can correct for all that (and in fact if you use it with a test set the test set will dominate the response anyhow), you should be fine to just use it as is. You can hook up a power splitter and measure A/R and B/R to compare the two receivers (you always have to have a signal in the R channel; no such thing as freq offset mode back in the day).
Likely the A receiver has a blown 50 ohm resistor (well, I think it more like 47 ohms) in the front stage. People were always blowing it up by overpowering the receiver. In fact we did a study on the kind of damage we can find vs. power applied, and we could tell if it was 1 W, or 10W or 100W that was put into the receiver by the kind of damage on the that thin-film resistor (e.g., if there was no resistor but charcoal on the lid => more than 100 W).
The lid is epoxied on so it will come off if heated to about 125 C and you can look inside. See if the input resistor is intact. The next thing would be the common base transistor that follows (it was an HP Tech center part TC16). The final thing to look for is a bond wire from one of the DC feeds that might have popped loose. If it is the transistor, any chip transistor with Ft over about 20 GHz should work. If it is the resistor you can try bodging in an 0201 or other small 40-50 ohm resistor (47 was the design value to make up for the 3 ohm input looking into the emitter of the CB stage) and see if it comes back to life.
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