Electronics > Repair
Rohde & Schwarz - IQ Modulator A240 (1084.9800.02) - Level Problem
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rg58:
Problem found. V103 (SHF-0186K) in the output stage is blown. Already ordered the part but will take about 2 weeks to arrive.

So our early assumption was correct that we have too low gain, hence ALC is turning N35 fully up to +14V on its output pin 6.

I did check this transistor several times, but as D-S already conducts if Vgs=0 V it's really hard to find when the board is sitting on the table. Unless you apply external voltage to it.

Rds at Vgs = 0V (in-circuit measurement)
V144 - 8.6 ohm
V103 - 6.8 ohm (faulty)
V104 - 12.9 ohm

Of course I found out when checking the voltage on the Drain which was only 0.89V instead of 7.0V, again by soldering wires to it (as well as to the other 2 FETs) and plugging it back in the machine.

I should have checked that Vds earlier, but that N35 thing brought me off track. Not to forget that this is a loop, making things harder. Plus, lots of other signals added along the path, summed and/or switched. There are many ways to get lost during troubleshooting.

Thanks Harry for your help!
Harry_22:
rg58 my congratulations!
Finding a fault in a looped circuit is very difficult. Everything affects everything in the circle.
Great!
rg58:
Exactly!

Guess what? Someone DID mess up! I wouldn't be surprised if it was Rohde & Schwarz themselves during their last calibration.

Remember I said that the date of calibration handwritten on the calibration sticker on the front plate is equal to the date when the error occurred for the first time, as stored in the machine?

Apparently they took off the heat sink from the output stage. There is some heat-conductive black rubber pad underneath, but only covering one third of it!! The 2 FETs were not touching the heat sink and couldn't dissipate their heat. In case of the blown V103 this was 840mW! In this tiny package! Seems these were 2 pads and the larger one fell off. Only V104 made contact to the heat sink through that pad, dissipating 1.2 W of heat.
rg58:
I must correct myself. The pad probably shifted down because of the heat as it overlaps the heat sink. The length can cover the 2 last FETs with the highest power.

But that still tells me that the pad didn't make contact, otherwise it can never shift like that if it would have been pressing against the 2 transistors. I have tried to move, shift and remove it, but it broke off. Will replace it with something better.
Harry_22:
When disassembling the module I also discovered an aluminum plate that hung in the air above HFET. The gap was 2.6mm. The radiator was coated with heat-conducting paste on top which clearly indicated its purpose. The other module had no radiators at all.
It should be noted that the heat was supposed to be removed through the Source terminals. To do this HFET was placed close to the bulkhead on which the radiator was attached.
R&S does not repair modules immediately but replaces them by new or refurbished. Maybe your module is one of these. It shows smudges from the flux being washed off.
What about your second working module?
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