Electronics > Repair
E4433B RF sig gen repair
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microbug:
I put the synth board back in with the lid loosely resting on top, and the PLL was still losing lock. However, when I screwed the lid down tightly, the PLL stopped losing lock -- so I will ignore this error for now.

I had a look at the VCO output, which is well marked on the PCB with a socket for an RF probe. It's all over the place -- I expected to see 0.5-1GHz, but it was a mess of peaks that were almost never in the right place. This testpoint was on the output of RF amp U16, just after a 4dB pad. I also checked directly on the output of U16 and saw the same.

Next I checked the input to U16. I swept the sig gen from 2GHz-4GHz, and lo and behold, a clean peak moving across the spectrum! Power level is consistently -16dBm, which is about right if we assume the coax I soldered on forms a splitter that drops another 6dB. The coax is quite thin and lossy too.

Then I googled 'Agilent "A06" repair'. What do I find but another thread pointing the finger at the same part number. Very suspicious...

I will probe around a bit more and check the other Avago MMIC (A03/A06/A07) parts on the board.

Edit: will take a better picture of the problem area later, but U16 is visible on the zoomed picture of the synth board a few posts ago.
R-1125F:
It looks like you found the problem. This should not be too dificult to fix.  MSA-0686 is available on ebay but who knows if you get real of fake.  The TSP repair video has a similar repair with a part that needed a different bias.   
vaualbus:
Cool!
Seems you found at least something "wrong"! I would first check the BIAS T of that amplifier maybe some the resistor/inductor got open? 
Also I would check for soldering cracks, I had a failing 8643A generator failing self test, I track it down to the output amplifier than I try resoldering the main output RF transistor amplifier and be hold afterwards it works and passed self test, I guess there was a cracked join in the soldering! The HP 864x series were the first RF generator made by SMD components that HP designed, very beautiful machine!

microbug:
The Minicircuits MAR-6SM+ looks like a promising replacement that is available from Mouser.

Thanks vaualbus, I'll check that too. I don't know if a broken U16 explains all the symptoms -- why would the PLL still lock? Maybe it is less broken than it looks when it's loaded correctly.
vaualbus:
I think is because the lock is before the divider!
As you see the schematic block F of that page, we have the VCO output that is 3db splitted into two paths: one is than feed to U15 and go to the prescaler and than eventually yo the phase detector locking the loop, the other side is feeding U16 and than the dividers/filters and eventually generate the Frac-N module output signal.

As it can be seen, U16 that seem suspicious is on the other side of the splitter so the instrument is "deaf" of the problem. I guess they can only figure out it by having the ALC detect an invalid level of signal?   

Never mind I confused U15 with U16  |O U16 is feeding the lock, U15 the dividers and output.
Still dunno what happen if one that amplifiers are bad and could create a problem on the others. :-// :-//
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