Repair RS SMIQ03B with Error 211 Summing Loop Unlocked

[mcckevin]

Hi –

I recently pulled an R&S SMIQ03B signal generator from my “To Fix” pile. I’ve run into a problem that I could use some advice solving. Some details follow:

The internal battery in the front panel control unit had petered out, which caused the loss of the  calibration memory. I replaced this battery, then ran the Utilities/Calib/All/Calibrate All calibration routines. These routines returned the following errors:

• ERROR IQCON no convergence
• 180 Calibration failed; Level preset

Checking the logged errors, using the ERROR key on the front panel produces:

• ERROR -313 Calibration Memory Lost; LVL PRESET – run internal calibration;
• ERROR -313 Calibration Memory Lost; ALC OFF MODE = TABLE; run LEARN TABLE;
• ERROR 110 Output unleveled; ALC failure

Looking at the signal generator’s output on a spectrum analyzer showed me that the frequencies were  accurate but that the output power was about 15 dB too low. Also, the harmonic content of the output was through the roof – the second and third harmonics were only about 10 dB below the fundamental.

These data pointed toward V104, the SHF-189 NFET in the IQ-MODULATOR block, as others have had this NFET fail. Probing with a multimeter revealed that the biasing on this transistor was incorrect, compared to the data sheet and to the other FETs in the the RF chain. Replacing the SHF-189 fixed the power output and harmonic distortion problem.

But now, I’m stuck. When I look at the signal generator’s output on a spectrum analyzer, the power and harmonic content are reasonable but now the frequency is sometimes not correct. The SMIQ03B occasionally flashes an

• Error 211 Summing Loop Unlocked

error that coincides with the incorrect output frequency. I hadn’t seen this error before working on the IQ-MODULATOR board.

So, I’m not sure if the 211 Error was masked by the bad SHF-189, or if it’s a new error (something that I caused when replacing the NFET). Or, the problem could be rooted in the ERROR 313 Calibration Memory Lost error caused by the dead battery in the control unit.

The table displayed under Utilities/Calibrate/VCO Sum looks reasonable (i.e. close to the one displayed in the user manual).

However, the tables under Utilities/Calibrate/Lev Preset (for the IQMOD-CW, IQMOD-VM and IQ-CON) are all filled with +128.0, which seems incorrect to me.

So, there is stands. I’d appreciate any help or hints (or even sympathy). Thanks!

Kevin